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Discussion Starter #1 (Edited)
Hey guys,

I'm rebuilding my L67 because it was losing about 40% compression in a leak down test and was running really rough.

I re-ringed the pistons but didn't re-bore because the cylinders looked good with no ridge at the top and the factory cross-hatch pattern was still visible (did a light re-hone though). I thoroughly cleaned off the cylinder heads and valves paying close attention to the valve seating surfaces.

So I put the cylinder head back on with a new head gasket (just one side) and connected the leak-down tester to each of the 3 cylinders and I'm still losing about 40% compression and most of it seems to be blowing by the rings again. A fairly noticeable amount is also escaping though the exhaust valves, I'm not sure how much is normal though.

My question is, when the engine is cold and the cylinders are somewhat dry, is it normal to lose that amount of compression in a leak-down test? Is it normal to have a bit of air escape though the exhaust value when it's closed? Is there a break-in period after rebuilding an engine where things start to seal up? Whatever advise you guys can give me on what to do from here would be much appreciated.

EDIT: I decided to cover up the exhaust port with my hand to see how much compression I was losing from the exhaust values. It dropped down to 20%! After very close inspection, I found that the exhaust value seats have very tiny pits all around them. I compared them with the intake valves which had none. So I'll be getting a new set of exhaust valves which will take care of a good portion of the leak. Now the question is, is the remaining 20% of compression loss acceptable considering the state of the engine? (cold, dry, and not broken in with new rings and a fresh re-hone).
 

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Need more info.... Since you did that work (why oh why did you only do one bank!)? Beyond that, have you run the engine since doing the work? If not, the test you ran will be very squed. First of all, it takes a while of running before the new rings will seat in to the cyl walls. Also, if you haven't run it yet, you may well have a few lifters that have not pumped back down to operating range/dimension which can partially hold valves open. I would run the engine until it seems to be operating normally. Then I would take it for a good test drive. Along the way, do a few hard accel (like in high gear to put heavy load on engine) without going to red line on tach....maybe 4500/5000 rpm. After a few of these, drive it normal back home or to shop and allow some cool-down but not all the way to cold. Re-run the leak rate test while engine is still warm (the hotter the better). Now lets see what the numbers say............

Good luck, Jake L.
 

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can you verify the gauge isn't leaking? Flip the heads over and put some liquid in the bowl. If it disappears you know where it's going!
 

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My favorite visual leak-down test was when my friend asked me to bring over my auto tap scan gauge to find out why his car was missing.

After lifting the hood I saw the dip stick was out about 3 inches, oil was sprayed all over the top of the engine, and when he started the engine there was a definite cylinder miss.....the Visual Cylinder leak down test!....Cost him about a $1000 for a cylinder 0.010 overbore and new number 4 piston.....cheers KEV
 

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Discussion Starter #5
@jakesq I haven't run the engine yet - it's still out of the car and on the stand. I also haven't connected the connecting rods and rocker arms yet so the valves are supposed to be 100% closed. I've already found that the exhaust valves are pitted which is why they're leaking.

So I hope your right in that it just needs some break-in time for the rings to properly seal as I suspected.
 

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Guaranteed that rings need break-in time before a good seal is obtained....

When checking for leaky valve, I invet the heads and fill enough carb/choke cleaner in the combustion chamber to cover the valves. Let them set for a while to see if the fluid goes down. Then turn the head over to look at the intake and exhaust ports for any sign of fluid leaking past the valves in to ports. Carb/choke cleaner is VERY thin and if they can hold this the seal is good.

My personal thoughts are as long as you have gone this far, why not get a good valve job done using some good seals etc. Maybe even set them up for that bigger cam you may buy down the road......

My 2 cents, Jake L.
 

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Do you plan on having the seats in the heads ground and if so who will you have doing them?
 

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Discussion Starter #8 (Edited)
Using carb cleaner is a good idea. I'll do that when I get the new valves in to test both the intake and exhaust.
I've put in new valve seals already. As far as everything else goes, not really interested in swapping in performance parts. Everything is stock right now and will probably stay that way. The car I'm working on is a 99 Buick Regal GS with a quarter-million miles on it. I probably won't be keeping it for more than another 30K miles or so.

@Caleditor - The seats on the heads seem to look okay, but if they don't pass the aforementioned carb cleaner leak test, should I have my local machine shop service the head?

Thanks for all the input thus far everyone!
 

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Who might that be?
 

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I worked for AC Nutter for awhile aka Nutter Racing Engines. I worked for A1 Cylider heads in Vancouver and Pacific Car Tech another machine shopin Vancouver. I have a friend named Gene Lynch that has a machine shop in Portland. Maybe its Lynch Racing Engines.
Hometown Washougal :icon_lol:
 

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IMHO do yourself a favor and have a machine shop do a valve job on your heads. Take them in and see if the pitting is bad enough to replace the valves. If not, just have them ground and save yourself the $$$ of the new valves. You are going to have to have the new valves and seats ground to match them to each other anyway.
 

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Guaranteed that rings need break-in time before a good seal is obtained...

My 2 cents, Jake L.

........that's for sure!!
 

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Discussion Starter #14
IMHO do yourself a favor and have a machine shop do a valve job on your heads.
Crap...

Looks like I am going to need to take it down to the machine shop after all. I got the new exhaust valves in (they were only $57 shipped from RockAuto) but it's still leaking badly. So they're gonna need to grind it to match as you said. :icon_sad:
 
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