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I was reading about oil control rings in the piston ring topic. Most modern engines use a 3 piece design for oil control rings where you have top rail, bottom rail and spacer-expander (sandwiched between the former two).

My question is what is a "spacer ring"? Is it different from the term "expander ring" or is it just different name for same thing (This is because various sources use the terms interchangeably while some distinguish it. Source1, Source2).

Also what is the purpose of spacer & expander ring (assuming they refer to different things) ?

From What I have read, I understood (seems pretty much wrong) it as follows. Spacer/expander (same thing) ring basically acts as a spring. When the spacer is sandwiched between the two rails and inserted into the ring groove, both of the rails press against the spacer. Since spacer has corrugations it can slightly expand (increase in radius maybe locally) and thus form a tight seal with the cylinder. Thus from what I understood it is the expander that scrapes of oil mainly in oil control ring. Also having corrugations allows oil to easily flow through the oil drain holes as well.

Update:

This is a website which seems to solve my doubts on how spacer works

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When people refer to the "spacer" or "expander", they are talking about the same thing. The spacer/expander has two different names used synonymously because it does both jobs at the same time. Why they are called the two different things:

Spacer: It's called this because there are two oil control rings in the same groove of the piston. These rings are too thin to be in their own ring grooves on the piston, so to better control them, the spacer is used to keep them a uniform distance apart as well as to keep them square to the bore.

Expander: It's called this because in order for the oil control rings to actually control the oil, they must be gently expanded so they press against the cylinder wall. Without it, they wouldn't much control the oil.

For the most part, it seems your understanding of what the spacer/expander does is spot on.

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  • Just to confirm again, is it the spacer ring that presses onto the top and bottom rails and thus forcing those rails to press against the cylinder i.e oil scraping is done by top and bottom rail. Or is it the top and bottom rails pressing against the spacer ring and thus causing the spacer ring to press against cylinder i.e oil scraping is done by the spacer ring. Commented Oct 29, 2022 at 8:29
  • The spacer/expander presses against the oil control rings. The oil control rings maintain contact with the cylinder and do as the name implies: controls the oil. The spacer does nothing with the oil itself. It is there to keep the oil control rings equally spaced and pressed against the cylinders. Think of the spacer as what controls the oil control rings. Commented Oct 29, 2022 at 11:05
  • In that case what I thought was wrong. Commented Oct 29, 2022 at 11:44
  • After a bit of search with some luck and proper keyword a website cleared up the remaining confusion also (I guess). I was confused about how a stretch in the spacer/expander forced the oil control rings onto the cylinder. I think it is actually the small vertical protrusions provided at the inner periphery of the spacer which actually causes the oil control rings to get pressed against cylinder when spacer gets stretched. I will add the link in the question. Please let me know if this is the correct explanation. Commented Oct 29, 2022 at 11:52
  • The spacer/expander doesn't get stretched. It is very flimsy. The oil control rings, when in place, provide structure for it. The cylinder walls press in on the control rings, which presses in on the spacer/expander, and it in turn pushes back. It's sort of a spring action, yet presses back enough to provide the tension needed to keep the rings seated against the cylinder wall. The ring groove on the piston provides extra support by limiting how it can behave. Commented Oct 29, 2022 at 12:28

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