I just replaced my old valves and had the head saved by less than 10 thousands . It no longer leaks around the head but the first spark plug is leaking at the treads. The engine starts fine but the rpm will not develope. As a note, this hole never did take a plug as deep as the rest of them. What do I try next?
Wrap the threads with Teflon tape or aluminum foil. many T's have a little blow by at the plugs. Unless you have MAJOR blow by, the upper end RPM should not really be affected by a little leakage. Did you adjust the valves?
What lifters do you have, solid or adjustable? If solid, how worn are they? If the cam is old and worn adjusting the valve timing by piston travel might be your best bet.
Did you do a vacuum leak test on the intake if you had it off, did you install new copper crush rings?
Mark, I just put in a new plug since writing and it went all the way down, no longer leaking. I have solid lifters and they had a little wear but used them. The cam? I timed by piston travel. I should have vacuum test it but got a little ahead of myself. And I did reuse my copper crush rings and I now exhuast it leaking. Dollar saved makes for more work. LOL
If all else fails and it still leaks a little, uses a small amount of silicone around the threads, the exhaust people do it all the time with great results..... do know why it works, but it does.