It's to do with dissimilar materials. Stretch bolts accomodate the block and head heating up and cooling down at different rates.

Canada1, spending more doesn't always mean better. I fitted an MHG and ARP's, was the first and only head gasket I have ever blown. I went back to stock gasket and bolts and never had a problem thereafter.

The problem that I had and I know Karl's lot have had the same, is that the head lifts at much over 350bhp. IMO, that is more down to it being the physical limitation of the SIZE of the bolts and going to ARP's won't help.

Karl, if OEM bolts are good enough for me/Tim/Oli and probably (I would guess) Stu Collins, then why aren't they good enough for you??!!