Piston rings

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monzadon
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Piston rings

Post by monzadon »

I just finished putting new rings and rod bearings in my/140 motor, i fitted the rings to 15 thousand gap after putting the heads on the.motor i air pressure tested the cylinders and i am getting blow by in the crank case, is this normal because the rings aren't seated yet, this is my first time.of doing this kind of work
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azdave
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Re: Piston rings

Post by azdave »

Some air will always leak into the case when you do a pressure check such as a leak-down test. If the rings are not seated yet you really don't know a whole lot about whether the amount of leakage is normal or too much. That ring gap should be good so unless you have one cylinder that is leaking way more than the others, it seems like you should continue the build and engine break-in.
Dave W. from Gilbert, AZ

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monzadon
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Re: Piston rings

Post by monzadon »

Thanks, all the cylinders check about the same pressure , another thing my book shows 40 lb torque for the heads i am afraid with that much torque I'll strip out the threads or brake a stud
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azdave
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Re: Piston rings

Post by azdave »

monzadon wrote: Mon Aug 31, 2020 6:20 am Thanks, all the cylinders check about the same pressure , another thing my book shows 40 lb torque for the heads i am afraid with that much torque I'll strip out the threads or brake a stud
40# is way too high but was recommended in some early shop manuals. 30# is the max the experts say to use today if you have them lightly oiled or use anti-seize. Even less torque required if you use one of those slippery thread lubes like ARP recommends.
Dave W. from Gilbert, AZ

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66vairguy
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Re: Piston rings

Post by 66vairguy »

Dave is correct, ignore the shop manual for head nut torque as Chevrolet revised it a few times. The head stud threads should be clean and smooth. You can use a light lubricant, but wipe most of it off. Dirty or rough threads should be fixed. The ARP lube is ONLY for ARP bolts/nuts and requires using the ARP torque value.

The 30ft lb. figure is now used by most, BUT you should retorque the head after it sits (I usually wait a day). The steel collapsible gaskets tend to seal better in my experience, but if you need a thick gasket you'll have to use copper from Clark's.

Pulling a head stud can result in some work and many use a tube and washer with a nut to torque the head stud to 30ft lbs BEFORE installing the cylinders and pistons to test the studs. Easier to fix a pulled stud with no pistons/cylinders installed.

Good luck.
monzadon
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Re: Piston rings

Post by monzadon »

Thanks, i won't go over 30 lbs, i had to.use copper gaskets i needed 70 thousands.because.the.pistons were hitting the valves, originally they.had.70 thousands under the cylinders which i was.told that was the wrong way to do it thanks again for all.your information
joelsplace
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Re: Piston rings

Post by joelsplace »

You will end up with way too much clearance in the squish area doing that so unless you have really low compression you will have detonation problems. Unless you have a wild cam or the heads have been milled way too much you shouldn't have valves hitting the pistons. Are you sure your cam timing isn't way off?
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erco
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Re: Piston rings

Post by erco »

I'm open to piston ring recommendations for my 140. Another forum has polarizing opinions of Grant piston rings, which are widely available. Maybe that in itself is the answer.

http://corvaircenter.com/phorum/read.php?1,844912
Wagon Master
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Re: Piston rings

Post by Wagon Master »

monzadon wrote: Mon Aug 31, 2020 8:36 am Thanks, i won't go over 30 lbs, i had to.use copper gaskets i needed 70 thousands.because.the.pistons were hitting the valves.
What cam are you using to have that tight of piston to valve clearance with .032" head gaskets? How much does the piston stick out of the barrel at TDC?
nirvairna
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Re: Piston rings

Post by nirvairna »

monzadon - did you confirm the pistons were hitting the valves? or is that what the previous owner told you? it really is an uncommon situation to have piston-valve interference in stock motors.

how did the engine run before you started working on it?
erco
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Re: Piston rings

Post by erco »

Domed smog pistons hitting non-smog heads mayhaps? Or severely milled heads? The latter calls for custom length pushrods or you'll wear out your valve guides prematurely. I speak from experience.

You have something unusual going on. If you do a quick fix with thick head gaskets you are arbitrarily monkeying around with compression ratio and may end up with low performance or even worse, chronic detonation. I urge you to figure out what the root cause of your problem is. Corvairs are not interference engines. Leave that to the Citroen SM crowd. Another oddball car I covet.
joelsplace
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Re: Piston rings

Post by joelsplace »

Erco,
"Corvairs are not interference engines"
Are you sure about that? I've only broken one cam gear and I haven't torn it down yet but I've read numerous times about broken cam gears and bent valves.
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erco
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Re: Piston rings

Post by erco »

Yes, you are right joelspace, I misspoke. What I meant to say is that under normal conditions (no timing chain to break or jump, I had never heard of cam gears breaking) valves should never hit pistons.

A broken cam gear would certainly ruin your day. How common is that?
joelsplace
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Re: Piston rings

Post by joelsplace »

Pretty common apparently. There were a couple of different aftermarket gears that would break with relatively low miles.
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erco
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Re: Piston rings

Post by erco »

Yikes, I was wondering why Mike offers 3 different gears. Repro, failsafe and billet. https://mikescorvairparts.com/?s=cam+ge ... pe=product
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terribleted
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Re: Piston rings

Post by terribleted »

Cam gears can and do come loose and self destruct, I have seen a few. All valves hitting pistons sounds like a slipped gear which has the same effect as a cam improperly indexed to the crank so that valves are up when the pistons come up and they contact. The engine will generally not run with the cam timing this far off as there will be no compression. Of course bent valves and damaged pistons (crushed upper compression ring grooves often result as well). Severely machined heads or shortened cylinders can cause interference like this also.
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