MATE Building Instructions - Page 7

 

PISTON (5-1)

  1. Centre the cast-iron stock in the 3-jaw chuck and extend 35mm from jaws. Place a folded paper trough under the chuck to catch the swarf and keep it off the lathe bed. Cast-iron is machined without coolant. Face end, centre-drill and bring up tail stock centre. Finish turn the piston 0D to the diameter of measured cylinder bore plus .02mm, an allowance which will be lapped away to achieve the final piston/ cylinder fit. Remove support and drill piston with ascending drills to Ø7mm x 11.6mm deep at drill tip. Bore piston Ø7.5mm x 11.6mm deep.
  2. Counterbore piston skirt to Ø11.8mm x 3mm deep. Semi-part piston from stock at 13.1mm from faced end, leaving it attached to stock with a Ø6mm spigot. Scribe a small mark on the piston at 6.3mm from the top (parted) face, utilising the topslide and a sharp tool. Remove from chuck and wash thoroughly.
  3. Drill a Ø6mm hole in the bottom of the vee in a vee block, at 25mm from one end. Fix the vee block to the milling machine table, using a dial gauge to align the centre of the vee with the spindle axis and the axis of the vee along the table axis. Place the piston and its chucking piece in the vee with the scribed mark directly over the hole in the bottom.
  4. Apply finger pressure to the piston and chucking piece to hold it firmly in the vee and place a drop of instant cyano adhesive on each side of the part. Traverse along the axis of the vee only to bring the scribed mark exactly under the machine spindle. Before proceeding, make sure that you have the piston axis precisely at 90° to the spindle axis and centred under it at 6.3mm from the top face. This is a most critical part in terms of alignment!
  5. Centre-drill piston for wrist-pin boss and follow with ascending drills to Ø3.9mm. Use new drills only at minimum 1500rpm, dry, to avoid wander when entering the lower boss. Pass drill right through piston. Use either a solvent or a sharp tap to release the piston from the vee block and clean all traces of adhesive from both parts. Mark the hole on one side with a felt-tip pen.
  6. Using a hand-held 4mm hand reamer and entering the marked side, line-ream the piston bosses. Do not allow the tip of the reamer to extend more than a few mm through the other side of the piston.

    Try the wrist-pin. It should be an excellent smooth fit in the marked boss, but not enter the other. Then continue to ream this boss until the wrist-pin will just enter it.

    Always turn the reamer smoothly and in one direction only while both cutting and withdrawing. You are using the tapered lead of the reamer to ream an undersize hole in effect, into which the wrist-pin will be a press fit, so it can't float and score the cylinder walls. Scribe an arrow on the smaller boss inside the piston indicating the direction in which the wristpin will have to be fitted during assembly.

  7. Return the mounted piston to the chuck and centre carefully. Finish parting off from the stub, with the tool a fraction further from the top surface this time. Do not chamfer or break any edges. File any remaining parting pip away carefully and put the piston to one side.

CONTRA PISTON (5-2)

  1. Face the end of the cast-iron stub remaining in the chuck. Centre-drill and drill the recess in the contra piston to a maximum tip depth of 3.5mm. Bore to Ø10mm x 3.5mm deep. Semi-part the contra piston in the same way as the piston, at 6mm from the faced end. Do not waist the material behind the contra piston. Leave it at the same 0D, so that it acts as a guide when later lapping.

NOTE: A later, alternate method of contra-piston preparation was devised by the Author and can be viewed under the title DCO Thin-Wall Tapered Contras. If this approach is used, the contra-piston should be bored Ø11.5mm to a depth of 4.2mm. All external dimns remain as per the drawing.

PREPARING THE PISTON LAP (5-3)

  1. The piston lap is designed to be held in an ordinary button die holder and the lap OD is matched accordingly. The adjusting screws are used to expand or contract the lap, just as they would a die. Turn the lap from brass or aluminium scrap and bore to suit the piston OD. Prior to parting off at 12mm long, place a block of wood under each of the three chuck jaws in succession, scribing a line along the lap each time. These lines will then be equa11y spaced at 120° intervals . Place the lap in a vyce (protected) with the first mark uppermost and hacksaw a slot to within 2mm of the bore. Repeat for the next slot. The third and last slot is cut right through to the bore and the 2mm wide chamfer filed before removing from the vyce.
  2. Place the lap (it's called a ring lap for obvious reasons) in the die holder with the centre adjusting screw in the chamfered recess. Back the remaining two screws right off at this stage.

LAPPING THE PISTON AND CONTRA PISTON

  1. Before lapping the piston, mount it on a simple mandrel. Any scrap material larger than Ø12mm - plastic, metal, even wood will do. Centre in lathe and extend 35mm from jaws. Face end and finish turn to a light push fit in the rear of the piston and some 30mm long. Reduce the end to a stub Ø7.5mm x 8.6mm long. Drill a Ø4mm hole right through the centre of the mandrel, in case the piston becomes stuck. Push the piston on the mandrel, mark the wrist-pin position, remove the piston and drill a cross hole to suit a dummy brass wrist-pin, some Ø3mm x 12mm long. Mount the piston on the mandrel with the pin and centre in the lathe. Cover the bed as before.
  2. You'll notice that the lap no longer fits on the piston, as a result of the stress relieving through-cut, so expand it slightly with the centre screw. Previous experience with the cylinder lap will tell you how tight to have the piston lap. Contract the lap by backing out the centre screw and increasing the pressure on the side screws. Start with moderate resistance. Use Brasso again. Hold the die holder lightly between the thumb and the forefingers, allowing it to float and guide itself as you stroke it backwards and forwards over the piston.

    Total stroke should be no more than 10mm. Do not allow the lap to extend past the ends of the piston more than a few mm. Clean and test the piston for fit in the lower bore regularly. As soon as the piston actually enters the cylinder bore excercise extreme care. It won't take a great deal more lapping to final size, which is reached when the piston top (crown) starts to get tight slightly above the exhaust belt and jams at about 12mm from the top of the bore. Use lard as a lubricant when testing this final fit. Remember that you can rectify a too tight fit, but you've had it if it's too loose! So err on the tight side. All fits are judged under hand pressure only, of course.

  3. The contra piston, still mounted on its stub, is centred in the lathe. Note that it will be lapped and tested actually facing backwards in relation to its ultimate position in the cylinder. Lap the contra piston with the piston lap (re-adjusted) until it enters the lower bore and jams just above the exhaust level. When happy with the fit, finish parting it off in the same manner as the piston.

ASSEMBLY OF THE PISTON AND CYLINDER UNIT

  1. Thoroughly clean the piston, contra piston, cylinder, wrist-pin and conrod. Place the piston in the vee block, its finish protected with a thin piece of paper, the largest wrist-pin boss uppermost. Now you know why you marked the small end with an arrow. Oil (3 in 1) the pin and enter it in the piston boss. Oil the little end of the conrod and push the pin into the rod with the fingers until it enters the lower boss. Holding the piston firmly down in the vee block with the fingers, tap the wrist-pin into the lower boss with a light hammer. Use a small 1/8" diameter punch when you get close to the piston and drive the pin just below the surface. Take extreme care not to hit or distort the piston in any way. Check that the wrist-pin is centred and does not protrude above the periphery on either side. Wrap the piston assembly in a tissue and put aside.
  2. Stand the cylinder upright on a block of wood, oil the contra piston and place it, recess up, in the top of the bore. It will only do so the slightest bit, but this should be enough to enable you to square it up with the bore.
  3. Place another piece of wood on top of the contra piston and tap this wood with a light hammer, driving the contra piston into the bore until it is flush with the top face. If it is far too tight, tap it out with a drift from beneath and mount it on a mandrel. Cyano adhesive will hold it for further lapping to size, but don't forget the through hole or you won't get it off again. Oil the piston assembly and place it in the cylinder bore, before wrapping it up again. The hardest work is done now and, following the completion of several straightforward parts, your "MATE" will be ready for final assembly and test running. So, back to the grindstone!

 

This page is Copyright © David Owen 1989-2006. All rights are reserved world-wide.
Reproduction in any form without the written permission of the copyright owner is prohibited.