I'm in the process of rebuilding the engine in my 98 Blazer with a 4.3 and the same bolt stripped on me. I've tried forcing an allen wrench into it with out any success. Ironically, the other bolt came right out.
turn the head of the bolt by tapping on the outer part of the flat of the b lol olt head with a center punch. takes some skill not to f it up. Bolt has lock tight applied to the threads. Then replace with a new GM bolt and retainer if it was damaged. Don't drill it
Trick to get them out clean in the first place is to put tension on (not so much to strip the screw head) with the torx bit while shocking the screw (rapping on it) with a small flat punch and a hammer. They will pop right loose! Note this takes more than two hands, so round up the wife or kids or a buddy before attempting!!
Getting this out will take some finesse with a center punch. Probably you will ruin the plate so get two bolts and a plate on order with your local GM dealer. These bolts have lock-tite applied at the factory. Why you need to shock the screw by tapping to get it loose.
Trick to get a stripped one of these out is to first punch a mark straight near (but not right on) the edge of the flat recessed screw head. Then angle the center punch and tap in the punch mark you made to loosen the bolt. For this size screw, don't try to use too big of a center punch. Maybe one only 4-5 inches long at the most.
If you are taking one of these out and haven't stripped the torx head yet, make sure you have the correct torx bit and have someone rap on the bolt head with a small flat head punch and a small hammer, while you apply torque to turn it. They will come right out.
I was able to get the bolt out using one of these and a torx bit. I found a replacement bolt kit from Rock Auto. Finding a decent torx bit from a local parts store was a challenge. I busted one taking the last bolt out. Then I busted another one just as I got the last bolt torqued down. I put some 242 loctite on the bolts for some extra insurance.
last year a guy I know which runs a body shop made me a gift - an oooold toolbox. Found one of these impact drivers a bit stuck and slightly rusty. Disassembled cleaned greased and reassembled. Works like a charm again.
Looks exactly the same - Some Milwaukee company. Sturdy 1960's tool