Direct quote "My cutoff for repairing a sooby HG is 160,000 miles. Anything above that (some have done it with mixed results) may lead to blowby from the piston rings. The rings have been there for all those miles. As engines get older, they do loose a little compression (alot if mistreated). When replacing a HG the heads get machined (sometimes a little sometimes more). That raises compression and adds stress to the rings"

Mine is at 170,000 miles.....