I got some time to work on Savannah again this weekend, and the first thing to tackle was the broken subframe stud. I've been thinking about the best way to go about this, since the convertible has considerable structure built up above it that would need to be cut away, and then re-welded to preserve the additional convertible structure. If possible, I'd like to salvage the current stud, especially since as far as I can tell, the threaded end and portion that broke only serves to clamp the subframe to the unibody - the base of the stud has a bump-out that locates the inner bushing within the subframe mount so there will hopefully be lower loads on the bolt itself.
Anyhow, since the original threads were M14 x 1.5, I wanted to use the same size in a high 10.9 grade to try to keep comparable strength and a relatively wide cross-section in case there were bending loads too. This requires cutting down the stud a bit to get enough of a cross-section for threading. I wanted about a 2mm thick wall all the way around at a minimum, which meant cutting it at the point where the taper brought it out to 18mm.
There was about 30mm of the stud remaining (originally it had been about 75mm of un-threaded taper, with another 25mm or so of threading on the end), with an end diameter of just under 18mm. Drilling the stud precisely down the middle was very important so that the threading doesn't poke through the side, to keep a constant cross-section in the stud, and to allow threading the bolt once the subframe tube is around the remains of the stud. Since all of this was still on the car with the rear up on jack stands, I needed to get creative in making a setup that would keep the drill aligned properly, while still allowing a number of different drill sizes to work up to the final 1/2" bit (12.7mm, while M14 x 1.5 wants a 12.5mm initial hole). So... here's a pretty janky solution that worked surprisingly well...
First, I cut an "X" in the surface of the stud, with the center of the X being the center of the stud. This was to keep the tip of the bit located at the proper spot. In the above image, I had already done a little initial drilling with a smaller bit, and had come back with a larger bit which unfortunately wandered a bit off-center - this had to be cleaned up with a dremel grinding tool to make it sit concentrically again.
However, locating the tip is only part of the problem... to control the angle of the drill, you need to fix the other end of the drill bit in space too, which brings us to the jankier bit...
Yes, a 17mm 12-point deep socket, pounded over the end of the stud, and a series of spacers to locate the end of the drill bit within the driver square at the far end of it. Each bit required its own spacer or spacers depending on the bit length and diameter, so I ended up with a small pile of little tubes made out of lighting rod, aluminum tube, brake line etc. So, now all I need to do is put the right bit in the drill, slide the spacer over it to fit in the drive square, spray it all down with WD40 as cutting fluid and to lubricate this spacer, slide it up into the socket and feel around for where the tip falls into the hole in the center of the stud, and then keep pressing this heavy ass drill into the underside of the car for 15 - 30 seconds at a time, then remove it, clean out the chips, spray it again, and reinsert until I get to the right depth. Also, to get up to a 1/2" drill bit going deep enough into the stud, I needed to do this with I think 6 different sized bits, periodically sharpening the bits as necessary. My arms and upper body were in no shape to be doing all this lifting and pressing, so now its time for the jankiest part of the whole thing (just when you thought it wouldn't get any worse...)
Yes, a jack to apply pressure to the back of the drill to do the cutting. Through all of this, trying to keep the stud cool was important, so I was spraying that with WD-40 too to try to prevent it from work-hardening... This worked pretty well up until the 1/2" bit, which got hot extremely quickly and actually burst one of the two bits in that size I had. The other one I was eventually able to make work by drilling out the center of that 17mm deep socket to make it large enough to put the remaining bit through, by frequently re-sharpening it, and through a lot of persistence. In the end, I had a properly sized hole about 40mm deep to the shoulder of the angle at the tip of the bit, looking pretty close to centered & concentric.
The bad news is that all of this took around 4 hours, and that trying to tap it is an absolute bear - it took about 40 minutes to even get the tap started, and another 2 hours to get 3 full threads cut into it since I suspect that it did work-harden and kept grabbing the tap, forcing me to go very slowly. I ordered a harder cobalt-steel tap and a bottoming tap to try again, hoping that the quality of my harbor-freight-grade current tap is just not cutting it (heheh), but we'll see when they arrive.
In other news, I finally got around to painting the diff teeth with marking compound and checking the engagement. By the looks of it, the coast-side pattern is indicating that the pinion is too far away from the ring gear (too shallow), which would be consistent with bearing wear and the looseness of the pinion nut. I think I'm resigned to ordering a few shims and a new crush collar from Mazda to try to get it set up properly since I don't want to have to go through all of this again.
The drive side, shown below, is also off and really biased towards the toe, although this may be a consequence of adjusting the ring offset to get the backlash correct. Either way, we'll see when I get the new shims (need to measure what the current one is first)
