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Pressurized crankcase - ring gap?

Old 06-27-2020, 09:23 AM
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Default Pressurized crankcase - ring gap?

I have a new build that has a lot of crankcase pressure. The motor is a 555 BBC with 10:I compression and a brand new Dart block with a really good hone. I have been reading on the causes of this issue and a lot point to worn rings. I have new rings but I am questioning my ring gap. My ring brand instructions called for .0045 x bore. That comes out to be .020. The instructions said this is the minimum gap. Since a marine engine is under constant load I increased that to to .025. This gap is the minimum gap for a nitrous engine according to the instructions. Could that ring gap cause the high crankcase pressure? It blew a press in oil cap out and doesn't seem to have the power it should. I also have a pcv valve.

Any ideas?
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Old 06-27-2020, 09:52 AM
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ring gap is correct if not on the tight side ring break in we use cheap 10-30 oil and run it on the stand for a while to get ring seal (1500-2500)........we use a bit more ring gap because the block is often on the to cool side so the bore has not expanded like it does at 150 160 degrees do not run synthetic to break in
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Old 06-27-2020, 09:54 AM
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It is said alot (engine builder write ups) ring gap has less to do with crankcase pressure than does piston to cyl wall clearance.
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Old 06-27-2020, 10:29 AM
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who did the engine assembly?
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Old 06-27-2020, 10:31 AM
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I ran Brad Penn straight 30 wt on break in. I am not sure on the piston to wall clearance as the machinist set that up.

I also have the coated piston skirts and thought those were supposed to wear in to the cylinder wall?
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Old 06-27-2020, 10:41 AM
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Are you running breathers in both v covers ?
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Old 06-27-2020, 12:20 PM
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I assembled it. I have done quite a few drag and boat motors. All big cubic in. I just assembled this motor with no issues except hauling ass.

I ran a pcv valve and a press in oil cap. I will be changing the oil cap with a K&N breather.
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Old 06-27-2020, 02:54 PM
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the reason i asked who assembled it is i once had a friend do his own assembly and he had the same issue you are having.when i took the engine apart i found that he had installed the 2nd compression rings up side down.i am not saying you did that but i can see that type of mistake happening.btw,i have a very similar engine in my hondo flatty,572,16.71 blower with a superchiller like yours.my block is black and with all the polished alumunum it looks bad azz.

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Old 06-27-2020, 03:35 PM
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I may have to check that as I was filing rings and my buddy was installing and clocking them on the piston. My rings didn't have the dot that shows you how to install them face up but mine you had to look at the bevel. Very easy to screw up I guess.

Time to pull it again. Getting pretty good at it now.

Thanks for that info.
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Old 06-27-2020, 08:15 PM
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I once had high blow-by on a fresh SBC build. Also had hand filed rings and everything was checked and double checked. Compression test narrowed it down to a single cyl. Pulled head and found a small chunk of piston missing from just above top ring. Cyl looked fine and rings re-measured OK. Replaced the single piston (& it's rings) and blow-by was completely gone. 15 years later, it's still running great. Never did figure out exactly what happened. Best guess was that I caught a ring edge when installing pistons.
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