compression test
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Joined: Feb 2010
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From: parma
compression test
I've got around 400+ miles on my fresh rebuild. A friend of mine has gone on a few excursions with me. He's an old FE man. He kept telling me that it sounds like a miss when the truck is idling. I really couldn't hear it myself. So just out of curiousity I went to Autozone and borrowed a compression checker. I got 5 cylinders at 140, two showing 155, and one showing 110. I pulled the valve cover and checked the valve lash. Perfect. Checked it again, 110. All plugs are out and carb plate wide open. I squirted some oil in the cylinder, same result. I'm thinking the heads got a bad valve. Thoughts?
Cargo Master
Joined: Apr 2003
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That's probably it. Perform a leak down test to verify that it is an issue with sealing the cylinder. While the cylinder is under pressure, you can listen to the intake and exhaust for the hissing sound of a leak.
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From: parma
Thanks for that input. These heads came off another rebuilt engine. It had never been fired, and the heads looked fresh. I had them milled when I found a scratch under the head gasket when I removed them from the other engine. I'll do a leak down test maybe this weekend. I still suspect the head because I haven't used any oil, don't get any smoke from the exhaust.
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From: parma
Well, did my leak down test Sunday and found the exhaust is leaking. Pulled both heads. I took out the offending valve tonight. I found that hardened seats were installed during the rebuild. However, I don't believe they ground the seats, just lapped the new valves in. There is a punch mark in the seat right where the valve seats. The valves are new replacements, no FoMoCo stamping on them anywhere. Is there any way to tell if these valves have rotators on them? Might as well ask that while its apart.
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From: parma
It'd be nice to throttle the people, BUT these heads came off that never fired 312 I bought last year. The one that had the broke #2 web. So far any success of anything from that piece of Sh&t is about 20%. Broke block, reground cam with reground lifters that had gall holes in them. Now it's got heads with new valves but a bad job. I had the heads milled .020 after finding that scratch in one of them. That was on my 1st install of them. So I might as well get a valve job done so they are right. How many other issues are hiding if I don't? The only thing that came out halfway decent was the crank and rods. I put the rotating assembly in my 292 block. Had the crank mains turned to the 292 block. The throws were OK according to the machinist.
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From: parma
I was asking about rotators because the tip of the valve that was leaking has the rub mark from the rocker only straight across. I would think if it was rotating it would be evenly wiped and not show a definite line across. If I pull the valves out myself do I need to keep the springs and keepers with the same valves? Could I use valve grinding compound and see if I can get that bad spot out of the exhaust insert? Even though the other cylinders gave good psi readings, I'm thinking of pulling all the valves and checking all the seats.
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From: parma
Now that the cold weather has eased up, I worked on 1 head last night. I lapped that valve with the punch *****. Lapping got that mark out. So now I'm going to lap all the valves on both heads. I think whoever did the heads just installed the new valves and called it a day.
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Tuned
Joined: May 2008
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From: Gladwin, MI
I like to keep assemblies together whenever possible. Valve, springs, retainers keep them together and to the same position they came out of. I would also check the valve guides to make sure they aren't allowing the new valve to float around and confirm the seals are in good shape.
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From: Waterloo, Iowa
Now would be a good time to measure valve spring installed height, from the sound of it this is probably something they paid no attention to either, like as not. Various thickness shims .015" ~ .030" are installed under each spring to achieve the correct spring height & spring pressure. Did they install fresh valve springs? They aren't expensive, but that won't usually stop 'em from cutting corners. I think a set of the hardened shims cost about as much as the springs.
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From: parma
Well, I've lapped all the valves. As for the spring question, some of the springs did have shims under them. The springs were new, they are also double springs. With a mild cam I'm sure the springs are overkill, but they're in there. They put new valve guides in all the cylinders, and the valve stem seals have springs on them, not like the original umbrella style. I was real happy with my spring compressor, I dropped $35 and bought a Craftsman unit. I've tried those little one's that have the handwheel and grab the springs, waste of time. Plus I don't think the cheap one would've grabbed the double springs.
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