[C1] Cylinder compression issue
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
Cylinder compression issue
I've been searching for a misfire and checked the cylinder compression numbers.
1: 160 2: 150
3: 140 4: 150
5: 100 6: 190
7: 130 8: 150
Obviously, I have issues with compression but the real problem for me is not knowing what to do at this point. I'm thinking I'll have the heads/valves redone, hoping I won't need to go further into the block, but I don't know what to tell the machine shop or is there another solution? Do I get new valves, springs, seats or what? Maybe new heads? I just need some thoughts to help get started.
Thanks in advance, Jim
327/340 30-30 cam, I think. Solid lifters. Little bit of a Frankenstein make up.
Drivers side head: GM J 20 7 3917291, 10.20.67 build, 64cc chambers
Passenger side: GM 25 K 16 7 3017293, 11.16.67 build, 75cc chambers
Block: 3782870, 3.17.63 Build
intake: GM Winters. 3844461 '64-'65 L79
1: 160 2: 150
3: 140 4: 150
5: 100 6: 190
7: 130 8: 150
Obviously, I have issues with compression but the real problem for me is not knowing what to do at this point. I'm thinking I'll have the heads/valves redone, hoping I won't need to go further into the block, but I don't know what to tell the machine shop or is there another solution? Do I get new valves, springs, seats or what? Maybe new heads? I just need some thoughts to help get started.
Thanks in advance, Jim
327/340 30-30 cam, I think. Solid lifters. Little bit of a Frankenstein make up.
Drivers side head: GM J 20 7 3917291, 10.20.67 build, 64cc chambers
Passenger side: GM 25 K 16 7 3017293, 11.16.67 build, 75cc chambers
Block: 3782870, 3.17.63 Build
intake: GM Winters. 3844461 '64-'65 L79
Race Director
Joined: Apr 2015
Posts: 17,571
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From: Fresno California
Verify the readings with another compression gauge. Then, and this is critical: perform a cylinder leakdown test. This will tell you where the loss of compression lies. No point in freshening up the heads if you have damaged rings in number 5 cylinder!
Safety Car
Joined: Oct 2016
Posts: 4,617
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From: Olive branch Ms
At this point if your readings are correct I would remove the rocker arms from 5 and hook air to that cylinder. Then listen for where it's escaping. It will either be going out the exhaust, in the oil pan or in the intake manifold. This will tell you where the problem is.
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
At this point if your readings are correct I would remove the rocker arms from 5 and hook air to that cylinder. Then listen for where it's escaping. It will either be going out the exhaust, in the oil pan or in the intake manifold. This will tell you where the problem is.
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
i think I can manage the leak down test. About freshening the heads, wouldn't I want to do that also even if I had to pull the engine and do the rings?
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Melting Slicks
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From: Riverhead New York
Once you get things sorted out, if engine needs rebuild I would consider a Crate motor from GM. Put the OEM engine away for the future and drive the Heck out of the Crate with no worries and a Warrantee. Al W.
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
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From: Starkville Mississippi
Le Mans Master
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From: Maryland
St. Jude Donor '05, '09, '15
.
Drivers side head: GM J 20 7 3917291, 10.20.67 build, 64cc chambers
Passenger side: GM 25 K 16 7 3017293, 11.16.67 build, 75cc chambers
Passenger side: GM 25 K 16 7 3017293, 11.16.67 build, 75cc chambers
Tom
Safety Car
Joined: Oct 2016
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From: Olive branch Ms
a leak down is similar to what I was suggesting with putting the air in the cylinder. At this point you want to know where its going if it's the exhaust or in the intake that's one issue if its in the pan its a complete overhaul including the heads.
Last edited by Robert61; Aug 5, 2018 at 09:42 PM.
Thread Starter
Racer
Joined: Aug 2013
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From: Starkville Mississippi
I’ve been worried that should be done.
Safety Car
Joined: Oct 2016
Posts: 4,617
Likes: 1,547
From: Olive branch Ms
Look at the casting mark on the end of the heads. If they are both double hump the one is not a 293. The compression test should be quite different if you had a high compression head on one side and a low on the other. The low compression casting is 3917293. Also if you do have a 293 head on one side the intake valve size is different, the 291 would have 1.940 intake the 293 would be 1.725
Last edited by Robert61; Aug 6, 2018 at 10:44 AM.
Race Director
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From: Fresno California
Yes. And be advised that you need to know what you're doing to perform a cylinder leakdown test. The piston has to be on TDC. The rocker removal method Robert61 Mentioned is actually and easier, less technical way of heading in the right direction, although the official leakdown test requires no disassembly of anything.
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The shade tree quick check is to remove the spark plug in #5, squirt some oil in the cylinder and rerun the test on that cylinder. If compression returns, you know it is the rings. No need to debate any further.
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
Yes. And be advised that you need to know what you're doing to perform a cylinder leakdown test. The piston has to be on TDC. The rocker removal method Robert61 Mentioned is actually and easier, less technical way of heading in the right direction, although the official leakdown test requires no disassembly of anything.
Thread Starter
Racer
Joined: Aug 2013
Posts: 345
Likes: 29
From: Starkville Mississippi
Thanks
Race Director
Joined: Apr 2015
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From: Fresno California
Probably the easiest technique of all. Every time I do it though, I seem to ruin another special schrader valve in my tester.
Race Director
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From: Central Arkansas
The main safety thing in running a leak down test is if you are using a break over bar to turn the engine over to position the pistons at top dead center - always be sure to remove the socket off the crank pulley before airing up the cylinder. If the engine isn't right on TDC, the air will push the piston down (fast) and sling the break over bar somewhere or whack the heck out of something.