The tough job will be approached first. If you can make the parts shown on the drawing accompanying this part, the rest will be easy!

Materials are always a problem. Try to obtain a good alloy steel for the piston and cylinder. I found some 4-14-0 steel for the cylinder and used drill rod for the piston. I have considered heat treating the piston, but never could decide how to handle the inevitable distortions that accompany heat treating. For this reason my piston and cylinder are in an as-machined state of hardness.

Do not be reluctant to scrap a piece and start over. The second time through you will do a better job and it will be easier. I ruined a piston and cylinder before completing the parts. More than anything else, recheck your dimensions before you take a new cut. Small errors can be tolerated because the design allows for compensation. Work as close as you can to the dimensions shown and compensate for errors on mating parts.

Start with the cylinder. Rough turn the OD so that the bottom of the bore is toward the tailstock. Drill out the bore about 1/16" undersize to the depth required for either a spark or diesel ignition engine. Enlarge the bore with a boring tool until it is .015 to .020 undersize.

At this point a decision must be made whether to finish the bore in this set-up or leave it rough and finish in another set-up after the ports and bypasses have been cut. There are problems either way. If the bore is finished now, burrs will be raised when the ports and by-pass are cut. These burrs will take a lot of careful hand filing to remove (this is the method I used). If the bore is left unfinished, the cylinder must be carefully re-centered in a fixture set-up to finish it. In addition, there may be problems in grinding a bore that is perforated by ports and by-passes.

Assuming that the bore is to be finished in the present set-up, you are ready for grinding.

I used Dremel motor stones in my grinder with good success. The orange colored (garnet) stone is good for rough grinding and fast metal removal. The grey colored (emery) stone is best for finishing. During all grinding, wet the bore with kerosene. This keeps the stones clean, speeds cutting and provides a better finish.

Be more concerned with obtaining a round, straight and smooth bore than a bore of 0.500" diameter (mine is .003 undersize). Make many light finish passes during final grinding and obtain the best possible finish. At best, the ground finish will be a disappointment. However, use of a very fine polishing paper wrapped around a dowel and held by hand against the bore surface will yield a finish you can be proud of. Do not overdo the hand work or a taper will result.

While on the subject of taper, many commercial model engines have been purposely made with .001 to .002" taper in the bore with the larger diameter at the bottom of the bore. Hence the suggestion of working from the bottom inwards. By this orientation, any taper is likely to be in the right direction.

When you are satisfied with the bore, finish the OD and cut the threads indicated. Recheck your dimensions and cut the cylinder off at the proper length (this is where I went wrong on my first try!). The exhaust ports are cut with a 1/8" diameter end mill of fly cutter, as you prefer. Use a sharp cutter to reduce burrs. They will still be present and must be cut out with a needle file. The X-Acto files are satisfactory.

The bypasses are cut with a 3/8" diameter end mill. This will leave a sharp edge at the top of each bypass that is not conducive to good air flow. However, the length of the bypass is easy to control. I would have preferred to use a tapered or round end mill and obtain a bypass shape similar to the Cox engines. The problem of controlling the length of the bypass forced me to used a flat ended mill. Carefully remove the burrs and wash the cylinder. Now you are ready for the piston. Actually, I cut the exhaust ports and bypasses after the piston was finished and fitted to the cylinder. This is perhaps a better method because of the burr problem.

Chuck the piston material and turn the inside diameter of the piston first. It is important to obtain a sharp corner on the bottom of the cavity so that the sub-piston will seat against the piston top. You will have to grind a special tool to cut the groove for the sub-piston retaining ring. Do not let the depth of this groove exceed the .470 diameter dimension shown. Rough turn the outside diameter of the piston until it is about .010 to .015" oversize. partially cut off the piston to the proper length and leave enough cut off groove width to allow the OD grinding stone to completely pass over the piston surface.

The comments concerning the grinding stones and use on the cylinder also pertain to the piston. As you approach the finished piston diameter, make trial fits of the cylinder over the piston. This is a critical stage as you are trying to obtain a piston-to-cylinder clearance of 0.0002 to 0.0005"! Unless you have very good measuring equipment, this will have to be gauged by feel.

Before making a trial fit of the piston and cylinder, wash both parts carefully in kerosene and leave them wet. Remember that one small chip or grit from the stone will give a false indication of the fit. A very small dab of jewellers rouge or similar polishing compound on the wet piston will help smooth any burrs and roughness during trial fits.

Be careful because a large film of compound will give a false indication of fit. What you want is for the piston to enter the cylinder at either end with just a slight drag. Better to fit tight and go through a slow break-in than to fit too loose. This is especially important on a diesel. The Diesel contra-piston should be a rather tight fit in the top of the cylinder.

After a satisfactory fit is obtained, complete the cut off of the finished piston from the stock. Use care that the piston is not damaged in the cut-off. Better to leave a small stub for hand finishing than to touch the finished piston crown with the cut-off tool.

Turn the sub-piston from aluminium and make trial fits with the piston before cut-off. The fit is not ultra-important because all piston forces are down while the engine is running.