Big block Chevy's, 351 and 460 Ford's have been plagued with manifolds that were super prone to cracking for years.
Then it moved to the " mighty" (yeah Right) LS motors, and Ford 4.6s/5.4s ... I've dealt with more of those last mentioned engines than I can shake a stick at for the problem... The 4.6s and 5.4s really suck the worst to repair, most other engines I deal with have the exhaust manifold above the frame not directly in line with it. I've done a couple on newer Ford 6.2 gas engines, and on that application compared to the 4.6/5.4, on the 6.2s Ford finally upgraded the stud diameter from 8mm to 10 mm. Much better success getting them apart with no stud breakage, and I haven't seen one 6.2 yet that "pops*/snaps off
the studs in service just from heat/cold cycles like the 5.4 is so common for ...
On the older 318s/360s I've pulled my share of manifolds and I see more busted bolts in intakes than exhaust on these engines. I've never had to drill a single exhaust manifold bolt on a Mopar small block, whether LA or magnum.
I'm hoping my luck with Mopar small blocks continues when I tackle the 4.7 in my Durango this spring. (It ain't gonna happen until then)
If you can find a good used manifold, take it and the one that isn't broken to a competent machine shop and have the flanges milled flat.
The manifolds are green when new and they will try to twist and contort after a few bazillion heat cycles. Because they can't move they crack, break studs or both. A seasoned casting will have done all the contorting it's likely ever to do and a trip through the Bridgeport will square everything up and insure its continued longterm good health.
Kevin
