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Piston rings and the gap

Sizing piston rings

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Kevin Murrell01/08/2022 18:40:08
53 forum posts
6 photos


I am building a Stuart No 1 from castings. The piston has about a 2 thou clearance and feels very smooth in the cylinder. The piston rings when squeezed enough to fit into the cylinder have a gap off a little less that 1mm between ends. (The No 1 has a 2" diameter piston).

It certainly moves in the cylinder - although a little snug. I do expect to run it under steam. Am I going to have a problem when it all gets hot?



old mart01/08/2022 19:00:45
3885 forum posts
264 photos

1mm gap in a 2" bore would never close up, no matter how hot the piston and cylinder got.

JasonB01/08/2022 19:07:03
22999 forum posts
2757 photos
1 articles

Should be fine, a smaller gap by quite some way would also have been OK.

It will feel tight when new, just put a good slug of oil into the inlet before you run it first time and you will get a black sludge come out the exhaust as the rings bed in, keep adding the oil.

Kevin Murrell01/08/2022 19:34:28
53 forum posts
6 photos

Thanks Chaps. Quite relieved as it's such a nice fit and I didn't want to muck about with it!

Howard Lewis02/08/2022 11:24:07
6295 forum posts
15 photos

In automotive use, with higher temperatures and pressures,the "rule of thumb", to minimise leakage, is 0.001" per inch of bore diameter, so for a 2" bore, I would look for 0.002" (0.05 mm)

At the temperatures found in a model steam engine, there should be no fear of ring butting and the problems that would ensue, and minimal leakage.


noel shelley02/08/2022 16:09:25
1430 forum posts
23 photos

I would NEVER question Howards knowledge but I seem to remember reading that for normal IC engines ring gap was 4Thou per 1" bore. May be that was the max before changing or reboring ? Noel.

Nigel McBurney 102/08/2022 16:36:05
1004 forum posts
3 photos

watercooled i/c ,engines can have a tighter gap than hotter running air cooled engin, on a Lister D twhich has a 3 inch bore the handbook states ring gap should be 8 to 12 thou of an inch,so bottom limit is just over 2 thou per inch.On a steam engine with all cast iron parts and a fairly even temperature .001 ins per inch dia of bore is adequate.

bernard towers02/08/2022 17:10:18
684 forum posts
121 photos

back in the day we used to work on 4 thou per inch of bore dia, that's on water cooled ic engines.

Howard Lewis02/08/2022 17:52:50
6295 forum posts
15 photos

Many years ago, large gaps were the norm, but as the pressure on emissions grew, so did the need to reduce crankcase blowby.. With assistance of suppliers, we were able to reduce ring gaps down to 0.001" per inch of bore. This worried me, initially, having seen the damage that occurs when rings butt, but my fears were unfounded, and blowby was reduced dramatically..

These were 1 litre per cylinder, liquid cooled engijnes, both naturally aspirated and turbocharged. In some cases, we were looking at 50 bhp per lire outputs, without problems.

Of course with an engine having higher temperatures in the ring belt, such a small clearance might be disastrous.

What works well in one engine might be a disaster in another, supposedly, similar engine. It depends upon many factors; materials, ring groove widths, ring configuration, ring dimensions and position, surface finishes, temperature gradients. A dry liner engine may well behave differently from a wet liner, or even a dry liner with different wall thicknesses, piston profile, or combustion system.

VERY much, horses for courses!. That is why engine development takes so much time and money, even with computer modelling.!

As Nigel says, on a steam engine with lower pressures and temperatures, 0.001" should minimise leakage without problems...


noel shelley02/08/2022 20:59:09
1430 forum posts
23 photos

Hi Howard, does the name Charlie Childs ring any bells? He was just outside Thorney !

Glad someone else thinks 4thou was once right - memory still holds up ! Noel.

old mart02/08/2022 21:33:50
3885 forum posts
264 photos

My Suzuki 400 Burgman has an 81mm water cooled cylinder and the top and second rings fitted gap is 0.002" to 0.008".

Clive Brown 102/08/2022 21:51:40
862 forum posts
47 photos

Having just fitted the rings to my "Centaur" gas engine, I've been reading E T Westbury's advice. He states that for this 1.5" bore the gap should be between 0.002" and 0.005".

The OPs gap of !mm seems more than plenty but I'd think that a steam engine is less critical than an IC engine.

JasonB03/08/2022 07:05:11
22999 forum posts
2757 photos
1 articles

Interesting photo posted by Wayne Grenning on his FBpage showing a very worn ring from an early Crossly piano base engine. So worn that there is no more "spring" left in the ring yet the engine ran be it with some blow by. The cylinder bore has some matching wear!


Howard Lewis03/08/2022 19:36:13
6295 forum posts
15 photos

Since leakage is determined by pressure and time, often a worn engine might be difficult to start, but once running will perform reasonably well, (The better, the faster it goes ) because the time for leakage is decreased.

I've seen a diesel engine that was difficult to start, but once running seemed OK.

When checked, the injection pump was badly worn, and at low speeds could not meet the specification, but once up to speed could deliver enough fuel at 500 bar pressures for performance fall off not to be very noticeable!

Over time, Engineers have learned a lot about piston rings, wall pressures, ovality, or negative ovality, and gaps..

A very slight change in ring section or shape can have a big effect.

Who would have thought that compression rings could have a major effect on oll control!

Professor Dykes taught us a LOT!


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