I am researching the design of engines and I noticed that most pistons have piston rings. My understanding is that the piston ring helps create a gas-tight seal.

Is the piston diameter machined to exactly match the cylinder diameter? Or is the piston diameter undersized, and then the rings make up the slack and exactly match the cylinder diameter?

I am asking because I would like to try machining a crude piston/cylinder arrangement. My idea would be to machine some round stock steel and purchase piston rings off the internet.

  • 2
    $\begingroup$ Why not get an old engine and completely dismantle it to learn? Even a simple single cylinder lawnmower engine would be quite instructive - especially putting it back together. $\endgroup$
    – Transistor
    Commented Aug 29, 2022 at 21:37
  • $\begingroup$ The piston rings are split rings. They must be stretched to get into their slots in the piston, then squeezed to let the piston fit into the cylinder bore. $\endgroup$
    – stretch
    Commented Aug 29, 2022 at 21:58
  • 1
    $\begingroup$ Hah. This long-lived (not 'old') fellow remembers when machining tolerances were such that rings lasted maybe 50 k miles or so before needing replacement and probably a cylinder polishing as well. Modern ICE production is fantasticallly more precise. $\endgroup$ Commented Aug 31, 2022 at 14:05

1 Answer 1


The piston rings perform two functions:

  1. they seal the piston tightly in the cylinder bore, so the piston can compress the inlet gases and also develop power without leakage (called blow-by) when the gases burn, and

  2. they scrape the splashed oil off the cylinder walls on the crankcase side of the piston so the oil doesn't get up past the piston and into the combustion chamber, where it will burn with a lot of smoke and foul up the spark plugs.

The compression rings are near the top of the piston and the oil rings are beneath them, near the bottom.

  • $\begingroup$ Do you happen to know if there is any dimensional difference between the compression and oil rings? For example, do the compression rings form a tighter fit than the oil rings? Also, I noticed that due to the ring design there is a small "gap" in the ring. Does this allow gasses to pass by? $\endgroup$
    – nwsteg
    Commented Aug 30, 2022 at 17:09
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    $\begingroup$ the compression rings are machined from hard cast iron and the scraper rings are a complex design with springy steel inside them. I do not know what dimensions they are made to. The gap you mention is necessary to allow the ring to be opened up and slid into the groove on the piston. It leaks just a teeny bit. Note that there are usually at least two compression rings which makes the net leakage even teenier. $\endgroup$ Commented Aug 30, 2022 at 17:33

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