Click on drawings to download - for personal use only.


This is probably the most involved part of the engine but if you take it one stage at a time it should not present too many difficulties.

It is just possible to squeeze the cylinder out of a piece of 50mm dia cast iron bar, anyone in the States thinking of making this will have a little more to play with by using 2" stock.

Start by sawing off a piece a few millimeters longer than needed and then lightly face to clean up the ends. Blue the end of the bar and then lay it onto a flat surface and scribe a line at centre height, roll it 1/4 of a turn and mark a cross line 3.5mm below centre height. Accurately punch where the two lines cross.

Set the punch mark to run true by holding the bar in the 4-jaw chuck.
Then spot drill followed by drilling out most of the material with several drills of increasing size until you get to about 18mm when the rest can be bored out.
I like to do the last couple of cuts with a HSS boring bar as I find that springs less but even so take a few passes at the final setting to let any spring work its way out and hopefully you will get a parallel bore. Aim for something like 23.90mm which will allow for honing and lapping but as the piston will be made to fit the bore the turned finish is what to aim for not the exact size.

Talking of sizes I did not use a micrometer once on this engine all measurement was with a digital caliper.

Take a finishing cut across the bar and then take another 0.5mm deep cut to leave the small raised area 28.5mm dia, this raised area will make it easier to lap the end of the cylinder later.
The work can now be reversed in the 4-jaw and the bore set to run true. Make sure you can machine down to the top of the valve face which will probably mean the work is away from the chuck face. You may just be able to see that I have a piece of 1" material between chuck and work so it can't get pushed back by the tailstock. NOTE this must be prevented from flying out for which I have used hot melt glue. Bring up the tailstock centre for support and rough out to about 43mm dia.
Then with a tool that can get into a corner turn to 42mm finished dia and to leave 16mm for the port area.
Now form the spigot on the top of the cylinder to leave the 42mm dia section 50mm long.
Next put your parting tool in the toolpost, set the front edge to touch the work and zero the cross slide dial. Then with the topslide touch the edge of the tool against the end of the cylinder, retract and move half the width of the cutter towards the headstocjk and then zero the topslide handwheel. This will make it easy to set the spacing of the fins as they are all dimentioned to their ctr from the top of the cylinder OD.

First make a central cut on each 5.5mm deep.

Then increase the width of the slots to 4mm by taking a equal cut off each side of the first grooves.
Then swing the topslide around to 5degrees and cut the taper down one side of each fin. Swing to 5deg the other side of zero and make a similar set of cuts down the opposite sides.
Grind up a 4mm square HSS tool bit to a half round profile and then round out the bottoms of the grooves, a slow speed will be needed to avoid chatter. Then with a file the external corners can all be rounded over with the lathe running.
Remove the tailstock and trim the spigot back to its final 5.0mm length.
With the largest part of the lower cylinder uppermost mill it back flush with the edge of the fins or just a fraction proud. Use a nice sharp cutter as we want a good finish here to save excessive lapping later on.
Centre up the port face and drill two 4.0mm holes 10mm each side of ctr into the bore, then another four holes will remove most of the material from the inlet passage. Finish up by cleaning out the slot with a 4.0mm milling cutter.
The cylinder can then be held bottom up on the rotary table and the two sides of the port machined as was done to the base.
Then again like the base round the rest to 42mm dia
Then add the four holes but this time thay are tapped M3.
Finally for now the five M2.5 holes can be drilled and tapped in the top but make sure you drill them in the right place unlike me - the one above the port face should be left out.

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

Part nine




Part 3 by Jason Ballamy -

the only weekly magazine for model engineers