Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Hi,

I am working on a model steam engine, and this is the first time I am using VITtron O-rings on the pistons. My question is, do they wear in?
I fitted the pistons, as normal, with about .002 clearance, I installed the O-rings, but they seem make the pistons a rather tight fit. On the other engines I have built, [without rings] I made the pistons so, when held vertical, they would slide in the cylinders under their own weight. I am unsure how tight the fit should be, using the O-rings.  Thanks in advance for any insight.

My approach is to check the '0' ring in the bore, it should just fit and not fall through.  It should be possible to just rotate the '0' ring when in the slot in the piston, with a little side to side movement. When in the piston with '0' is in bore there should be no compression of the '0' ring. To check with the top cylinder cover in place and a little oil in the cylinder, at TDC with a finger over the port it should be quite difficult to pull the piston out.

This is how I do it. First I get an '0' ring and bore the cylinder until it just stays in place, I then make the cylinder covers and piston which is a sliding fit just dropping down the bore when vertical. I start making the slot in the piston using a parting tool; finishing using a hand held graver and using the '0' as a guide, I do very little measurement, just fit 'n fit.  '0' don't wear in just out.

As I understand it; the '0' ring when in operation seals against the bore and either side of the piston slot depending on the direction of travel, with no seal to the bottom of the slot.

I hope this helps; it works for me.

Take care Tony