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Thoughts on compression test

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Old 07-23-2017, 05:24 PM
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Thoughts on compression test

I was just curious to see what more knowledgeable people thought of my compression test results. A little bit of backstory: 1976 Highboy, 390 4 bbl from who knows what (measured stroke, yes it's a 390) I bought the truck 5 years ago, p/o said it was her dads that passed away and that the engine was rebuilt some time but who knows when with who knows what. the truck always ran great, but developed a lifter/valve tap last year and was parked. Not a rod knock as it's not load-dependent, it's only noticeable above ~1200 RPM. In trying to diagnose it, I cleaned all the lifters which appear to be brand new, checked & measured the push rods which for some reason are 9.680" rather than the factory 9.590" and the push rod on cyl #7 was ever-so-slightly bent, and the #7 exhaust valve stem is noticeably lower when a strait-edge is laid on the valves. I stuck a boroscope in cyl #7 and there appears to be a tiny nick on the piston and the edge of the valve appears to have a 1/32" chip off of the side. I would guess that I floated a valve and it got bent which caused the bend in the push rod and is the tapping culprit. With all that, I ran a compression test before tearing the heads off.

1) dry: 110 Wet: 120
2) dry: 107 Wet: 120
3) dry: 80 Wet: 90
4) dry: 90 Wet: 120
5) dry: 100 Wet: 127
6) dry: 105 Wet: 125
7) dry: 105 Wet: 105
8) dry: 90 Wet: 105

I had planned to pull the heads and replace the valve and install 9.621" push rods since they'd be much closer to the stock length. These numbers seem not only much too low but scattered all over. Thoughts?
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Old 07-23-2017, 06:14 PM
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I don't know much about FE engines, but those compression numbers aren't very encouraging. Since you know you have a chipped valve, and you were going to pull the heads anyway, I'd pull the heads.

not sure why you'd have pushrods that are too long-perhaps the rockers and/or cam were changed. Check the condition of the cam and the lifters. If you keep them, get the tool to measure the proper length for pushrods.

New valve springs should be part of the head job-especially if the cam is something's other than stock.

What is the condition of the valve stems? My 351m intake valves were all gummed up, and sticking in their bores. Caused a couple bent pushrods
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Old 07-23-2017, 06:50 PM
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It a lot less work to pull heads than to pull engine. I would yank heads and see how things look.
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Old 07-23-2017, 06:50 PM
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everything looks pretty darn new in there. I know some of the FEs had lifter noise, so if it did, the previous owner may have just bought the longest push rods he could to cure it... inadvertently putting ones that are too long in. speaking of which, I read another post that excessively long pushrods could skew compression numbers as the valves wouldn't be able to seat correctly. I didn't mention in my initial post that I replaced the bent #7 pushrod with a shorter 9.621" one. I know it was hitting the piston because i put a .020 feeler gauge between the valve stem and rocker and could feel the tapping get noticeably worse.

the valve stem heights differed slightly; a few thousandths. If the cylinders aren't out-of-round and look fine, would it be a bad idea to re-hone the cylinder walls and throw new rings on? ideally hone the cylinders without removing the crank? I would imagine any metal dust from the hone would be minimal and would be flushed out with an immediate oil change after the first startup.
I was going to pull the heads, replace any valve components that didn't look healthy, then lap all the valves and reassemble.... but with these sorry compression numbers I am wondering if I should change out all the pushrods with the new shorter 9.621" set I have waiting and perform a second compression test or just replace the rings and hone the cylinders.
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Old 07-23-2017, 06:55 PM
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After you pull the heads, you can measure the runout. If the crosshatch looks good, I would leave it as is. If the crosshatch looks bad, or the cylinders are oval then pull the block.

Until then, pray it's just the (too) long push rods...
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Old 07-23-2017, 09:38 PM
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You can't even really go by cross hatch in a cyl since someone was in there. For all you know the cylinder was honed wrong when bored and could be big in the middle. If you want it to just run a while longer than yea throw rings in it if the end gap is wide. Myself I'd dig a little deeper in that motor just because it sounds suspicious.
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