Piston ring grooves

Hi, I'm building the Stuart No 9 and am machining the piston, and need some advice please on the following. The rings I have, (which were supplied by Stuart) have a dimension of 1/8 wide and .0525 thick. The drawing has a groove depth of 1/8 (.125) Is this the right depth for a ring of this size. Thanks Garry

Are there two rings in one groove? If so then it sounds reasonable. Remember the rings need to be able to move in the groove to spring in and out when temperatures change or to accommodate slight variations in the cylinder bore.

Martin C

Yes, there are two rings in one groove. I know that the rings will expand out into the bore, so to me the gap underneath seems to make sense, but I was just getting a second opinion.

I think the root depth of the groove should be 1/16th giving 10 thou clearance under the ring.

In use - when pressure is applied - the gas fills the space behind the ring and pushes the ring out against the bore. With a petrol or diesel engine or a single acting steam engine, the important face of the piston ring is the one away from the pressure - normally the underneath with a conventional layout. This means that the opposite (top) face is not critical in terms of flatness. If the rings are stamped or etched with numbers, etc, this is where they are put for this reason.

With a double-acting steam engine, both faces of the ring need to be properly flat, and the gap in the piston needs careful control as the ring is forced, every stroke, to flop across to the other side of the groove.

My thanks to Mr Ricardo (saviour of the side-valve engine) who discovered this while sitting in a WW1 tank trying to find out why it smoked so much.

Cheers, Tim

Thats helpful, thanks for your replies.

Garry