I got a little done this morning before my day took off in another direction. ... My rocker assembly is not adjustable except by shims to raise the rockers. If the rockers are too high, the answer is longer push rods. It turned out that all of the push rods on cylinders 5-8 were loose enough to spin freely. I loosened and then retightened them to get at least a quarter turn after the push rods stopped turning freely. Six of eight push rods seemed right, but two of them are still not sufficiently preloaded. It looks like I need to get a new set of slightly longer push rods.
In my haste to do some testing, I did something stupid. I put a spoonful of
oil in cylinder 8 and repeated the compression test. The good news is that the compression jumped right up to the 210 PSI level of the other cylinders. The bad news is that I don't know if it is because I added
oil or because I properly preloaded the lifters. If I had done the compression test before adding the
oil, I'd know. .... So, tomorrow I plan to button up the engine and run it for thirty seconds or so to burn out any residual oil in cylinder 8. Then I'll repeat the compression test to see whether the oil or the lifter preload made the difference. ... If I'm lucky, I'll discover that the cause of my noise was insufficient valve opening due to normal wear and push rods that were just barely long enough when everything was brand new.