I seem to be picking up a lot of dissonance on this issue. All over the net I see people tell stories about how they took in their high mileage head for a valve job and milled flat, and they were thrilled that the motor ran like new thereafter, perhaps with a gasket shim to restore original compression. However, I also see many recommendations to never do such a thing without re-ringing, because the rings are sure to fail soon afterwards. It seems like the success side is comprised of anecdotes, whereas the non-success side is comprised of generalizations.
In theory, I understand how the failures are supposed to occur. But why do people bother with valve jobs then? Are these proclamations of successful top end work on an old bottom end simply naive as to the inevitable ring job that is coming to them?
Is a dry-wet compression test and a visual examination of how pronounced the cylinder ridge is enough to determine whether or not a given block is a suitable candidate for a valve job? If not, what else can you do besides guess at it with knowledge of the mileage and maintenance history?
I've got the block and piston tops all cleaned up waiting for either the new head, or the old head - which has four exhaust valves of four different colors.
What about using the new head but installing some or all of the old valves in it? Then I have the new guides, stem seals, springs and camshaft at least, but it would remain leaky enough to be "nice" to the bottom end. Or maybe install the old intake valves which look reasonably okay; since there's 8 of them and only 4 exhaust valves which would be new, would that be leaky enough to not bother the rings? This whole idea may be bad because I suspect the guides would need to stay with the valves they have worn to fit.
Another possibility is that I could get a HG shim to back off the compression a little, again to be "nice" to the rings. But I'm not sure if that would help with the drag on the oil ring as the piston moves down.
Anyway, please post your experiences in this area. Did a valve job finish off your rings, or did it renew things until it was time for a new car? What about customers or friends who did the same?
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Of course, I'd also like some comforting words, given that I've already bought an exchange head for my well-maintained but gasket-leaky 190k Isuzu motor. :-) Coolant leak in #4 pocked up the piston, hard carbon in #1 and #2 that I think came either from guides or the leaky valve cover - at least a dry-wet test didn't point to the rings if I understand the results correctly. I ran Pennzoil 5W-30 with never more than 3000 miles between changes and most often far less. The motor has never used any oil except at sustained high-RPM driving. (A leaking distributor seal and external HG leak may have contributed to this.) Recently, the internal coolant leak became significant enough to foul up my oil and give me occasional pops on the highway, probably where the piston #4 pock marks came from. So that got me on the stick to take care of it. Originally I planned to be done in two days. That's turned into four and still going... but I want to do it RIGHT.
Re-ringing is not an option at this point because of the time frame and my lack of experience or anyone to help me with it. (Just to get the oil pan off, front exhaust pipe and crossmember have to come out, then on the top, ridges reamed, cylinder honed or possibly pulled and re-bored due to uneven wear, new pistons and wrist pins, ring compressor needed, don't install rings upside down, correct breakin procedure, etc) I just need to figure out the right way to proceed. If I have to eat the exchange head to do the right thing, so be it.