Piston Ring Gap Wear Limit

echrisconnor · echrisconnor's Garage

I checked my piston ring gap yesterday for my existing Nickasil pistons and cylinders which I will be reusing with my rebuild. I pulled one of my old/existing rings for the test to see if I need to re-ring during the rebuild. If possible, I was hoping not to re-ring.

The check process is easy enough, but I was stumped at what was the proper range of acceptable values. I'm doing this on a Carrera 3.0, so when looking through Wayne's book, I believe the range was something like .15mm to .30mm (although several 3.0 engines are listed and I'm never sure where the C3 fits in). My measurement came in at .38mm. So, by that I'd assume I'm out of spec. Then came the following line... "Wear Limit." This value was .8mm. Now I'm stumped. Is the gap clearance limit for new rings, and the Wear Limit is the maximum acceptable value for worn/used rings. If so, isn't that an enormous gap?

Any advice would be appreciated.

Chris

echrisconnor · echrisconnor's Garage

Bump. What is wear limit? Does it mean that .8mm is the maximum acceptable gap?

Henry Schmidt · Henry Schmidt's Garage

The ware limit is for used rings. You want your new rings to fit within the specs.
If your cylinders are round and scuff fee, there is no reason that you can't get many more miles of trouble free use out of them. The relatively low cost of new rings vs. the high cost of tear down makes this purchase an easy call. If your new rings are out of specs it's time to think of other alternatives. Remember to check your pistons for ring clearance ( ring side play). If there is too much clearance you will get premature ware and pure seating. Also remember not to hone your cylinders. For reasons why check this data base for Nikasil honing.

echrisconnor · echrisconnor's Garage

My first intent was to simply let the proverbial sleeping dogs lie-- not to re-ring. Conventional wisdom seemed to indicate that if everything was seated, and the car had 100k on it, I should be good to go for the foreseeable future. However I have now purchased new rings and will be re-ringing.

.8mm is HUGE though, isn't it? With that much gap, I'd think you'd have terrible compression.

c

Henry Schmidt · Henry Schmidt's Garage

.030 ring gap is excessive. I believe oil consumption is more likely the problem, nor compression.
If you now have new rings and .030 ring gap it time for you to think about other avenues. Custom pistons ( JE will make a set to fit your cylinders and rings can be purchased as file to fit) , another set of Ps & Cs, or ?

echrisconnor · echrisconnor's Garage

I'm assuming that when I put in new rings, I will have an in-spec measurement. I'll take that measurement in a couple days when the new rings get here. The pistons and cylinders look great otherwise and show all original crosshatching.

I had hoped to reuse, but the .38mm gap caused me to re-think things and get new rings.

911pcars · 06-01-2004, 09:31 AM

Excessive ring gap with new rings indicates the cylinder bore might be over spec, yes? Usually one must file the ends of the compression rings to arrive at the correct minimum end gap with in-spec cylinders.

Another important spec is the ring side clearance with the piston groove. About 0.004"(.1 mm) is usually the wear limit with new rings. Verify with a feeler gauge. What's yours?

Sherwood

Superman · 06-02-2004, 07:37 AM

Yeah, there are at least two different specs we are talking about here. There is the ring lands on the pistons. Place a ring in the groove, and see what is the thickest feeler gauge you can put in there in addition to the ring. If the ring lands (grooves) are too wide, your new rings will beat the poop out of your pistons there.

The other measurement is end gap. There is a minimum and a maximum. Should be no problem. I think your cylinder wear is minimal, but I would also measure them if I were you, or get them measured.

88911coupe · 05-21-2008, 07:34 AM

I've been digging through old threads and still have a question regarding this issue. Is there any reason to resuse old rings if they are in spec? I was planning to trash mine but they still appear to be in great shape but I've never measured them. The p/c's have been measured and were well within spec so what do I do if the rings still measure in spec? The engine has 160k on it. I was assuming I'd be ordereing new rings but just wanted to check. BTW, I did break one oil ring, FWIW.
TIA

304065 · 05-21-2008, 07:42 AM

Buck,

Give old rings the "water test"-- throw them in the nearest convenient lake or stream and if they float, retreive and reuse. Bonus if this procedure is accomplished in front of some Sierra Clubbers.

But seriously, in 160k, somethin's gotta give. All those trillions of microwelding cycles where the ring adheres to the piston and then breaks free, coupled with friction from being forced against the groove, MUST have reduced the dimensions.

How did you measure the pistons? SOP is to buy new rings, mic them to be sure in spec, then install and check clearance with a feeler gage-- gage thickness plus the mic measurement is the groove width-- which has to be in spec or the new rings will wobble and break.

Only way I could imagine measuring the pistons without doing that would be with a go/not-go gage or a gage block of the appropriate thickness.

88911coupe · 05-21-2008, 08:08 AM

John, thanks for the advice...I'll let you know if they float! For some reason I'm thinking of the scene in Monty Python about the witch..."so, if she weighs more than a duck"...
Anyway, I did not measure the p/c's myself, I had them measured by Zims who confirmed everything was in spec. They wrote the numbers on everything but it's all since been washed off. I'll order new rings and then measure the side wall gap to see if that is in spec as well.