Thanks.
I also ordered some Grade 12.1 head bolts from Boltdepot.com
They are only 90mm rather than the stock 95mm but we'll see what happens.
When I was measuring the thread pitch on the old stretch bolts I was shocked by how much they had stretched.
I see no reason for upgrading the headbolts on the M42 if natually aspirated. Actually the S42(1995) engine used stock headbolts, as well as main bearing bolts. I would be more concerned about using extra high quality rod bolts.
Also i'm not an engineer, but your remarks about the stretch on the bolts makes me think that they have been over tightened. And not of too low specification for the application.
modern stretch bolts should be considered like a spring, a very stiff one too. When tightening the bolt you typically tighten it to abut 80% of it's yeild strength. Thereby stretching the bolt / tensioning the spring. If you tighten it above the yeild strength of the bolt. You will see permanent stretch (not good) as the clamping force has then been compromised. And the metal structure distorted, resulting in a weakened bolt thats ready to be scraped.
Very high quality fasteners are often precission measured and logged before assembly. And during rebuilds checked against the log, any sign of permanent stretch means the bolt has to be replaced.
