I agree that there may well have been a fault in the casting, but in retrospect and to be helpfull in the future it has been suggested that a torque wrench not to be used on the intake bolts. Tighten by hand, let set and retighten by hand, warm up the engine and retighten again (by hand).
If using alluminum heads let cool for a half an hour and re-tighten. I only mention this as I have buggered intake bolts, epecially easy to do on the rear corner bolts by not having a good feel on them.
Chris K
(the other one
