Now that I have 2 cylinders, it is time to make the pistons. These are supposed be graphite, which is easily obtained. But that only works if the cylinders are perfectly round inside. And regular borosilicate glass isn’t perfectly round. Close but not close enough. Option B is to make pistons out of brass and add a groove for an o-ring.
I’m a big fan of Plan B anything. First up, cut some hex brass to just over the final length of 2 pistons plus room for finishing the ends and the .05 wide cutoff blade. My 4×6 band-saw takes no time at all. Some time spent aligning its blade means that cuts are close to perpendicular. Less post processing that way. At same time I also cut the cylinder block. It amazes me sometimes just how much scrap material my dad collected from various places he worked. The waste was incredible. After 40 years I am still using the scrap leftovers.
The 3 jaw chuck came into play, the hex was turned down to .670 and then sanded lightly to get to .001 under the minimum diameter of the glass cylinders. A groove 0.10 wide was cut incrementally until the o-ring fit into the groove and cylinder with minimal friction. The hex was then flipped around and the now round part went in the chuck. The rest of the hex was turned down and a second groove cut.
The rod was cut into 2 20 mm long pieces and then each piston was finished to length of 19mm. The end result: 2 cylinders with pistons whose rings seat with just a tiny edge against the glass. Minimal friction but a nice pressure build up when you move the piston. If I need more pressure against the glass I can add a tiny shim strip under the o-ring. We shall see, that test is a long way off.
In the rh photo above you can see a thin line ‘on the glass’ in the cylinder lying flat. That thin line is how I know the o-ring is sealing, it is a bit thinner line in the area where the glass is a larger diameter but there are no gaps. The o-rings are Nitrile, not Viton as shown on Jan’s drawing. This comparison of o-ring materials indicates my choice should work ok.
I haven’t decided yet how the connecting rod will mount to the piston. Jan shows an option on his first version of this engine, it also used brass pistons.