Atom Minor Mk III
by Jan Huning
9. Cylinder Barrel
The cylinder was machined from a piece of free cutting mild steel. The majority of the machining was completed at one setting, which requires a length of bar long enough for the cylinder, plus parting off thickness plus enough to hold securely in the chuck.
The outer profile was first roughed out, leaving about 1/32" on diameter, then the bore drilled and bored to about 10 thou undersize. (If there is any distortion as a result of internal stresses in the cold drawn bar, this will all relax out before final machining.) The fins were roughed out using a 1/16" wide parting tool, as described in the ETW book. I machined the entire length of the cylinder skirt to a close fit in the crankcase. I did not reduce the outer diameter of the lower section, since I think it is already quite thin, and I do not think it is necessary to have a "tight wringing fit" for the top portion adjacent to the ports. A close fit will not leak between transfer and exhaust ports. A constant diameter for the skirt makes holding it to finish the top of the cylinder a little easier (added bonus).
A 3/32" wide parting tool was ground on the end of a piece of 5/16" square HSS and the end radiused. This was used to machine the gap between the fins, starting with the underside of the bottom fin. Axial spacing was set using the leadscrew handwheel. The top surface of the top fin was also machined at this setup. The crests of the fins were rounded using a form tool, previously made for another engine. Last operation was to finish turn the bore, leaving about 1 thou for final lapping. The cylinder was then parted off. This method ensures that everything is concentric and the face that seats on the crankcase is truly square to the bore, but does require the extra length for holding in the chuck.
The cylinder was held by the skirt for machining the head location spigot. It only needs very light clamping for this, to ensure the skirt is not distorted.
With all the turning for the cylinder and head completed, they were clamped together on the rotary table, (centred on a suitably sized spigot) for machining the cutouts in the head and drilling the clearance holes for the retaining screws. The cutouts in the head fins were first machined undersize, 6mm I think, before finishing with a 1/4" slotdrill. The clearance holes were drilled under size and finished with a sharp 3mm slotdrill, to minimise the burrs thrown up between the fins, which are not the easiest things to remove afterwards.
A George Thomas designed dividing head was used to hold the cylinder for machining the exhaust and transfer ports. A short piece of bar, one end 3/4" diameter to fit the bore of the dividing head spindle, the other a close fit in the cylinder bore, was used to locate the cylinder concentric with the spindle. A piece of threaded bar passed through cylinder and spindle clamped the cylinder firmly against the end of the spindle. The cylinder was positioned under the milling machine spindle as follows—A 1-2-3 block was placed on the table, resting against the cylinder skirt and the cylinder location face. The axis of the cylinder was set parallel to the table axis; an edge finder used against the side of the 1-2-3 block that was against the skirt to be able to set the axis of the cylinder under the spindle; an edge finder against the side of the 1-2-3 block against the cylinder location face set the axial datum at the location face. (Hope this makes sense!) The table could then be moved to machine the ports in the correct location relative to the cylinder location face. The cutout at the bottom of the skirt to clear the conrod on the transfer side was also machined at this setup.
(The distance from the location face to the ports was modified to correct for the actual position of the cylinder location face on the crankcase relative to the crankshaft axis.)
Last operation on the cylinder was to lap the bore. A split aluminium lap, with a tapered bore held on a tapered mandrel, as described by yourself, was used. I used 600 grit diamond paste. A small amount was spread on the lap, and pressed into the surface by rolling the lap firmly on a piece of steel plate. I used a thin penetrating oil as lubricant. In the past I have tried soluble cutting oil as a lubricant for lapping. This works, but dries out too quickly. I think a very thin (low viscosity) lubricant makes it easier to feel the "drag" on the lap. This is lost if a thicker oil is used. The thin lubricant also clears the lapping debris better.
I lapped the lower section of the bore, below the ports, to a slightly larger diameter than the top section, but to less of a taper than I would use for a diesel engine.