Anyone ever experienced this before?
Here's the scoop.
Vortec heads modified for screw in studs, guide plates and 1.6 rockers.
Comp guide plates, Pro Magnum 1.6 roller rockers and pushrods.
GM studs.
Thousands of miles of trouble free performance before this.
The stud broke off just below the poly lock. RPM at the time was about 2000 while cruising on the interstate on my way to Virginia.
In all my years of hot roddong, I've never had this happen before.

First for everything. Pull it out and replace it.

This happens to studs sometimes, especially 3/8's studs. You can replace the stud with a new one and be on your way or you can upgrade to 7/16's studs for a piece of mind. You will need 7/16's rockers too.

A possibility could be that although youve had guideplates this hole time, maybe the guideplate wasnt 100% aligned properly and it just took a long time to fatigue the stud? You should see signs of wear on the pushrod if this happened.

very common with pro magnums, you need the longer 3/8 studs to fix that or it will happen again.
if you notice where your allen key lock is in the rocker nut, its down too far, very little nut on the threads.

Right on the money not enough thread engagement in the nut . Like John I've seen it a few times [surprised that it doesn't happen more often ]

Would you say then that the allen key lock should be flush or slightly above the top of the polylock? With the present set up, the allen screw is about 3 (+/-) threads below the top of the nut. If I make the move to the longer studs which are .145" longer (at least in the ARP catalouge they are), it would push the allen key up another 3 1/2 threads (24 threads/inch).
I agree that the 3 1/2 threads would be better served with more stud engagement in the polylock however should that be at the cost of threads with the lock screw?
I agree with the move to a 7/16ths stud would solve any issue I might ever have with my current set up. As of now, I've picked up a set of the ARP 1.75" long 3/8's studs and will go over whole installation before I head out on a 8000 km road trip at the end of the week.
I just find the whole thing a little odd in that I've been running this combination for thousands of miles and then this happens. I suppose everything has a cycle life.

it doesn't take much to lock them. the trunions on the pro magnums are big, and i have changed studs on 3 sets of heads that had the same prob as you and happened like yours, went a long time and then when not beating on it they broke the tips off the studs. also check your nuts and see if the threads inside it go all the way down or if there its countersunk before the thread starts inside, some i changed the nuts too.

For what it's worth, the original screw in studs (manufacturer unknown) put the allen screw about 2 threads (.080") below the top of adjusting nut. The ARP 7101 studs (which I bought for the on the road repair) are 1.750" long which ended up being about 1/10" shorter than the originals. That put the lock screw more than 4 threads below the adjuster and reduced the thread engagement in the nut by that amount. Thinking was that I would pushing my luck with the new studs so I opted for the ARP 7104 which are 1.875" long. After adjustment, the lock screw is flush with the nut and there's more thread engagement than before.
With the longer and stronger ARP hardware I should be good to go.

were your nuts threaded to the bottom or not?
checked out a new set the other day and the nuts on those were all the way down, all the ones i saw break studs were not.

Full threads to the bottom.
I couldn't get more thread engagement if I tried.
Considering the original set lasted tens of thousands of kms with considerable abuse (6 grand every day I've driven it) I suspect the new set can exceed that.
Of course, I could have taken the advice of many and upgraded to a 7/16ths stud from the start but I'm a little blockheaded that way. I never thought I'd bust anything after the initial performance build.