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Before starting on the piston it is best to hone the cylinder. This can be done quite easily with one of the cheap brake cylinder hones. I run these in the bench drill at about 750rpm and just move the cylinder gently up and down making sure not to let the stones stick out too far and risk a bell mouth end, a little paraffin will help stop the stones clogging.

The piston will come out of a length of 25mm cast iron bar, face it off then turn the outside until it just enters the cylinder bore. Check against both ends of the bore, if one is smaller then use the hone some more to remove any taper. Once you have a push fit into the cylinder stop working on the outside and bore the inside, followed by drilling and tapping for the M3 thread.

Saw or part off the piston and then face off to 20.5mm finished length being careful not to distort or crush the now hollow piston. Face off and tap a bit of say 12mm bar to take an M3 screw and this can be used as a mandrel to hold the piston while it is lapped into the bore.  This is easily done at a slow speed with a small amount of 1000g silicon carbide powder and a liberal amount of oil. Just work the cylinder onto the piston from the top until it is a close but friction free fit. Clean off all traces of SiC powder.

The lug for the gudgeon pin can first have the end reduced down to 3mm and then be threaded M3

Then mill it to a square section, if you have not started with square bar. This can then be cut off the main bar and milled to length and a 3mm slot cut into it.

Followed by the drilled and reamed cross hole after which the end can be rounded off by filing or machining.

The gudgeon pin is a simple turning job. Once done blue the end, put it in the hole and mark off the position of the split pin hole. Then drill for a 0.8mm (3/32") split pin.

This pic shows the finished parts along with the conrod which is yet to be described.




Part 5 by Jason Ballamy


I made a start with the crank web and to make it easier to hold actually machined it on one end of a short length of 40mm dia steel bar and the cam on the other end.

Start by facing the end of your stock, then using a tool with a small radius on the corner turn the 3mm long spigot and also the 38mm OD. Followed by drilling and reaming the central 5.0mm hole.

Transfer to the mill, locate centre and then use the hand wheel dial or DRO to move 16mm and then drill and tap M3.

Saw or part off and then holding by the spigot clean up the other side but leave the web about 0.5mm over thickness at this stage. Mark out the two straight edges, saw off the waste and then mill back to the lines.

A bit of simple file work will round off the edges.

The crank shaft can be made from precision ground mild steel (PGMS) or silver steel, stainless would also be OK if it is a good fit in your bearings. Face it off either end but leave it 1.0mm longer and then turn the spigot also 1.0mm over length. The crank web can be used as a gauge to get the spigot a firm push fit into the hole. Once done clean up both parts and Loctite them together.

Once the Loctite has had a good time to set the crank can go back into the lathe to have the web faced down to the final 3mm thickness. Doing it this way will ensure the face of the web is at right angles to the shaft even if it went on a bit crooked when Loctited.

Part one here  Part two  Part three Part four  Part five  Part six -

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