Piston Gap Measurement

88911coupe · 01-21-2008, 08:14 AM

I'm sending them off to let a pro measure them, but I wanted to see about measuring the rings and have a question. I'm trying to clarify how to measure the up and down movement, not the end gap, to see if I am in the ballpark of allowable wear. Do I just keep inserting feeler blades between the ring and the piston ring cutout till one does not fit? I think I got to about .0027 on the ring closest to the crown of the piston. I assume this is the #1 compression ring....correct?
Or, do I remove the ring and just measure the whole gap? As I said, I'm sending them to a pro but I wanted to see what I could find out myself.
TIA

euro911sc · euro911sc's Garage

I just put the ring on and found a feeler that was the top end of the spec. If it fits I'm out and if it doesn't fit I'm in... I was in on all 6 accounts.

Best regards,

Michael

robmog · 01-21-2008, 09:29 AM

is there a way to do this w/o buying new rings?

euro911sc · euro911sc's Garage

Theoretically, I guess, if you took your used rings and measured and it was in spec then one could assume that you would be in spec with new rings as well... are you saying you wish to re-use your old rings? I thought that was a recipe for disaster?

Best regards,

Michael

robmog · 11:51 AM

sorry for the hijack buck!
mike, wasn't planning on reusing, but my engine is in pieces
and if i can't reuse my pistons i might upgrade p+c's.
so, its almost a catch 22, if my pistons are junk i just wasted some $$ if i buy new rings.
thanks for your advice.
any other opinions about this?

88911coupe · 12:47 PM

No problem...I'm wondering the same thing. Although just b/c pistons are worn does that mean you HAVE to replace the cylinders as well? Couldn't you possibly just get new pistons, assuming the cylinders were in spec?

euro911sc · euro911sc's Garage

Bob,

If you measure a go/no go with your used rings and you find that a piston is out of spec you wont know if it is the ring or the piston that is out (I suspect its the piston being of the softer material) If you find they are in spec then both ring and piston are in spec... kinda an all or nothing test.

Since you have the parts there in front of you, just do the measurement with the used rings. If you find that some are out and some are in then you can make your call on weather or not to buy new rings and re-measure. I think you could find a buyer here for close to retail for a new set of rings so it might not cost you much and if you are all in spec then you will be buying them anyway... ... or ... er.. um... were you looking for an excuse to upgrade

Buck,

I'm pretty sure that you can re-use pistons or cylinders independently as long as they are in spec. If your pistons are out and the cyls are in... go get new P's. You may wish to measure the side clearances to double check things, but if its OEM on both parts I might be comfortable making the assumption they will work together. If I'm wrong here someone smarter will correct me

Best regards,

Michael

88911coupe · 01-21-2008, 06:45 PM

Have another question. Why in Wayne's book does it say n/a for piston top/middle ring side clearance for all engines past 2.7? It only lists clearance and wear limit.

Eagledriver · 01-21-2008, 07:22 PM

The maximum allowable clearance is .004 inches. This is the wear limit. It can be measured with the old rings as they don't wear on the top and bottom, only on the edge that touches the cylinder. The top ring clearance is always the largest so if the top ring is good the rest will be good. The gap is largest at the 6 and 12 position.

I'm a big fan of re-using the old rings provided they are kept on the same pistons and cylinders. I've had brand new Goetz rings that had larger end gaps than the used rings that came off the pistons. I've used old rings on several of my engines with no problems. In addition if you use the old rings the wear limit on the ring land clearance is not as critical. It's new rings on pistons with excessive side clearance that cause problems.

You can buy replacement pistons but getting the proper running clearance is not easy. There are several different diameter groups for each size cylinder. You have to match the pistons to the cylinders by these groups. It's really best to keep the sets together if possible.

-Andy

88911coupe · 01-22-2008, 05:56 AM

OK Andy, let me see if I understand this correctly. If I can get a .004 inch blade between the ring and the side of the ring cut out on the piston, AND NOTHING LARGER, my rings are in spec? I may have screwed up since most of my rings have rotated around the pistons a bit, some I moved substantially when I was trying to check for any broken rings...have I shot myself in the foot since they are no longer in the same relative position?
I assume the ring groove/cut out is what gets larger over time with the ring slamming into the sides millions of times, correct?
Thanks

304065 · 01-22-2008, 08:44 AM

Buck, what I think is actually happening is the ring is microwelding itself to the piston due to the intense heat and pressure, then separating, millions of times.

Anyway, the ring gap check is always performed with new rings and a feeler gauge. No reason you couldn't use a stack of feeler gauges to determine the actual ring gap using the go/not-go method, but the conventional feeler gauge might not lend itself perfectly to that approach because all the feelers are riveted together. Thats why most guys use a new ring and then add the tolerance with the feeler gauge over the top.

Eagledriver · 01-22-2008, 05:00 PM

The first engine I built with used rings I was careful to keep the orientation the same. Since then I've re-oriented them to the positions that I use with new rings. The important thing is to keep the rings on the same pistons and in the same cylinder, not the orientation.

If you can get .004 feeler in but not a .005 feeler in then your pistons are within spec. You can use new rings on pistons with that spec. My spec book shows a wear limit of .006 for using the old rings.

-Andy

euro911sc · euro911sc's Garage

Quote:

Originally Posted by Eagledriver

The first engine I built with used rings I was careful to keep the orientation the same. Since then I've re-oriented them to the positions that I use with new rings. The important thing is to keep the rings on the same pistons and in the same cylinder, not the orientation.

If you can get .004 feeler in but not a .005 feeler in then your pistons are within spec. You can use new rings on pistons with that spec. My spec book shows a wear limit of .006 for using the old rings.

-Andy

Wow, learning lots of stuff all the time. I always thought that if you split piston from cylinder you needed to get new rings. Guess not

Thanks for the education!

Best regards,

Michael

Eagledriver · 01-24-2008, 06:15 PM

This has been my experience but I don't mind taking apart my engine if something doesn't work. (only had to do that once....I chipped a ring installing it). The rings move around on the pistons at least sometimes. They are never where you put them when you installed them after running for awhile.

-Andy