The last 20 years all the American manufacturers have had broken exhaust manifold bolts. My Dakota 3.9 Magnum V6 has a few I need to deal with.
When they did their design updates from the LA, SBC and SBF they all came up with the astoundingly brilliant idea to save material on bolt construction. Now we do have to understand that an ounce of steel.saved per exhaust bolt spread over the better part of a million engines per year keeps the bean counters happy. Well the bean counters might be happy, but I would like to stuff a handful of broken bolt remanents up the place the sun don't shine. The bolts that break are 8mm, basically 5/16". Metric sizing is not the issue. When virtually every engine had 3/8" exhaust manifold bolts we very rarely had broken bolts in operation. I have broken a few off trying to remove rusted in bolts though.
If rebuilding an engine I might be tempted to drill and tap for 3/8" and then drill the manifolds out to fit.
