Engine: Piston Ring Gap

[T-CAB]

Tested the ring clearance in a new set of .020OS V8 pistons. The edge gap on the 2nd compressioin ring was less than .001 in. The book says a minimum of .002. The ring, howeer, can rotate easily. Would it be advisable to take the pistons to an engine shop and have the grove widened?

T-cab

[Dan Timberlake]

Does this picture show the clearance you are referring to?

http://www.aircooledtech.com/piston_rings/ring-side-play.jpg

What is the measured ring width?

Machining ring grooves is not a easy task. The sealing face (side of the groove) must be perpendicular flat and smooth. Grinding is preferred to "turning" with a cutting tool.

Perfect circle says 0.0008 inch minimum. Nearly everybody else sys 0.0015 minimum.

I'd spin the oiled ring around a dozen times, remove and clean, and measure again

[T-CAB]

Dan:

Yes, that is the clearance that I am referring to. I think I will take the piston's and rings to an engine shop (hate to spend the money) to see what they say. My thinest gauge is .002. so maybe .0015 would fit if I had one.

Bruce

[Mike Van Veghten]

Agreed, you likely ruin a new set of pistons trying to remove .001".

More likely the ring is correct and the piston is a little tight, but I know that doesn't help.

Actually, "both" sides of the ring AND ring groove are sealing faces...! The piston does need sealing when it goes up also.

I think I'd get a new piece of 600 grit or finer wet or dry sand paper and run the ring around in a figure eight pattern a few times using the palm of your hand (NOT just your finger tips !). Remove just a few tenths off of each side, clean well and...carefully measure again.
As Dan says, .0015" should be fine.

Question, what brand piston do you have ? A quality cast or Hypereutectic piston will accept a tighter gap. A forged or lesser quality cast piston might require more gap.

Go slow, you can't put back lost material..!

Mike

[Dan Timberlake]

Many, maybe most of the feeler gage sets I've bought from Sears or NAPA have a 0.0015 inch leaf. I'm guessing you'll probably find the clearance will measure fine, but I'd measure the ring width with a mike anyway. As I recall the width should provide correct clearance in a "nominal" groove, so If by chance the rings are fat, another set would probably provide the proper clearance.

The first problem encountered as ring groove clearance is reduced in top rings is delayed pressure buildup BEHIND the ring necessary to seal on the power stroke. AIR in some piston or ring tech article Kevin Cameron mentioned the late new England cam grinder Domenic Ravesi was developing an engine and among other things tested very "tight" ring side clearance. I believe the result at some tighter than stock value was power shutting off at high rpm.
For over 30 years Drag racers played with reduced side clearance in a quest for better ring stability at crazy rpm. To make the rings seal in "too tight" grooves "gas ports" are added (and needed) to pressurize the volume behind the ring. Topic 14 here. http://www.wiseco.com/FAQs.aspx

I don't think gas ports would be applicable for a "too tight" second ring, even short term