[john walker's workshop]

the ends of the broken studs are black at the break, indicating a penetrating corrosion. the two places they break, are where they are exposed to the atmosphere at the head to cylinder joint, and between the cylinder base and the engine case. these areas generally have a rust ring around the stud. i've snapped some that were not broken yet, in the vise, and noticed the brownish/black penetration in the fresh break. the replacement black coated studs had a breaking problem about 10 years ago, which was deemed to be a bad batch. you're probably safe enough using new ones these days, and if they did break, it would probably be in 20 years or more. i still don't really trust them though, after all the problems.