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Piston End Gap???

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Old 11-13-2010, 07:04 PM
  #1  
FLMustangMan
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Default Piston End Gap???

I can't really find too much on some exact piston end gap clearances? I've read a bunch of different articles and can't really find something solid. So can anyone give me around an exaxt measurement for the top, bottom, and oil ring rail should be? My book only says compression rings and oil ring so this doesnt seem specific enough for me.

My application:
96 Mustang GT. Stock bore, but recently cleaned and honed so bore may be slightly bigger.(I forgot to have the shop write down my bore sizes after honing but the shop measured the pistons to the block and said it was good.). The car is supercharghed so i need ring gaps for blown setup.

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Old 11-13-2010, 07:05 PM
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This is something i found in another article.

http://www.manleyperformance.com/dl/...ston-rings.pdf
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Old 11-13-2010, 07:08 PM
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"You need a file fit ring for a boosted motor. It won't live without proper ring end gap. You need more end gap with boost than the rings will come with.

.025-.026 top and .016-.017 second ring. I know lots of guys who have used standard iron rings for both the top and bottom, but I heard that steel rings are better for boost. PTW clearance is critical too. I had mine set at .0035.

I am no expert on engine building, but PTW and ring end gap clearances are the secret to engine longevity."

Found this too.

Anyone agree???
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Old 11-14-2010, 10:30 AM
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Steel rings are good for big power/ high boost, but if you are on the stock shortblock then the standard file fit rings will be fine for what the sb can handle safely. .025 is a pretty big gap I set my top ring gap ~.022 and bottom gaps at ~.018 for 18psi. Bigger won't really hurt anything, but you will have excessive blow-by the bigger the gap. I've got quite a bit of blow-by with my setup, but I guess that comes with having a boosted car. I'd set ptw clearance where the piston manufacturer specifies, which is usually around .003-.004.
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Old 11-14-2010, 02:07 PM
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Originally Posted by Chris98Gt View Post
Steel rings are good for big power/ high boost, but if you are on the stock shortblock then the standard file fit rings will be fine for what the sb can handle safely. .025 is a pretty big gap I set my top ring gap ~.022 and bottom gaps at ~.018 for 18psi. Bigger won't really hurt anything, but you will have excessive blow-by the bigger the gap. I've got quite a bit of blow-by with my setup, but I guess that comes with having a boosted car. I'd set ptw clearance where the piston manufacturer specifies, which is usually around .003-.004.
Ok I'm confused. My haynes manual says that the compression rings should be set at .0394. The oil ring should be set at .0493.

And just to clarify, because now I feel really lost, the Oil ring is the Very top ring. The compression rings are the Middle ring and the two oil rail rings at the bottom?
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Old 11-14-2010, 05:08 PM
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No, when I said bottom ring I meant the second ring. The top and second rings are the compression rings. The very bottom setup is the oil rings. I cannot recall for sure, but my haynes manual said to gap the rings somewhere like ~.012 top and .008 second I think. Which is for stock n/a which would be way too tight for a boosted setup anyway. I don't remember it saying to set them up in the .030's thats crazy.

Last edited by Chris98Gt; 11-14-2010 at 05:12 PM.
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Old 11-14-2010, 06:40 PM
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See thats what I dont get. In the book, which is a Haynes manual, 94-04, says:
Piston Ring End Gap
-Compression Rings~0.0394
-Oil Ring~0.0493

That doesnt make sense to me either. It is on the spec sheet for the 4.6L v8 Engines on page 2C-4. Unless I'm reading or misunderstanding it wrong, but thats whats there. What is on yours and/or what page?

Also I went out and measured my rings in my cylinders at the bottom.

Top Ring~0.026
2nd Ring~0.024
Oil rails~0.030

As that was done to my block was a hot tank, hone and hot tank again. The machine shop even had my pistons to check and mic everything. They said it was good. I'll have to call them tomarrow and see if they have the bore measurments. I also used my feeler gauge to get a somewhat close measurement of my piston to wall clearance and i was able to slide a 0.0015 in with the piston tight to the opposite wall. if i tweaked the piston in the cylinder i could manage to get a 0.0020 in but dont know if its accurate to look at that reading. Piston to bore clearance limit on my book says 0.0008 to 0.0018. So it looks like i am right at the limit with the cleance which is probly why my ring gaps are slighlt big. What should I do, besides have the block bored.020 over and have to get new pistons and rings?????

Please help. and ask if you need any more info from me to help me solve this stupid problem. Thanks to all.
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Old 11-14-2010, 08:50 PM
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For you engine the top compression ring should be .005xbore size, 2nd compression ring should be .0055xbore size and the oil ring should be .015 gap on any engine.

You can even open the 2nd ring up a bit more and make a bit more power.
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Old 11-14-2010, 11:14 PM
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So then with my current set of rings what should I do? Do you think that they'll be just fine to run or what?

Again my rings are:
Top Ring~0.026
2nd Ring~0.024
Oil rails~0.030

And you guys say, based on the calculations, they should be around:
Top Ring~0.017-0.018
2nd Ring~0.019-0.020
Oil rails~0.015

The differences are:
Top Ring~0.008-0.009
2nd Ring~0.004-0.005
Oil rails~0.015

Are these differences too much too deal with or do I need to attempt to get bigger rings and then file them down? Mabe rings that are 0.020 over and file down to what I need. Please help.

Stock bore is 3.552. My block was only honed so I calculated at 3.56 as an estimate till I call the shop.
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Old 11-15-2010, 04:55 AM
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Ring gaps are suppose to be measured ~1" down in the cylinder, not at the bottom. Check them again ~1" down and see what you get.
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