Building the Morton M5








Click on images to view them in larger size.

See the M5 Reference Page for a description and history of this engine.



It's easy to tell that the M5 is a labour of love for Bruce Satra. Along with his outstanding investment castings, Bruce sent me very detailed, hand written machining sequences and fixture diagrams. Some of his approaches are not the way I'd go about a task, but unless I have serious misgivings, I'd be crazy not to follow the advice from a far wiser, more experienced source.



Crankcase - Stage 1

Straight away, Bruce's instructions save me from disaster as he instructs me to machine the inside of the crankcase first to just clean-up for about 3/16" to the plan dimension of 1.620", or larger, if that's what it takes, then machine the gear case flange to suit. The fit of gearcase flange and locating spiggot neads to be very close in the crankcase. Bruce obviously knew his casting ID was very close to 1.620" already, so he'd instructed me to machine it first. I'd probably not have checked, machined the gearcase down to a precise 1.620" and have been royally screwed as my crankcase cleaned up at 1.630"! In this photo, the case is held by the machining stub and tapped lightly with a soft mallet to get the runout on the ID to a minimum. I managed to get it to 0.0015".

Machining was straight forward, apart from the case's tendency to ring like a bell during turning--probably due to it being shaped like a bell! Magnified examination however showed no chatter, so no problem, otherwise I've been told plasticine in the cavity will deaden rigging/chatter. The locating flange on the gearcase will protrude 0.062" into the crankcase, so my clean-up depth of the ID could have been less, but I followed the master's words. Variations show that the raw ID was slightly distorted at the cylinder flanges which accounts for the 0.0015" my DTI indicated, but I seem to have averaged out the variations well. The case is faced back until the distance to the center of the cylinders is 0.571". If you look closely, you may see the center-pop mark arrived at using odd-leg calipers in the uppermost cylinder flange. Also note the hole in the rear face. This is a cast-in index for the bottom most screw hole and assures correct orientation of crankcase and gearcase. Nice touch, that Bruce! Now to prepare the mating gearcase casting.

Gearcase - Stage 1

The front face of the casting is lightly filed so it will sit flat for drilling of the mounting holes. There are 3 mounting holes and 9 attachment holes. Bruce says to drill the mounting holes #33 to clear 4-40 so the casting can be secured to a plate to machine the gearcase cover flange. These later get tapped 6-32 and the tapping size for that is #36. I decided I could clamp the casting down so went with the correct tapping size, though I'll tap them later in case they have to be opened out to clear 4-40. All the other holes are drilled #44 to clear 2-56. The cast-in dimples for these holes are as good as center drilled starting holes and I trust their location to be accurate (they need to be to ensure the mating holes that will be drilled in the rear face of the crankcase are centered in the thin wall). One step omited from the instructions, possibly because it is so obvious, is to spot-face the 2-56 holes after drilling. You may be able to see the drilled and spot-faced holes at the left and the dimpled, undrilled holes at the right in the badly flared photo.

In this shot, the clamping has been moved so the rear face where the gearcase cover plate will fit can be machined. This is important as it will form the mounting face to machine the flange that mates with the crankcase. Bruce's instructions say to remove only enough metal to get the face to clean up completely, so that's what I did here, using a 1/2" end mill. The face was then lapped on 600 glass paper and a surface plate.

The gearcase is now bolted to a flat plate which can be clamped to a lathe faceplate. After light clamping, the plate (not the casting) is tapped with a soft mallet to get the OD of the spiggot to indicate true. Again, the raw casting is not perfectly circular (neither would I expect it to be), so the errors were averaged at 4 mutually orthogonal points. Total runout again indicated less than 0.002". The clamps are then tightened and the run-out rechecked to ensure it did not move during tightening of the clamps.

Finally for this stage, the spigot is machined to a close but not tight fit in the crankcase for a length of 0.062". Having had close fitting aluminum parts weld themselves together before, the case rim was smeared with oil before offering it up to the gearcase. As the rim cleaned up, the location of the mounting holes relative to the flange center could be checked (but not changed). They are good--not perfect, but good. This shows that the dimples were accurately cast, thank goodness! I'm not mentioning it each time, but after turning, all sharp edges are "broken" by setting the top slide over 45 degrees and taking a cut of 4 to 5 thou feed from there the tool first touched the metal.

Boring for Bearings

Bruce's notes gave a smart way of boring the crankcase and gearcase for the forward and aft crankshaft bearing journals--I don't know if I'd have thought of it independantly. First the mounting holes for the crankcase bolts need to be drilled and tapped. This starts by drilling the cored hole (mentioned above) #50 for tapping 2-56 to a depth of 1/4". The core location correctly orients the crankcase and gearcase allowing the remaining 8 holes to be spotted through the gearcase with a #44 drill as seen in this photo. I used a rotary table and wound each hole in succession around under the drill, but the case could have been juggled around.

With the gearcase removed, the spots can now be drilled #50 1/4" deep and tapped 2-56. I was worried that they may end up so close to the inner surface of the crankcase that tapping would raise that surface (as has happened on the Morton case machined by that unknown worker an unknown number of years ago). However, despite my wall being 0.005" thinner than the drawings, the holes came out close to center and tapping caused no problems. On operations like this I always start the tap by placing it in the drill chuck (keyless chucks are marvelous) and rotate it by hand to ensure a true alignment. The thread is finished with the small tap handle visible above. Small handles on small taps provide the "feel" necessary to prevent tap breakage (so how come I still break taps? ...nevermind...)

The gearcase and crankcase are firmly secured together with an adequate number of screws and the assembled unit set to run as true as possible around the opening in the rear of the gearcase that will eventually take the bronze bushing for the cam carrier and its gear. After this is done, it is important that the position of the crankcase in the chuck not be changed until all further operations on it are complete. The journal can now be bored out to 0.5625" as measured with a telescoping hole gauge and a micrometer (actually this dimension is not that critical as the bushing can be turned to suit).

The gearcase is now removed and the seats for the front crankshaft bearings bored out. This assures axial alignment of the front and rear journals--the register between the two parts can fall where it will and mine tracks a thou or so off normal. The plans call for a five tenths (0.0005") interference fit on the bearings. Bruce says that's ok for the rear bearing, but to reduce it to two tenths for the front bearing as the case is thin there, so that's what I did. How do you machine to tenths of a thousandth when the crossfeed dial is calibrated in thousandths? One way is to set over the compount slide 6 degrees (5.7392 actually, but who's counting). This "magnifies" the dial reading by factor 10, effectively recalibrating it to tenths, and use it to put on the final cuts. I didn't do that though--I just ran the boring tool in again at the same setting and let tool springback take another wisker off. Repeat if necessary. Smells of VooDoo Black Magic, I admit, but I hit both dimensions spot on, so I'm happy. Oh, I also mike'd the bearing OD's to calculate the ID required in case they were different from the plans.

Crankcase - Stage 2

The chuck and crankcase are now moved to the rotary table on the mill to complete the cylinder mountings. But first, I decided it was time to re-swing the mill head as I'd noticed that the large diameter end mill I'd used to face the gearcase had been taking just a wisker off on it's trailing edge when cutting right to left, indicating that the mill head was not perfectly perpendicular to the table. The drill chuck holds parts liberated from a magnetic base used to mount a plunger type DTI so it can be swung through an arc about 18" across. The head is rotated so the DTI reads the same at both sides. Well worth doing more often--damn thing was out by 0.04 in 18". That's only 0.12 of a degree, but significant nevertheless.

The rotary table is aligned to the table axis using an engineer's square (I've previously convinced myself this is accurate by clocking across the flat face of the table when aligned this way) and the plunger DTI mounted in the quill to set the #1 cylinder seat flat. On my table, 72 degrees (the angle between cylinders) is exactly 18 turns of the crank, so the micrometer collar on the handle is zeroed at this setting--you can see it on the extreme left of this picture--and we're ready to mill the cylinder seats flat and set the deck height relative to the table axis. The Morton drawings give the deck height as 0.938" +0.001" -0.000", but how to measure this? At first I thought, easy! This is why Bruce's instructions said clean up the gearcase register for 3/16" when the gearcase only intrudes 1/16". Half the ID, plus the thickness from ID to cylinder seat gives the deck height. But alas, when the crankcase was mounted last, it was adjusted to get the center hole of gearcase running true, so the gearcase register in the crankcase could be anywhere!

This photo shows the first attempt. All seats were milled flat, but left oversize 5 to 10 thou as measured from the case ID as described above. My plan was to bore all the cylinder holes, then average the readings and mill the seats to deck height by rotating the case with the quill locked at this averaged height. The Heath-Robinson support was cobbled up from clamping kit bits and while it provided support against downward drill loads, the rotating radial load produced a noticible movement of the case. Clearly, a better arrangement was needed.

The solution I arrived at was to machine a sholdered plug, the small end being a close slip fit in the gearcase bearing hole and the other end, a concentric 0.6000" diameter. This was done in the 4 jaw chuck as my 3 jaw was quite obviously tied up! The plug was parted off, reversed with the 0.6000" end set to run true, and center drilled. The oiled plug was pressed into the gearcase which was securly bolted to the crankcase. A mill table tailstock center was then brought firmly into the plug and a DTI used to check that the stub turned true as the rotary table was cranked around (it did). Now I had a reference point (the stub OD) which is 0.3000" above the case axis allowing all cylinder seats to be milled to the required deck height. Additionally, the assembled case being effectively supported between centers, was absolutely rigid in all axes.

This photo shows the steps for boring the cylinder liner holes. These are supposed to be positioned 0.571" back from the case rear, but this face is not running true (due to following the instruction to clock on the gearcase bushing ID, not the gearcase register!) I could either average out difference (nearly 0.003"), or set each one individually meaning my cylinders will not all be exactly in the same plane. For better or worse, I chose the former; so the process is:

  1. Rotate case to locate the quill at the averaged joint location
  2. Wind across to 0.571" from edge, then center drill, followed by 5/16 and 5/8" twist drills.
  3. Bore for slip fit of cylinder liner (0.688") with micrometer boring head
  4. Wind around 72 degrees (18 turns) to the next location and repeat
  5. Finally, set an end mill 0.638" above the 0.6000" plug and finish machine all seats to height.
As a precaution, the center of each flange was located approximately with odd-leg calipers before step 2 so I'd know if the point I was about to drill was off flange center. Fortunately, none were off to any degree worth talking about, or so I thought (see later). Visible on the mill table is one of the cylinder sleeves obtained from Vernal Engineering. These are chrome molly tube of the correct ID (0.625"), ground on the OD to the correct diameter. They will be lapped before being glued into their cylinders.

The final operation for this stage was to machine the carburator flange flat and drill the inlet and mounting holes. This is located between the #3 and 4 cylinders, directly opposite and under the #1 cylinder, so the position was easy to derive as 9 turns from a cylinder face. It's interesting to note a drawing change here. The Morton drawings I got from Vernal Engineering show a 1/4" inlet bore with mounting holes either side, 1/2" between centers. An earlier drawing (in the set available from ECJ) have the mounting holes 0.563" between centers. I'm guessing the dimension was reduced because the 9/16" spacing placed the holes too close to the carby casting edges. Regretably, I drilled the Morton case to the old drawings before discovering the change. The Satra case is to the new dimension, so my carbys will not be interchangable.

Note To Self:

If I had it to do over, I think I'd clock the crankcase true on the ID, then bore the gearcase to diameter for the 0.563" bushing, or larger if it did not quite clean up at that diameter. The plug mentioned above would be machined to fit and used as described. The advantage is the case join would track true making the location of the cylinders relative to this face consistent. Hindsight is a wonderfull thing, right? Oh well, next time...

Gearcase - Stage 2

There are 15 holes to be drilled in two bands in the gearcase. The first 10 are the tappet guide holes. These are 0.109" back from the join line (my reference point previously set) and have an uneaven spacing. Each pair is spaced 40 degrees apart, with 32 degrees between the outermost holes of each pair (5 times 32, plus 5 times 40 equals 360 degrees). As the carby mount in bang in the middle of a 32 degree spacing and one turn of the crank exactly equals 4 degrees, we are in business: 4 turns from the carby to the first hole; 10 turns (40 degrees) to the next; 8 turns (32 degrees) to the next, and so on. Each hole is center drilled, followed by a #27 drill and a 5/32" reamer.

The tappet guide holes are followed by the fuel feed pipe spider holes, 0.343" from the reference plane. The feed pipes will be bent (and I'm anticipating great difficulty here) from 3/16" brass, or aluminum tube. The ends will be lightly swaged to be a wring fit in the spider and cylinder holes. Now, the Morton drawings show the 5 spider holes as coincident with 5 of the tappet guide holes, but I found out on the Morton case (and later, examining a factory built Morton) that is just ain't so and there was an error in the casting. Happily, the Vernal casting corrects this error and the holes can be drilled as drawn.

This photo shows how the feed holes break into the gearcase cavity, behind where the valve lifter cam will reside. Note that each hole ends on a step, the depth being 3/16" from the case surface on 4 of them (one being drilled through the extension of the gearcase that houses the distributor drive shaft). A twist drill encountering this step would be pulled off center and you'd end up with an oval hole. To prevent this, the holes are drilled with a 3/16" ball end slot drill as seen in the previous photo. These are far more rigid (and shorter) than twist drills, so the problem is prevented from occuring.

Crankcase - Stage 3

I could have drilled and tapped the cylinder mounting holes using coordinate drilling, as the cylinder centers can still be reset using the mill DRO, but having made this drill jig for the Morton case, why not re-use it, especially as it will be used to make 2 jigs that will hold and orient the cylinders for machining. The two pegs at the rear of the jig register against the join face to accuratly position the hole pattern (and I can double check against the DRO at a glance). Each hole is drilled #50, then #44 to just break the surface, then tapped 2-56. The #44 dimple prevents the tap raising a burr above the surface as taps are wont to do.

With all cylinder holes tapped, the case can be parted off from the machining stud. The tailstock chuck here holds a length of rod which will catch the case as it is parted off. I had one case (the Original Ohlsson replica) part off, bounce up and get totally mangled between spinning chuck and lathe saddle. Ever since, I've taken this little precaution when parting off large objects that took a long time to make.

Last operation for the crankcase is to reverse and grip lightly on the gearcease internal register to bore out the nose opening and clean up the case front. Sadly, all my cylinder holes have missed the exact center of the crankcase boss-I absolutely don't know how--considering all the trouble I took to prevent this! It became evident only after the drill jig was removed and showed the hole in the south east position being closer to the edge than the others. It's not bad, but it's not perfect <sigh>. I probably should have wound the boring head out to boss diameter and watched how it tracked as the head was rotated by hand. More hindsight...

Gearcase - Stage 3

For the next operation, I figured that the mill vice needed to be square with the table travel. I do this by mounting a small DTI to indicate the rear vice jaw as the table is cranked over the jaw surface. Zero at one end; wind to the other, then tap the the vice around to halve the error indicated. Re-zero and wind back to the other side, repeating until the DTI indicates less than 0.0005" over all.

In the next operation, the hole for the distributor layshaft will be bored to 0.359". As my commercial boring head tools are not that small, I needed an adapter bush so I could fit one of my shop-made boring cutters to it: the commercial cutters having 12mm shanks, while mine are 1/4". I thought this shot worthwhile including because it shows a very simple way to position in the center of barstock for cross-drilling. Just mount the stock to be drilled horizontally under the mill (it's sitting in a V-Block here) and take a cut across the high point of the bar with a milling cutter to just kiss the bar. Now position your center drill in the middle of this cut and you will be so close to on center, it's not worth talking about. This tip from Model Engineer circa 1991, by Guy Lautard, the author of the Machinists' Bedside Reader series.

The layshaft bushing bore must be exactly 0.500" from the case axis to give the correct gear mesh, so I decided to mount the gearcase to a plate using the motor mount holes. As the plate will be mounted on parallels, the screw heads can be below the plate, so the holes were tapped 6-32 and I had avoided drilling them out to clear 4-40. To center the mill, I'm using a co-axial indicator. This is a handy tool I can't use often because of my "mill" geometry. The tool takes a lot of vertical space, but after setting, if I change the head height, table position is lost (don't buy one of these round column mill-drill creatures!).

With the case axis located, the mill is would across 0.005" and the hole for the bushing bored and the gear cavity cleaned up. For this operation I was depending on the gearcase vertical axis being lined up with the mill axis (hence the care taken to indicate the vice jaw). After the bad experience with the cylinder bosses, I swung the boring head around the gear cavity figuring it was more likely to be in the correct location and found I was 0.002" out, probably due to lack of precision in the drilled mounting holes, either in the case or plate. I adjusted and the result looks good. There's a piece of note paper under the case in this shot to protect the gearcase flange when it is clamped down.

The last operation on the gearcase will be to drill and tap the mounting holes for the gear cover plate, but this will have to wait until all the cam and distributor gear shafts and bushings have been completed so everthing can be assembled and the cover plate screw holes spotted through the plate into the gearcase in the correct relation.

Timing Gears and Stuff

The bushings are now turned from bronze to fit the gearcase bores. These can either be a press fit, per the original, or fitted with an anerobic adhesive, like Locktight. If the latter, they need about 0.001" of hole clearence so that pushing the bearing in does not wipe all the glue off. I've never liked this much, but read that an alternate is make the bush a firm, finger press fit, but file three or four light flats on the bush OD to act as adhesive resivours. These only need to be 0.001" deep, so they are easy to make and the bush will be firmly and accurately held in place. This shot shows the bushes before flat filing, together with the cam carrier and gear.

The other bushing required is a press (or adhesive) fit inside the cam carrier gear which allows the rear "slave" crankshaft to run co-axially within the cam carrier. The photo shows all the components for this assembly. In the top row from left to right are the slave shaft, cam carrier with integral 36 tooth spur gear, the 16 tooth layshaft drive gear and the bearing which will reside in the gearcase cover plate. Below are the captive, internal cam carrier bushing, cam and gearcase bushing. The gears and cam were purchased from Vernal Engineering in 1996. Today, I would make these, along with the cutters to form them--amazing how jobs once viewed as hard become easy--or at worst: tedious. The cam has been nitrided (fully hardened). Now this is beyond me, but I could case harden one adequately, I think.

The coaxial cam is driven by a layshaft (which I think means a shaft suspended over another) that revolves at 1/2 crankshaft speed and also drives the distributor and moves the points. To do this, one end carries a 5 lobe cam for the points and a flat for the distributor arm. The shaft is machined from 0.3125" drill rod. A quick spurt of CAD tells me that 0.030" needs to be milled off the 5 sides to form the cam. I milled to 0.028" to leave slight rounded edges. The distributor arm is drawn as trailing one of these flats by 30 degrees, so 18 turns each to mill the flats, then back 7.5 turns from the last flat for the arm flat. Actually, the Satra distributor arm has no molded in key. This is built up with epoxy when the engine is "timed". So the position does not really matter, however, for the sake of authenticity...

And lest you think this is all plain sailing, it took two attempts to get this simple piece of turning right: there are five concentric, close tolerance diameters on that shaft: 0.1260", 0.2971", 0.3000", 0.3125" (stock), and 0.1875". Tolerance are respectively, +0.0002", +0.0005", -0.0005", -0.0003", and -0.0003" (0.0000" where not stated). Easy to screw up on any one (why is it always the last?)

Here's the layshaft (or distributor drive shaft) components. From left to right we have the 12 tooth cam carrier drive gear, the 32 tooth gear that drives the layshaft from the slave shaft, then the layshaft itself and the distributor arm. Above the shaft is the bushing for the gearcase that runs between the two gears. All the Morton gears are neatly selected so the pinion gears are smaller than the shaft diameter that carries them. This allows them to be permanently pushed onto their shafts, yet still be easily disassembled.

Just to put the engine timing gears in perspective, here is the exploded view. The slave shaft (top) runs inside a captive bronze bush in the cam carrier, which runs inside a bronze bush in the gearcase. The aft end of the slave shaft carries a 16 tooth spur gear and is supported by a ball race (not shown here) in the gearcase cover casting. Above this shaft in the engine (but below in the photo) runs a layshaft with a 32 tooth gear driven by slave shaft spur gear. This drives the layshaft at 1/2 engine speed providing drive for the distributor arm and points via a milled in 5 lobe cam. At the front end of the layshaft is a 12 tooth gear that drives the 36 tooth cam carrier at 1/3 layshaft speed. This means that the cam rotates at 1/6 engine speed, in the same direction as the slave shaft reducing friction and wear a little. The layshaft is supported in a bronze bush between its two gears in the gearcase, and at the other end by an unbushed hole in the gearcase cover. The outer surface of this journal will also carry the distributor body. Now we can machine the gearcase cover plate casting.

Gearcase Cover Plate

The rear cover for the gearcase on the M5 has two bearings that must line up with those in the gearcase. These pose some workholding problems. The uppermost is a plain, unbushed bearing for the layshaft. The plan for it was to bore it and face the flange at the same setup. This assures that the rear part of the layshaft bearing will line up with the front (bushed) section in the gearcase. To accomplish this, the case was gripped in the 3 jaw chuck as seen here, with the boss for the ball race carrier resting against a chuck jaw to prevent flexing of the casting as the interrupted facing cut was taken.

In case you are wondering, I've no idea why Bruce cast in that cavity at the base of the layshaft bearing either. It is not present in the original M5, nor the factory drawings. Maybe he had plans to insert another ball race there? If so, these cavities would just touch as the shaft spacing is 0.500" and so is the bearing diameter and that does not sound like a really great idea. I cleaned it up while boring the journal to give me a larger diameter to indicate on in the next step, so it was not a total waste.

Without disturbing the part in the chuck, it was moved to the rotary table under the mill and the bored hole set to run true using the coaxial indicator. With the DRO zeroed at this point, the mill table was wound across 0.5000"--the same distance as used when boring the bearings in the gearcase. The rotary table was then rotated so that the cast in cavity and case were centered laterally under the spindle.

The ball race that supports the end of the slave shaft poses a small difficulty. Normally, the casting would be bored for a press fit on its OD, but the ID is also a press fit on the shaft. This means that the cover plate, once fitted, would be just about unremovable. Bruce Satra recommends that like the original M5's, the cavity be bored for a slip fit on the bearing OD. This allows the cover to be removed, while (hopefully) allowing the ball race to do its job. The setup I used here is, as you can see, quite "delicate", so cuts with the micro boring head were kept very light. Eventually I achieved the required fit without disaster. You can see how close the bearing cavity came to the mystery cavity--scary stuff.

Now the OD of the extension that carries the layshaft needs to be turned down to 0.437" for the distributor body. Bruce said mount it on a "true running mandrel" for this job. Experienced machinists are prone to saying things like that, which leave we amateurs scratching our heads. What's required is a tapered mandrel, which is easy when you know The Trick--which is that a "tapered" mandrel is parallel over most of its length; only the last bit (near the headstock) actually tapers. This allows you to accurately position the work on the true running parallel section, then jam it against rotation on the short tapered bit. I make the taper less than 2 degrees included angle and form a small under cut from which the taper is cut. The beginning diameter of the taper is cut slightly less than the paralled diameter allowing the part to jam up about half way along the tapered section.

I don't see perfect contricity of layshaft bore and distributor spiggot as very important, but this is the neatest workholding solution. The jig was turned and used in place, at one setting as it would never go back in the chuck and run true without a lot of trouble, if ever. If it needed to be reused, the best choice would probably be to clean up the OD and grip it in a collet before turning the mandrel.

The cover is now complete. All that needs to be done is spot through its mounting holes into the gearcase. These are first drilled #44 to clear 2-56 under a drill press using the cast-in dimples as guides (as can be seen in the previous photo). The alignment is achieved by assembling all the bushes and shafts into the gearcase and cover and remounting the gearcase on the carrier plate made earlier--with a 1" hole drilled in its center to allow the web of the slave shaft to poke through. The first hole spotted, drilled and tapped was at the 6 o'clock position. After spotting, the cover was removed and the cavity (with exposed ball race) protected by a piece of masking tape as the hole was drilled and tapped. The cover was then screwed down and this process repeated at the 11 and 1 o'clock positions. At this point the shafts were checked for free rotation with the cover firmly held by 3 screws. Finally, the remaining 6 holes were drilled and tapped without removing the plate.

That completes the machining for the M5 case assembly. Time to put the bits together, make a cup of coffee and begin the obligatory admiration-of-own-work process. Well, nearly anyway. My layshaft goes decidly tight at one point in its rotation. This indicates that my plan to machine it from a piece of drill rod whose diameter was the final, major OD (0.3125") was no so good. The other end of the shaft (0.2971") is obviously slightly eccentric, even though it was turned in a collet. I'll have to make another layshaft. If I can't clock the material to run 100% true before turning it down, I'll have to make it from larger stock and machine all 3 diameters at one setting--that will nail it! Regardless, this little shaft is resulting in a lot of scrap...

Cylinders (Flange)

Machining the cylinder castings is easily the most complex job on the Morton and the most complex aspect of this is workholding. The Vernal cylinders will need 5 different jigs to complete this task; the Morton casting four. With luck, I can make three common to both--and before anyone thinks these jigs are my idea, I must set the record straight: the basic idea for each and every one of them originates with sketches and words provided by Bruce Satra of Vernal Engineering. He's the real Morton Guru.

Unlike the Morton castings, the Vernal ones have neither cast-in liners, nor attachment bolt holes. As the bolt pattern sets the cylinder alignment, drilling these is the first task, requiring a unique jig that will align the cylinder casting for hole drilling and guide the drill. Before mounting the cylinders, the base must be cleaned up with a fine file to remove the casting gate. The cylinder in the middle of the bottom row is "as received"; the others have been cleaned up.

Here's the jig for step one. The steel plate forms a drilling jig with the flange bolt pattern accurately coordinate drilled and an accurate center hole reamed 3/8". A plug turned to a close fit in the as-cast cylinders is centered via a 3/8" spiggot in this hole and secured with a 6-32 bolt and washer from the "top". The plate attaches to a thick aluminum plate whose sides are made parallel (in the shaper). I made provision to dowel the two together, but found it was not necessary. This plate carries two adjustable pillars made by facing back round head 8-32 screws until the screwdriver slot cleans up. These are slotted so their height can be adjusted from the "back" of the plate and fitted with lock nuts to fix their position once adjusted. The cylinder is clamped to the steel plate by two "finger clamps" whose step height of 0.100" is just higher than the cylinder flange to assure a good, positive clamping action--same as you would when using mill clamping kit parts--ensuring that clamping is applied by the tip of the clamp, not the edge of the part being clamped.

The jig is adjusted to set a cast cylinder so that its rocker posts are at equal heights from the aluminum plate by adjusting the pillars with a screwdriver from the rear, then locking them in place with the lock-nuts. As there was no perceptable difference in the as-cast cylinders, this adjustment is a one-time operation. The plate is then secured to an angle plate with clamps so that the plate and angle plate bases are flat against the mill table. This trues up the cylinder flange relative to the vertical axis. The plate can now be jiggled around under a #42 drill in the mill for each hole, then lightly clamped to drill each hole (note the clamping kit parts to the left--finger tight on the nut is quite sufficient). Drilling all cylinders is then a simple, fast operation. Once more, far more time is spent making the jig than using it.

The results speak for themselves. Notice how close the hole pattern is to the edge of the casting, yet all are perfectly aligned and concentric with the central core (more a tribute to the castings than anything else). This may sound like a lot of trouble, but it is essential that the hole pattern be consistant and that it perfectly align the cylinders rocker posts as the holes will be used position the cylinders for subsequent operations (and ultimately align them on the crankcase).

Cylinders (Bore)

This is, beyond doubt, the most complex and time consuming jig required by the Morton project. It uses the flange bolt pattern (which was centered on the cored hole) to position the casting and bore out the cored hole for a press, or slip fit of the steel liner. I chose to go with the slip fit, which will require that the liner be secured with a high temperature anerobic adhesive. This avoids any danger of a pressed in liner splitting the thin casting, or of a liner working loose.

The jig was constructed mostly from 3/8" and 1/4" aluminum plate, doweled together and secured with socket head screws. The top plate is from 1/4" steel and carries three 3/32" pins that fit into the cylinder flange holes to position the casting for boring. The casting is retained by finger clamps, similar to those used in the previous jig. After boring the hole in the plate, the inner surface was faced with a boring bar out to beyond flange diameter (about 1") so it will be normal to the lathe axis.

The bore is already within 0.006" to 0.008" of required size, but all cylinders cleaned up perfectly--ie, no "as cast" edges remain in the bore--another tribute to the quality of the castings. The liners are lengths of 4140 chrome-molly tube with an ID of 0.625" and the OD reduced to 0.686". These are available from Vernal either finished, or with the outside diameter needing turning. In both cases the ID will need honing or lapping. Chrome-molly tube cuts well using insert tooling and high speed. It does not seem to like HSS tooling all that much though (don't ask me why--I'm not a trained machinist). All in all, a day to make the jig; about an hour to use it on seven cylinder castings.

Cylinders (Rocker Posts)

The jig to position the cylinders to drill and ream the rocker posts for the pivot pins is a lot simpler and can be used on both the Vernal and original Morton castings. It's just an aluminum block, positioned on parallels in the mill vice and milled with a 1/4" slot drill 0.110" deep, almost the the height of one post. A finger clamp of 1/8" steel carries one tapped and one clearence hole for 6-32 screws. These act like the clamp screws in a machinists' clamp.

Both clamp and base are drilled #29 to give a little clearence when drilling #33, then reaming 1/8". I'll be using straight pins, secured with E rings, so the operation was very simple. As mentioned earlier, the Original Morton design had a very short 2-56 threaded length (about 4 threads) on the end of the 1/8" pins that I found too hard to make without specially grinding away a perfectly good die (that's my excuse, anyway). My modification is much simpler and more positive, I think.

Valve Cage Recess

The Morton vale design is complex, but neat. Each valve rides in a bronze cage that drops into astepped hohe in the head and is retained against the step by a locking nut. A hole in the side of the bronze cage registers with a hole in the side of the recess for exhaust and inlet. Each valve assembly is inclined 30 degrees off the vertical, positioned on a lateral diameter of the head casting (if that makes sense). The recess diameters, progressing from the combustion chamber are: 0.025", 0.313", and 0.344" tapped out 3/8"-32 TPI. The inlet/exhaust holes intersect the 0.313" bore at 90 degrees, so they are effectively inclined 60 degrees off the vertical in the opposite direction to the valve cages recesses. All this requires One Last Jig.

And here's the jig. It's simply a block of aluinum, bored for the cylinder liner with three pegs to locate and align the head casting via the flange bolt pattern. Tapped holes for the well used finger clamps hold the cylinder units in place for machining. One side of the base of the jig is milled off at 30 degrees, allowing easy positioning of the jig in the mill vice.

The recess is cored in the Vernal castings as a tapered hole, minimising the metal to be removed at the three different diameters. Each diameter requires a sharp step at the transiton, so if the jig (and casting) is positioned so the 0.344" bit is symetrically disposed to cut metal at the top, all the others will fall into line. The operations are:

  • Drill through with a 1/4" slot drill
  • Counterbore 0.375" deep with a 5/16" slot drill
  • Counterbore 0.156" deep with a 0.344" D-bit (or 11/32" slot drill)
  • Tap 3/8" 32TPI to 0.156 deep

3/8-32 is an unusual size, but it is covered by the British "Model Engineer" series. Unlike UNF/UNC and metric threads which have 60 degree included angle teeth, the ME series use 55 degree teeth. This has no significant impact on anything. To speed up the process, I fitted stop collars to the 5/16" slot drill and 3/8" tap. The funny spiral on the slot drill stop serves, no purpose; it's just a left over from an earlier purpose on the lump od material selected from the scrap box for this purpose--a 2-56 grub screw holds the collar in place. The collar on the tap is two 1/2" brass rings, cross drilled 1/16" allowing a tommy bar to be inserted to spin the nuts, there being not enough room in the casting recess for hex nuts. Even bottoming taps have a few threads of "lead". TO get the thread to the bottom of the short 0.156 recess, I ground away all but one thread of lead. With the sliding tap handle seen earlier, this was sufficient to start the tap as the 0.344" hole is oversize from the thread minimum diameter, providing only 75% of thread "engagement".

Cylinders (Exhaust/Inlet Openings)

The inlet and exhaust pipes are a "wring in" fit to 3/16" holes intersecting the valve cage resesses. The Vernal casting Has cored these holes very close to finished size (the original Morton castings does not core them at all). The same jig used to the valve cages can be used by positioning the milled off part of the base vertical in the mill vice with a square. With some carefull jiggling, the casting is positioned so a 3/16" slot drill cleans up the hole equally on all sides (almost). I'd have been happier if Bruce had left more material to clean up, or even left them undrilled, like the Morton castings.

Cylinders (Sparking Plug Hole)

Again, the casting can be oriented using the flange bolt pattern. To save a bit on tooling, I made another plate that bolts to the opposite end of the first jig. This plate is bored for the cylinder liner (allowing it to accomodate Morton and Vernal casings) and fitted with three 3/32" dowel pins that position the casting. Another adjustable pad supports the casting against drilling forces. None of the pads interfear with the casting when the jig base is used for either of it's functions.

The first operation is required only on the Vernal cylinders. These use the center of the spark plug boss as the the sprue for casting. The broken-off stem (looking like a volcanic core) needs to be milled down in easy stages to within a wisker of the plug seat location. Having had cutters "snatch" work like this before and send it on a trip around the workshop, I took off about 0.030" per cut.

Next, the hole for the tap is drilled for tapping 1/4-32 using a 7/32" diameter drill. The hole will break through the wall on a step in the hemispherical combustion chamber and continue in a way. The plans say drill 9/32" deep, bu the cast-in hole in the original Morton cylinders goes considerably deeper than this--almost to the center in fact.

I've compromised and gone in 5/16" deep. The allows a 2nd taper tap to cut a thread or two to guide the plug tap. For more confusion, the Morton plans say spot face the plug seat 1/2" diameter. A supplimental sheet from Bruce Satra says spot face 3/8". Inspection of an actual Morton shows a 5/16" spot face! I went with the 3.8" dimension.

I learnt to fly (full size) in the late 60's. This entailed a lot of study for the ground exams, going into more detail than is required today on engines and aircraft systems. In those days, spark plugs were officially called "sparking plugs"--I've no idea why, but I like the term, so this shot shows the way the sparking plug recess intrudes into the combustion chamber. It all ended up very neat with the tapped hole just clearing the top edge of the liner.








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