entropy
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posted December 18, 2006 01:18 AM
Muzzy 87mm rings: wide gap on #2???
I just measured up/gapped a new set of rings for my ole heap:
top ring from Muzzy / after sizing: #1: .0135; #2: .0135; #3: .0135; #4: .0135;
2nd ring from Muzzy / as received : #1: .022; #2: .022; #3: .021; #4: .023;
I thought that conventional wisdom says that the gap on the 2nd ring should be set to same as #1. But no can do; they came from Muzzy WAY looser than #1.
any thoughts??
(i mention Muzzy specifically to draw Rob or DMeyer's comments)
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Texas12R
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posted December 18, 2006 01:30 AM
I just thought this was interesting  Most piston ring manufacturers recommend a minimum end gap of .004 inches times the bore diameter for the top piston compression ring. So for a 4 inch bore, the standard end gap would be .016 inches.
For the 2nd compression ring, the standard end gap recommendation for most stock engines is typically .005 in. x the bore diameter. So for a 4 inch bore, the minimum end gap on the 2nd ring would be .020 inches.
For a modified street performance engine that generates more horsepower and heat, the end gap should be opened up a bit to compensate for increased thermal expansion. The recommendation would be a minimum top compression piston ring end gap of.0045 to .005 inches times the bore diameter. For a 4 inch bore, the ring end gap on the top ring should be increased to .018 to .020 inches.
For the 2nd compression ring on a modified street performance engine, the recommendation is typically open up the end gap to .0055 inches times the bore diameter. For a 4 inch bore, the 2nd ring would be gapped to .022 inches.
For a nitrous or blown racing engine, the top ring end gap sould be opened up to as much as .006 or .007 inches times the bore diameter. Now we're looking at a 2nd ring end gap of .024 to .028 inches in an engine with 4 inch bores.
For the 2nd ring on a nitrous or blown motor, the recommended ring end gap is even wider: .0063 to .0073 inchestimes the bore ( or 0.025 to 0.029 inches with a 4 inch bore).
The recommended ring end gap for oil rings regardless of the engine application is typically .015 inches.
Some racers believe that opening up the 2nd end gap even more (say an additional 10%) can improve overall ring sealing by allowing trapped gases to escape before they blow past the top ring and cause ring flutter at high rpm (say above 5000 to 6000 rpm).
NOTE: These recommendations are rules of thumb only. Always follow the end gap specifications recommended by the piston ring supplier or engine manufacturer.
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Texas12R
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posted December 18, 2006 01:35 AM
I know this is not what you are looking for, but it is interesting that the second ring has a larger end gap...much like your observation.....Id like to think all info is good info....
just may not be helpful today 
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entropy
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posted December 18, 2006 02:32 AM
Texas12R,
good info, and like you say all info is good, just gotta put it in context.
Thanks!
BTW the factor you show for street performance for my 87mm application gives:
top ring = .015-.017
2nd ring = .019
I get advice from many hi-end experts and write it all down 
Based on that advice I previously had my #1 gap at .015-.016 but this gave 15%+/- leakdown after break-in.
Another expert says try .013-.014, thus my .0135 shown first above.
(i could be talked into ignoring the leakdown data and going back to .015-.016.
I'm guessing that the difference in expert opinion may be that ring type may impact gap setting???
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krexken
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posted December 18, 2006 07:38 AM
.0135 is tighter than any specs I've been given but those are for generic applications. I'm guessing you've seen no signs of ring butting before. If they butt, you'll know it and it's pretty obvious when you see it. Good chance your exhaust valves/seats could be adding to your leakdown #s. They carbon up pretty quick and can leak a bit.
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entropy
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posted December 18, 2006 08:44 AM
quote:
... Good chance your exhaust valves/seats could be adding to your leakdown #s...
and right you are, Ken!
After the leakdown I had the head looked at and ended up having 6 exh guides and all valves replaced, and all seats re-cut. 2.5 yrs of beating on the head, i guess it was time for a re-fresh, eh?
I will open up the 1st gap a bit, but am still wondering about why Muzzy's 2nd ring gap is so big???
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dougmeyer
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posted December 18, 2006 03:01 PM
Second ring is iron, right? Greater expansion. Needs more gaposis.
Ring end gaps don't play very much into leak rate unless they're really big.
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Y2KZX12R
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posted December 18, 2006 05:59 PM
quote:
Ring end gaps don't play very much into leak rate unless they're really big.
+1
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entropy
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posted December 19, 2006 01:47 AM
quote:
... If they butt, you'll know it and it's pretty obvious when you see it....
Ken,
what does evidence of ring butting look like????
(for future reference)
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osti33
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posted December 19, 2006 01:12 PM
quote:
quote:
... If they butt, you'll know it and it's pretty obvious when you see it....
Ken,
what does evidence of ring butting look like????
(for future reference)
Scaring on the cylinder walls. Been there done that on nitrous car motors.
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entropy
Moderator
Posts: 8671
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posted December 19, 2006 08:11 PM
quote:
quote:
quote:
... If they butt, you'll know it and it's pretty obvious when you see it....
Ken,
what does evidence of ring butting look like????
(for future reference)
Scaring on the cylinder walls. Been there done that on nitrous car motors.
Ry,
i'll call you tommorrow.
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osti33
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posted December 19, 2006 08:29 PM
 Edited By: osti33 on 19 Dec 2006 20:35
Ok.
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dougmeyer
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posted December 20, 2006 04:21 PM
At a MINIMUM, the ends of the rings get mushroomed and polished and there is a scar on the cyl wall where the gap is. Next worse is excessive wear on the ring faces all around and scoring all around, next worse is broken rings. Given enough time, this always happens.
Doug
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