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Top piston ring gap - whats really required?

General engine tech -- Drag Racing to Circle Track

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Mark O'Neal
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Re: Top piston ring gap - whats really required?

Post by Mark O'Neal »

There is only one rule that counts. Don't let the ring ends butt.

Past that everything is opinion.
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Re: Top piston ring gap - whats really required?

Post by ptuomov »

Mark O'Neal wrote: Mon Oct 05, 2020 5:21 pm There is only one rule that counts. Don't let the ring ends butt.
Past that everything is opinion.
If that’s true, why not leave the ring out altogether? The. It won’t butt for sure.
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Re: Top piston ring gap - whats really required?

Post by gmrocket »

In a perfect world where cooling systems keep every cylinder an identical temp or never ever spike in temp from a problem , you can probably take a chance on closing the gaps up... if you have the slightest issue, you could be in big trouble

It may even be possible to measure the increase in power at the low end of your power band where time may be on your side, but as the rpms climb, time for pressure to escape just isn’t there.

on lower rpm , lower hp builds I have closed up the top ring gap some.

Be Leary of testing smaller gaps on the dyno, where you can keep coolant to a nice ideal constant with air circulating all around the engine .. it’s a different world for the back cylinders in an engine bay with a V8
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Re: Top piston ring gap - whats really required?

Post by n2omike »

german4inline wrote: Sun Oct 04, 2020 1:46 pm don´t know about one single engine failure caused by this. As long as both cyl. and ring material is iron-based, the elongation of both is identical with the same temperature.
That's all fine and good for something running at a steady, easy state. But, the ring has far less mass than the iron cylinder liner, and will heat up much more quickly under heavy load. Therefore, it will grow faster than the liner and seize if there is not enough clearance. The greater the load/more heat... the bigger the gap needs to be.
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Re: Top piston ring gap - whats really required?

Post by jed »

Speaking of piston oiler jets on the latest PERA webinar Lake Speed Jr. and Keith Jones from total seal were talking and
Lake Speed jr let it slip "you'd be surprised how many piston oilers NASCAR engines have"
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Re: Top piston ring gap - whats really required?

Post by Krooser »

Does alky vs gasoline change the desired gap?
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Re: Top piston ring gap - whats really required?

Post by Mark O'Neal »

ptuomov wrote: Tue Oct 06, 2020 3:21 am
Mark O'Neal wrote: Mon Oct 05, 2020 5:21 pm There is only one rule that counts. Don't let the ring ends butt.
Past that everything is opinion.
If that’s true, why not leave the ring out altogether? The. It won’t butt for sure.
It's your motor. I am all in favor of you doing whatever you want to do.

However the subject matter, and I realize that I am relying on English comprehension here, is ring end gap. At no point was the subject changed to ring usage. Had it been so, my statement would have reflected that alteration.

So, in adhering to the spirit of the conversation....how can one possibly have a ring end gap....if one does not have a ring?
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Re: Top piston ring gap - whats really required?

Post by Mark O'Neal »

Krooser wrote: Tue Oct 06, 2020 1:11 pm Does alky vs gasoline change the desired gap?
EGTs are what, in the 1,200 range. So, probably, but why?

Still better to have too much than too little.

Measuring static leak has nothing to do with what the engine is doing at 8,000 rpm with the cylinder going oblong, the piston flexing like a 2 minute softboiled egg and traveling at Mach 6, the rod whipping like a piece of pasta, and the crank doing whatever cranks do at that speed. All while we worry about how much air leaks through a gap of about .020 x .015 in 3 milliseconds.....with about 60 lbs of actual pressure.

So, I never put much stock in it...but I do get the muddy copper head gasket law.
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Re: Top piston ring gap - whats really required?

Post by ptuomov »

Mark O'Neal wrote: Wed Oct 07, 2020 8:46 pm
ptuomov wrote: Tue Oct 06, 2020 3:21 am
Mark O'Neal wrote: Mon Oct 05, 2020 5:21 pm There is only one rule that counts. Don't let the ring ends butt.
Past that everything is opinion.
If that’s true, why not leave the ring out altogether? The. It won’t butt for sure.
It's your motor. I am all in favor of you doing whatever you want to do.

However the subject matter, and I realize that I am relying on English comprehension here, is ring end gap. At no point was the subject changed to ring usage. Had it been so, my statement would have reflected that alteration.

So, in adhering to the spirit of the conversation....how can one possibly have a ring end gap....if one does not have a ring?

English and logic are both difficult. If there’s no ring, the ring ends can’t butt. After all, “There is only one rule that counts. Don't let the ring ends butt.”

Joking aside, I think that the tightest gap that never butts is the best, but a slightly larger gap is only slightly worse while a slightly smaller gap is much, much worse. Because of this, my engine has ring end gaps set to 0.024"/0.61mm for both top and 2nd rings on a nominally 3.937”/100mm Nikasil plated aluminum cylinder towers. I consider this looser than what I think would be the absolute “best” in expectation, but it was more of a risk management decision.
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Re: Top piston ring gap - whats really required?

Post by turbo camino »

I seem to be getting in the habit of buying engine parts and then never assembling anything. Durability is approaching infinity with this method as long as stuff is stored correctly, it's incredible.
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Re: Top piston ring gap - whats really required?

Post by jed »

Mark O'Neal wrote: Wed Oct 07, 2020 8:55 pm
Krooser wrote: Tue Oct 06, 2020 1:11 pm Does alky vs gasoline change the desired gap?
Measuring static leak has nothing to do with what the engine is doing at 8,000 rpm with the cylinder going oblong, the piston flexing like a 2 minute softboiled egg and traveling at Mach 6, the rod whipping like a piece of pasta, and the crank doing whatever cranks do at that speed. All while we worry about how much air leaks through a gap of about .020 x .015 in 3 milliseconds.....with about 60 lbs of actual pressure.

So why make a big fuss over using a torque plate at room temperature???
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Re: Top piston ring gap - whats really required?

Post by Mark O'Neal »

jed wrote: Thu Oct 08, 2020 8:54 am
Mark O'Neal wrote: Wed Oct 07, 2020 8:55 pm
Krooser wrote: Tue Oct 06, 2020 1:11 pm Does alky vs gasoline change the desired gap?
Measuring static leak has nothing to do with what the engine is doing at 8,000 rpm with the cylinder going oblong, the piston flexing like a 2 minute softboiled egg and traveling at Mach 6, the rod whipping like a piece of pasta, and the crank doing whatever cranks do at that speed. All while we worry about how much air leaks through a gap of about .020 x .015 in 3 milliseconds.....with about 60 lbs of actual pressure.

So why make a big fuss over using a torque plate at room temperature???
I'll bite.

Why?
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Re: Top piston ring gap - whats really required?

Post by Mark O'Neal »

ptuomov wrote: Thu Oct 08, 2020 3:51 am
Mark O'Neal wrote: Wed Oct 07, 2020 8:46 pm
ptuomov wrote: Tue Oct 06, 2020 3:21 am

If that’s true, why not leave the ring out altogether? The. It won’t butt for sure.
It's your motor. I am all in favor of you doing whatever you want to do.

However the subject matter, and I realize that I am relying on English comprehension here, is ring end gap. At no point was the subject changed to ring usage. Had it been so, my statement would have reflected that alteration.

So, in adhering to the spirit of the conversation....how can one possibly have a ring end gap....if one does not have a ring?

English and logic are both difficult. If there’s no ring, the ring ends can’t butt. After all, “There is only one rule that counts. Don't let the ring ends butt.”

Joking aside, I think that the tightest gap that never butts is the best, but a slightly larger gap is only slightly worse while a slightly smaller gap is much, much worse. Because of this, my engine has ring end gaps set to 0.024"/0.61mm for both top and 2nd rings on a nominally 3.937”/100mm Nikasil plated aluminum cylinder towers. I consider this looser than what I think would be the absolute “best” in expectation, but it was more of a risk management decision.
That is as decent an assessment as any.
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Re: Top piston ring gap - whats really required?

Post by PackardV8 »

Two factors which definitely affect ring gap haven't been mentioned thus far; top ring location on the piston and the piston-to-deck height.

When emissions became the prime directive, OEM car engines moved the ring as high as possible on the piston and the piston as high as possible in the cylinder. The goal was to reduce the dead space above the ring. An unintended consequence was that put the top ring up in the solid area of the block. Because there is no coolant there, the piston is dwelling in a very hot area of the block. The OEMs had to do extensive testing to determine ring end gaps.

Only slightly OT, but I use a Ford 370" dump truck piston which has the top ring waaay down on the piston and also has a cast iron insert in the top ring groove; both necessary to have the engine live while running against the governor all its service life.
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Re: Top piston ring gap - whats really required?

Post by engineguyBill »

[/quote]

If that’s true, why not leave the ring out altogether? The. It won’t butt for sure.
[/quote]

A well-known crew chief once told me that he was pretty sure that a Top Fuel engine would run much better without any rings. The only problem was getting the engine to start without rings . . . . . . . .
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