Are you sure the o-ring is 1.5 mm cross section? The nearest UK standard size would be 1.6 mm. Tubal Cain recommends (in his Model Engineer's Handbook) a groove width of 2.3 to 2.5 mm and depth of 1.18 to 1.25 mm. The width is greater than the ring cross section to allow for the compression of the ring. I would start at the tighter end of the tolerances, try it, and adjust to get a good feel for the fit. Yes, silicone is good.

Russell

I've just used cotton kitchen string dredged in LM3 moly/graphite grease in some of my engines, when I couldn't get o-rings or nylon 66 to work smoothly. I also think brass will do fine unless you're expecting to run it for hundreds of hours at high temperature.

Edited By Mick B1 on 23/01/2020 10:05:56

HI Ron,

Piston fits for team engines don't need to be airtight, as you usually use packing or an o-ring.

Personally I would bore the bore rather than ream it, then turn the piston down very slowly so that it passes through the bore without any binding, but don't panic if there's a thou or more clearance as long as it isn't 'rattling'.

With a small engine like this that won't be doing any work, aim for the piston (with ring) to be able to move pretty easily, as this will help slow running (especially on air) which is more important than getting maximum efficiency in a display model.

Neil

Ron' I'll work out the correct groove width and depth for you this evening, on site now

I thought a brass piston in a brass cylinder would cause galling ?

As geoff says you could get away without a ring and maybe just cut a couple of shallow oil grooves with a Vee pointed too, if it is good enough for all those Stuart 10 series and smaller engines it will be fine for this one and the reduced friction will also help with slow speed running on low pressures. Or cut the ring groove and run it without a ring for display and fit the ring if you ever want it to do light work.

I like to use aluminium pistons which again help with smoother running and as this engine has no counterbalance weight on the crank every bit of weight saved helps. Aluminium would work both grooved or with a ring 6082 will be OK.

Fit will be as per that video I posted the other day and hardly measurable but if I had to put a figure on it -0.01 to 0.02mm, a little more if running on steam rather than air.

I tend to use 2.4 cross section rings so for that size cylinder a 11.6mm ID x 2.4mm ring will work well. Groove 2.6mm wide and 2.3mm deep giving a diameter at the base of the groove of 11.4mm. I'd also suggest Viton as it is not sensative to oils.

Stuart 10 series flywheel would be fine and look nicer or you could make one like my Preston's oscillator.

Aluminium for any flywheel is a bit light unless the diameter is increased but it is easier to machine. I'd stick with steel or iron particularly as it is only single acting and the energy stored in the flywheel is what drives the non power stroke.

An alternative to the Stuart casting would be this one from the same guy that does the one I used on my Muncaster