To valve job or not to valve job...

[pbsch]

So I ran a compression test on my donor engine to see where I was starting from. 3 cylinders came in at 150psi – not bad. 3 cylinders came in at 90psi or less – ugh. I decided to go ahead and pull the heads to see what was going on in them. Interesting – fel-pro LIM and head gaskets – someone has been in here before, and fairly recently from the looks of things. I flipped the heads over and filled the combustion chambers with water. The three cylinders that read low on compression all leaked out through the intake valves and fairly quickly. Everything was pretty carboned up and I wanted to just do some smoothing of the intake and exhaust runners anyway, so I figured I’d clean them up real nice and see what I found. After cleaning the carbon and junk off the valves & heads, I could see that the seats and the valve surface that meets the seat looked to be in pretty good shape – no pitting or radial grooving, no erosion of the metal. So, lapped them in and got nice, uniform continuous rings of matte finish on both the valve and the seat. None of the valve stems had appreciable wear either. I’m thinking about picking up a set of oil seals (the o-rings were hard plastic film when I removed what was left of them from the valve stems) and reassembling them.

Questions:
Assuming I reassemble these heads in super clean condition and re-test the combustion chambers for fluid holding, do any of you see a need to have new valve surfaces cut?
Upon disassembly, I only had oil seals on the exhaust valves – is that correct? I also thought I read somewhere that there is a certain seal that is much better than others, but now that I am looking for that information, I can’t find it anywhere. Which ones are the optimal for my 3.4L Iron heads?
If I reinstall the heads and find that I now have 150psi in all six cylinders would that be acceptable compression to not re-ring?

Thanks for your insights,
Paul

[pbsch]

correction - the three bad cylinders leaked out through the exhaust valves. Not the intakes

[86FieroSEv6]

Gawd, that sounds so much like somebody mixed up their pushrods during an intake manny gasket job. 150 is okay for a motor with some miles on it, but it ain't gonna be no hustler.

[ForcedFirebird]

Iron heads have all the same sized pushrods.

It is common for the exhaust valves to pit severely on a 60*, 99% of the time I have to kiss the valves on the surface grinder to get them back to sealing condition (lapping also works).

Use small block seals on intake and exhaust, I don't know the part number off the top of the noggin.

[86FieroSEv6]

"Iron heads have all the same sized pushrods."

I stand corrected. my bad.

[pbsch]

so I did a little more research and now under stand that a compression test isn't really all that good for the depth of diagnosis I would like to do before deciding to tear this engine down or not. It looks like I'll be fabbing up a leak-down tester and going at it with that once I have my heads done and reinstalled.

[pbsch]

I also started my port and polish tonight on the heads. After two intake runners and an exhaust I ended up with a pretty imrpessive pile of filiings. I'm just cleaning up the machining around the valve guides, the shadowing around the valve openings and doing a general smoothing - nothing too aggressive. The exhaust ports will be final polished with something in the 120-200 grit range and the intakes will be done with a coarser finish for swirl and tumble. I can see why these things don't flow very well now. I believe that even the work I'm doing on them should have some pretty decent improvement - so many sharp edges and so much casting flash...

[pbsch]

Hey ForcedFirebird - by small block seals - do you mean i can use seals for the 350 z28? I noticed that the valve stem specs are the same for the 3.4 and the 350 at .3413 oem od. Is there a vendor that seels the fel-pro units individually? I spent my lunch hour googling and only found them in sets of 6 or 8. Either way I'd end up buying two sets to get the 12 I need that way. Rock Auto sells individual ones, but they are not fel-pros. They are viton sealed power brand. Would they be similar durability and quality fit?

[pocket-rocket]

Since a 4.3 is a 350 with two cylinders chopped off, you could probably get a 4.3 valve stem seal kit and have it work out fine since it's a v6 as well.

[pbsch]

unfortunately this is a 3.4 which is a direct descendant of the 2.8 which I believe was not derived from any v-8. The similarity of the valve stem sizing makes me suspect that they used a lot of the same tooling for things like valve guides. This makes a lot of sense from a simplicity of manufacturing and cost control standpoint.

I'm going by the Advance Auto parts to compare the seals in set SS72530 (v6) and SS72527 (v to see if they're the same. Since the v-8 version contains 4 seals and 8 orings, I can buy three and have exactly enough seals to do all the intake and exhaust valves. They are about $10 a set. The v6 version which has only 2 more seals and 4 more orings is $20. I'll report my findings later this evening. Are we sure that the Viton seals will handle the higher heat the exhaust valve must carry?

[pbsch]

Well, I stopped by advance on my way home today and made another interesting discovery. The item numbers of the two types of seals are identical for these two kits. the only difference is quantity. SS72527 has 8 of the orings and 8 seals and is priced at $11.97. SS72530 has 6 of the seals and 12 of the 0rings but costs 21.97. The only way for this price disparity to be explained is that the unit price for the seal is -.67 and the oring is 2.16. If you want to seal both the intake and exhaust, buy two of 72527, it costs $2 more to have enough seals to do all 12 valves and have 4 left over in case of mistakes or to give to your friend who meeds a few.

[pocket-rocket]

I made my comment based on Forced's comment on the small block seals. Reason being, you don't need as many as a v8, and like I mentioned, a 4.3l v6 is a 350 missing two cylinders...