I have a friend with '66 289 that has broken 2 rocker arm studs (on different cylinders). This is a stock rebuild with a very mild hydraulic cam, Edelbrock intake and carb, & headers. He has only put a couple hundred miles on it, and it has not been driven hard. The first stud broke while cruising at 35 mph. After that, he had all the studs replaced with screw-in studs. The second broke this week at idle. All valve train components, except the cam, are stock. The engine runs very quiet and there is no sign of interference wear at the ends of the rocker arm slots. Has anyone else heard of or experienced this? My guess is some type of valve train geometry issue, but what?

sounds like too much lift for the factory rocker arms, may need to go to roller rockers, when the push rods lift the rocker, it can only pivot until the springs compress no further, it may be exerting more lift till the stud stretches and eventually breaks under fatigue, you may also end up breaking valve springs, I'd quit driving this till you can tear the top end down, measure travel, clearences, and reassemble correctly, or who knows, may just be inferrior parts,or incorrect part application, don't overlook the obvious sometimes.

i broke one stud running a crane cam .488/.512 lift 218/224 found it to be the rocker slot was hitting the stud (i used a dremal and stones to open up the slots a little) see if you got little nicks on the ones not broke iv had people tell me this cam is mild and outhers say it huge. what do you guys think?
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66 289 mild port work edelbrook 600 cfm proformer intake msd/duraspark long tube headers roller tip rockers crane cam flowmasters 3.55 gears b&m transpack

i broke one stud running a crane cam .488/.512 lift 218/224 found it to be the rocker slot was hitting the stud (i used a dremal and stones to open up the slots a little) see if you got little nicks on the ones not broke iv had people tell me this cam is mild and outhers say it huge. what do you guys think?
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66 289 mild port work edelbrook 600 cfm proformer intake msd/duraspark long tube headers roller tip rockers crane cam flowmasters 3.55 gears b&m transpack

I broke one as well...it was due to the adjusting nut not having enough threads to bite down on...should have at least 3/8" down on the stud.

Just a thought.
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Red 1967 Ford Fairlane convertible with built 289, 4.11s and T5!

Thanks for all the replys. He found out today that "Joe Machine Shop Guy" installed the wrong (too short) valves.

hmmmmm, incorrect part application....where have I seen that....lol