Interesting ring end gap phenomenom
Thread Starter
Rennlist Member
Joined: May 2001
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From: Puyallup, WA
Interesting ring end gap phenomenom
Doing some more work on the stroker motor.
Because of my custom bore size I had to go with rings that need to be cut (filed) to fit.
Stock Porsche spec calls for a ring end gap of .20-.40mm which equals an "incredibly tight for a domestic motor" end gap of 0.0079 - 0.0157 inches. For comparison sake, a typical domestic ring has an end gap of 0.018".
Porsche measures its bores at 62 millimeters (equal to 2.4409") from the top of the cylinder, and I would assume that this would also be the point where you would measure the ring end gap, but I could be wrong.
Since there is no mention of using deck plates anywhere in any literature I've seen regarding the machining or building of a 928 engine, I'm assuming that all measurements are taken with the open deck.
Now we go to the stroker, which is nikasiled and is using domestic pistons.
My Total Seal rings require that end gap be measured at 1" from the top of the cylinder and also states that the measurement should be taken with a deck plate. I cut the end gap on my rings, and drop the ring in to 1" and measure the end gap. I'm only at 11 thou, 7 thou short of my target 18 thou. I decide to go ahead and bolt the head on at this point just to see how much difference there will be in the eng gap once the cylinders are stressed, not really expecting to see any.
Couldn't believe it. After torquing the heads, the end gap spread to 13 thou, an increase of 2 thou (!). I figured I mis-measured my initial 11 thou with the open deck. After letting the head sit on overnight, this morning I pulled it off and remeasured the ring end gap. Back to 11 thou.
Don't know if a stock bore cylinder would show that much flex, but pretty wild knowing that the 104'ish cylinders can distort that much under torque.
Comments?
Because of my custom bore size I had to go with rings that need to be cut (filed) to fit.
Stock Porsche spec calls for a ring end gap of .20-.40mm which equals an "incredibly tight for a domestic motor" end gap of 0.0079 - 0.0157 inches. For comparison sake, a typical domestic ring has an end gap of 0.018".
Porsche measures its bores at 62 millimeters (equal to 2.4409") from the top of the cylinder, and I would assume that this would also be the point where you would measure the ring end gap, but I could be wrong.
Since there is no mention of using deck plates anywhere in any literature I've seen regarding the machining or building of a 928 engine, I'm assuming that all measurements are taken with the open deck.
Now we go to the stroker, which is nikasiled and is using domestic pistons.
My Total Seal rings require that end gap be measured at 1" from the top of the cylinder and also states that the measurement should be taken with a deck plate. I cut the end gap on my rings, and drop the ring in to 1" and measure the end gap. I'm only at 11 thou, 7 thou short of my target 18 thou. I decide to go ahead and bolt the head on at this point just to see how much difference there will be in the eng gap once the cylinders are stressed, not really expecting to see any.
Couldn't believe it. After torquing the heads, the end gap spread to 13 thou, an increase of 2 thou (!). I figured I mis-measured my initial 11 thou with the open deck. After letting the head sit on overnight, this morning I pulled it off and remeasured the ring end gap. Back to 11 thou.
Don't know if a stock bore cylinder would show that much flex, but pretty wild knowing that the 104'ish cylinders can distort that much under torque.
Comments?
Last edited by atb; Oct 13, 2008 at 09:37 PM.
Racer
Joined: Jan 2003
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From: Berkshire. 'Old' England. U.K.
Adam
Interesting observation, sorry more questions than answers !
What tool have you used to square the ring in the bore ? very important that its 90 deg. all the way round & checked it again after torquing the head, & have you torqued the mains as well ?
Whats the thoughts on ring gaps with our alloy blocks v cast iron blocks , is the 18 thou recommended for both ?
Interesting observation, sorry more questions than answers !
What tool have you used to square the ring in the bore ? very important that its 90 deg. all the way round & checked it again after torquing the head, & have you torqued the mains as well ?
Whats the thoughts on ring gaps with our alloy blocks v cast iron blocks , is the 18 thou recommended for both ?
Thread Starter
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Joined: May 2001
Posts: 4,871
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From: Puyallup, WA
Hi Martin,
The "tool" is a 4" diameter black PCV pipe end. Thanks to Ken Osage for cutting down my original which was a 2.5" tall end cap (which would have been just about perfect for OEM spec) and making two 1" tall cylinders out of it. (So now I can do two cylinders at once without having to remove the head). He used a chop saw and then a DA sander. Got the heights to be consistent to about .04", which ain't bad.
The cradle has been torqued. Since the engine is on the stand, you don't have too much choice since the engine stand bolts to both the upper block and the cradle.
The instructions that came with the rings from Total Seal gave two different formulas for calculating end gaps, one for iron blocks and another for nikasil'd aluminum blocks. I don't recal the exact formula, but for my bore and the nikasil'd cylinders it came out to a little more than 18 thou. I think it was bore x .0045 for the first ring, which would be 4.087 x .0045 = .0184", which sounds about right. The second ring was bore x .0035 which would be a little tighter at .0143".
The "tool" is a 4" diameter black PCV pipe end. Thanks to Ken Osage for cutting down my original which was a 2.5" tall end cap (which would have been just about perfect for OEM spec) and making two 1" tall cylinders out of it. (So now I can do two cylinders at once without having to remove the head). He used a chop saw and then a DA sander. Got the heights to be consistent to about .04", which ain't bad.
The cradle has been torqued. Since the engine is on the stand, you don't have too much choice since the engine stand bolts to both the upper block and the cradle.
The instructions that came with the rings from Total Seal gave two different formulas for calculating end gaps, one for iron blocks and another for nikasil'd aluminum blocks. I don't recal the exact formula, but for my bore and the nikasil'd cylinders it came out to a little more than 18 thou. I think it was bore x .0045 for the first ring, which would be 4.087 x .0045 = .0184", which sounds about right. The second ring was bore x .0035 which would be a little tighter at .0143".
