Low compression
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Low compression
Long story here, but now that the machine shop has finally got my 390 heads done correctly now after the 3rd time 
. I have found I have one cylinder that is only pumping up to 55psi, wet or dry, all the others are between 128-140psi. The cylinder walls are all nice with no gouges and no big lip at the top. I am on really tight budget and need to get this thing running again. So is it possible to change just one set of pistons ring. 
Yes I know that this should not really be done, but I am in a big pinch.
.
. I have found I have one cylinder that is only pumping up to 55psi, wet or dry, all the others are between 128-140psi. The cylinder walls are all nice with no gouges and no big lip at the top. I am on really tight budget and need to get this thing running again. So is it possible to change just one set of pistons ring. Yes I know that this should not really be done, but I am in a big pinch..
Last edited by aj3816; May 12, 2003 at 09:16 PM.
Post Fiend
Joined: Feb 2001
Posts: 6,624
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From: Missouri
Low compression
If your compression doesn't come up when you add oil to the cylinder, you have a valve that isn't seating right. Who is doing the heads? Have them pressure checked before you install them. It only costs about 15 bucks and should be done anyway by the guy performing the headwork.
Posting Guru
Joined: Aug 2002
Posts: 1,214
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From: Minneapolis, MN
Low compression
I agree with ratsmoker. It's probably a valve. If you do a leakdown test before you pull the head(s) back off you should be able to tell where the compression is going.
Just a thought... If the pushrod or the valve stem is too long the valve won't seal and the compression will be low. If you do a leakdown with the rockers in place (make sure you're on compression) and it shows leakage but with the rockers removed the leakdown doesn't show leakage, the valve train is holding the valve open.
Quick and dirty test for valve sealing... With the head off and the combustion area dry, spray enough penetrating oil through the port to cover the valve sealing surface. If there's any leakage into the combustion area, the valve isn't sealed.
Just a thought... If the pushrod or the valve stem is too long the valve won't seal and the compression will be low. If you do a leakdown with the rockers in place (make sure you're on compression) and it shows leakage but with the rockers removed the leakdown doesn't show leakage, the valve train is holding the valve open.
Quick and dirty test for valve sealing... With the head off and the combustion area dry, spray enough penetrating oil through the port to cover the valve sealing surface. If there's any leakage into the combustion area, the valve isn't sealed.
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Low compression
Well I tried a leak down test without the vavle train on and I couldn't even build up enough pressure to check it. Sound like it was going right through the exhaust valve. All other will air up and hold pressure. So I just took the head off again and will be dropping it off to the shop. This will be the last time I do any business with this shop. And if it is the valves, the nice people at the shop said they would actually cover my labor for this deal. 
and will be dropping it off to the shop. This will be the last time I do any business with this shop. And if it is the valves, the nice people at the shop said they would actually cover my labor for this deal.
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