By Jason Ballamy - Part six

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At a shade under 24mm dia the piston will come nicely out of a piece of 25mm or 1" bar so pop a bit in the chuck with enough poking out for the part and turn down to the required 23.95mm which will leave a little room for the piston to expand due to the heat if you decide to run on steam.

At the same setting the end can be faced off then starting with a spotting drill most of the waste can be drilled out  to say 12mm before changing to a boring bar to open up the hole to the required 16mm dia by 24mm depth.

The piston ring grooves can also be added at this stage, I have given dimensions from the end of the piston to the left of the groove so simply touch the edge of the parting tool against the end of the piston and then wind the top slide along 8mm for the first groove and 26mm for the second. Bring the top slide back as you make a couple more parting cuts until the grooves are 2.6mm wide. Ease all the external corners so the O-rings don't get nicked when fitting.

You will notice that I cut three grooves as per Stan Bray's sketch but two will be quite adequate and is what I have shown on the drawings. My engine runs fine on air with just one ring in the groove furthest from the crank.

Over to the mill now, locate the centre of the piston dia, the ruler trick will do and then drill and ream a 4mm hole 17mm from the open end. I don't have many machine reamers but find that a hand reamer held in a collet and run at slow speed works perfectly well and in these sort of sizes tend to drill 0.2mm under dia so in this case drill 3.8mm.
Saw or part off the piston a little over length then with something to protect the finished surface lightly hold in the chuck while the end is faced back to get the final 28mm length.


I used Vitron O-rings: 19.6mm ID x 2.4mm section, bought on e-bay # 371503929586

Wrist Pin

This is just a short length of 4mm silver steel (Drill Rod) faced to 21mm long and with a 2mm dia x 5mm deep hole in each end.

Wrist Pin Pads

These can be from Acetal, nylon, or any other similar material or bronze if you prefer doing them in metal so long as it is softer than the iron so they protect the bore from the wrist pin. Turn a short length to 4mm and then the 2mm spigot and finally part off to give a 1mm thick head.

Click on drawings to download - for personal use only.


I used a piece of 12x25mm black mild steel bar for this a few mm longer than the finished size. Start by machining one end down to 10x22mm and then the other to 6x10mm, square off the 10x22 end and then drill and ream 6mm dia 80mm from that end.

Put a small BS0 centre drill hole in each end and also drill the two 3mm holes in the big end.
Use a hack saw to remove most of the waste then set up on the lathe between centres, firstly rough the rod down until its parallel and a little over 8mm then set the top slide over and finish to the final taper going from 8mm down to 6mm with a round nose tool.
Finally the small end can be rounded over which will remove the centre drill hole.

Big End Bearings and Keep Plate

I will describe these together as some of the work can be done on both at the same time.

For the bearings machine two pieces of bronze to 6mm thick and then soft solder together, once cool machine the pair to the final 22mm x 12mm. I find it easier this way than trying to line up two pieces that are finished size when soldering.

The keep plate is just a piece of 3mm thick material machined to 10mm x 22mm.

Drill the 3mm holes and include another small center drill hole in the keep plate.

Lightly hold the two bearing halves in the 4-jaw and drill, bore then ream out to 10mm
A small round nose tool can then be used to remove 1mm of material on each side blending nicely into the 12mm dia.
The parts so far can be assembled with a couple of temporary nuts and bolts, held between centres and then the big end turned down to 22mm dia and the round nosed tool used again to cut the 20mm dia waist in the bronze. Mark the parts so they can go back together the same way. Unsolder the bearings and give the mating faces a light rub on some wet & dry.

Small End Bearing

This is a straight forward turning job, the bearing can be pressed or Loctited into the conrod.

Finally, thread the ends of a couple of bits of 3mm rod and add a nut each end to hold things together. Test the fit on the crankshaft as the bearings may have closed up a bit when the solder was cleaned off, if so just run the reamer through again.

Part one here  Part two  Part three  Part four Part five  Part six  Part seven   Part eight