Why must the piston ring move up/down and around the piston? I assume this is on purpose, but why?

 

I think it's the scraping/wiping/sealing action. If it were completely solid, it may break, or break the ring lands.

Wiki really didn't offer too much insight.
http://en.wikipedia.org/wiki/Piston_ring

 

The amount it must be allowed to move up and down in the ring groove is extremely slight- a couple thousandths of an inch or so. If it's flopping up and down a good bit in the ring groove then chances are you have the wrong thickness rings for your pistons. It should be so slight that you basically can't see any movement except that the ring can move in and out of the ring groove as your squeeze it against it's own tension. GM used different thicknesses on their stock rings/pistons over the years- 5/64", 1/16" and metric sizes in later years.

It needs just the slightest bit of play becuase the ring itself must be allowed to contract and expand to seal against the bore. Definitely NO up and down movement in the ring groove that you should be able to notice!

 

There has to be more of a reason besides it contracting and expanding. Is this really it?

 

The aluminum of the piston and the ring material have different expansion rates. I have not studied metalurgy but suspect the vertical "slop" of the ring in the piston will decrease as the parts come up to tempature.

 

Strictly from a materials standpoint... iron alloys (including steels) generally have lower thermal expansion coefficients than aluminum alloys. That means that as the parts heat up, the aluminum expands more than the steel, and the gap would increase...
I don't think it's solely a matter of thermal expansion, again, strictly from a materials standpoint.

 

So the material on the top and bottom of the ring expands and the gap gets bigger? I think you've got something backwards.

 

Nope, he's got it right. The gap would get larger (picture the INSIDE wall of the ring land expanding in all directions... including top to bottom).

Likewise, the bore of the cylinder gets larger, but not as much as the aluminum piston... hense piston slap is needed when cold, or the piston would gall the cylinder wall when up to operating temperture.



If the rings fit any tighter to the piston you'd probably have issues with tolerance stacking... i.e. if a ring was a hair too thick, and the cast piston's ring lands were a touch too thin, you'd risk piston damage and stress risers forming during installation.


Other reason besides tolerances? Installation. Putting a ring into a ring land is a delicate process to avoid galling the piston, and will most likely (from when I've seen it done at least) require a little slop to allow the ring to be slipped in at a small angle. Even after the ring is in the piston, it needs to be compressable with a ring tool or you'll never get it in cylinder bore. If the ring was "fused tight" during installation, you'd have a hell of a time compressing it and installing it without damage.

Last reason? Spring pressure. Rings are designed to expand against the cylinder walls, and "jamming" them into the ring lands coudl affect the outward force they could produce for sealing. There are benefits and draw backs to different spring pressures (low tension vs. high/normal tension springs) but using a low tension spring that was "jammed" into ring land may make it prematurely fail or fail to seal at I suppose.

I just seems to make sense to open the gap a bit to allow for easy installation and proper ring operation without piston damage... but I've work at a piston or ring factory so I couldn't ya for sure. Just my best guess.

 

Hey Steve post up some pictures of that crap they sent you for piston rings