This is going to be a generic Faulty Parts Report for substandard threaded fasteners. There have been all too frequent and repeated reports of weak threaded studs for manifold bolts and head studs. I recently pulled threads off an engine mount stud with a small wrench. There are so many reports that I don't have the time to post them all.

Apparently a lot of current "cheap" manufacturing sources for threaded fasteners are delivering substandard parts, and the parts distributors have insufficient Quality Assurance programs and often do not catch the junk before it is sold. This leaves it up to the individual customers to bear the brunt of QA testing after receiving new parts (and possibly having no spares).

I have come to the reality that I cannot trust the integrity of fasteners from any of the common British car parts suppliers, so I have allowed time in my life to torque test at least one of every batch of fasteners that I receive before using them or putting them in inventory. The test is easy and quick, and there is no reason the vendors shouldn't do it before shipping parts to the customers, but apparently they chose not to bother.

To test any bolt, install a spacer or some flat washers on the bolt along with a good quality nut. Clamp the bolt head in a vice and apply the prescribed assembly torque to the nut with a properly calibrated torque wrench. For double threaded studs put nuts on both ends (with matching threads), put a spacer or washers in between, and apply torque in the same manner. If any fastener breaks or strips the threads, send the whole batch back immediately with a firm note to the supplier requesting that they test every production batch of fasteners before shipping any to the customers.

For non-critical applications it may be sufficient to test just one fastener from each batch you receive, and hope the rest will be the same. For critical applications such as cylinder head studs, flywheel and clutch bolts, it may be a good idea to torque test every one of them before installation. One single head bolt breaking during the final round of tightening can really spoil your day when you have to remove the head to extract the broken stud, worry about whether you can use the same head gasket after this, and possible having to order more parts when you don't have a spare stud. A lot of people have been discovering this the hard way. Let's hope you can circumvent some of the pain.