So, I realize this has been covered before and I've read a few threads of recommendations - most of them don't make any sense to me and it sounds like they are dealing with subframe bolts as opposed to the radius arm bolts...here's my experience so far and I am hoping for some advice:
I watched 2 different videos on how to remove the rear radius arms. Both started with older Minis than mine and more corrosion form the UK (mine is '95 that has been garaged most of the last 20 yrs) and neither had much issue with the bolt heads breaking off so I approached this job cautiously but not as cautiously as I should have! I soaked with PB-Blaster and Aerokroil for a couple of hours before I started trying to remove the bolts. I also used a torch to apply heat on the second side. Of the 8, I had FIVE bolt heads twist off. In retrospect, I should have hammered on the bolt heads a lot more to knock them loose, used a lot more heat (but I was worried about the rubber at the rear subframe mount) and gone slower but I don't know if it would have changed anything.
So, there I am with 5 broken bolts in the subframe mountaing holes and none of them were the ones with exposed captured nuts....awesome. So, I read forums and learned that most people strongly recommend against try to use extractors in this case since the back of the bolt is rusted and you are likely just going to end with having to drill out a hardened broken extractor. So I ordered a 5/16-24 helicoil kit and proceeded with center punching the bolts and drilling them out. This has been a nightmare!!!
I've broken off more drill bits in the last 24 hours than in my last 40 years! I'm still not quite sure why - either it is when the edge of the hole reaches a different metal type (I'm hitting the nut instead of the bolt?) or a piece of a drill has gotten stuck back inside the space behind the subframe and is grabbing the drill bits? But I've managed to extract enough of the drill bits (or push them through) to get the first hole tapped, the second one is drilled to the right size for tapping and I'm working on the third.
1). Should I just be dropping the subframe at this point? I watched some videos of that process and determined that it is quite a bit of additional work and I'm likely going to end up with broken subframe bolts so I was thinking I should avoid it if I can (and it seems feasible).
2) Should I helicoil or just drill and retap the existing hole. I'm leaning towards re-tapping because I don't know if there's enough material or depth there for a helicoil and I think I'm going to break more drill bits trying to get the holes big enough for helicoil?
Here's what I've learned and I'll add to this as I move further along:
1) Go really REALLY REALLY slow with everything. The metal seems to want to gum and grab every step of the way. I've used a ton of lubricant and learned to keep my drill speed so slow that I can stop before it grabs and breaks the bit (most of the time). This means incredible patience (get a good book on audio).
2). Go very incrementally slow with the tap - like a quarter turn or less and then clean the tap. Repeat. I broke the end of my tap off on the first turn - I've never seen anything like this. I think it must be the effect of the hardened nut material????
3). Although I saw more than 1 person saying use a helicoil, I'm not convinced that is the right call in this case. Since these appear to be captured nuts, they may not have enough material for a helicoil. I may be wrong here and will change this if someone has information that disagrees?