Repairing Rear Subframe Studs... The Janky Way
08-16-18, 09:58 PM
#1
Repairing Rear Subframe Studs... The Janky Way
So, about a week ago I had to pull my convertible FC off the street since the differential was making a ton of ugly noises on deceleration. To get to it, I started dropping the rear subframe, and the driver's side stud snapped right at the base of the nut... well, crap! I've been thinking about the best way to go about repairing the stud, 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 in order to replace the stud in the way documented here on the forum. 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 *** 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). Fortunately, that was the case, as the new tap cut right through, doing the job in a fraction of the time! It was still a bit tedious, since I was tapping upwards into a blind hole, meaning all the chips settled onto the tap itself, which required me to unthread the tap and clean it every couple turns, but ultimately it worked! The hole was tapped 40-45mm deep, while the threaded part of the 80mm, M14 x 1.5 bolt is about 35mm. When it is fully tightened, the distance between the chassis and the top of the washer is about 3mm tighter than the height of the tube inside the subframe bushing, which should give adequate thread engagement while still allowing the bolt to tighten onto the bushing tube.
I have not put the subframe back on or gone for a test-drive yet, since I'm still waiting on a few parts to rebuild the diff. So, for now, let me know why you think this will or will not work!
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 *** 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). Fortunately, that was the case, as the new tap cut right through, doing the job in a fraction of the time! It was still a bit tedious, since I was tapping upwards into a blind hole, meaning all the chips settled onto the tap itself, which required me to unthread the tap and clean it every couple turns, but ultimately it worked! The hole was tapped 40-45mm deep, while the threaded part of the 80mm, M14 x 1.5 bolt is about 35mm. When it is fully tightened, the distance between the chassis and the top of the washer is about 3mm tighter than the height of the tube inside the subframe bushing, which should give adequate thread engagement while still allowing the bolt to tighten onto the bushing tube.
I have not put the subframe back on or gone for a test-drive yet, since I'm still waiting on a few parts to rebuild the diff. So, for now, let me know why you think this will or will not work!
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mr_vaughn (06-04-20)
08-18-18, 10:10 PM
#3
Oh man, triplets! I'm not sure I can handle that... a single child plus our dogs has me extended to my limits as it is! (of course, I know another child is coming in the next few years, so I better get to training those dogs better!)
Anyhow, in the end, after cleaning up the end of the stud, the rear subframe and differential seemed to just slide into place as if nothing was wrong, and the 80mm bolt threaded in with no issues. I had also gotten a 90mm bolt, just in case, but it bottomed out in the threading before tightening onto the subframe, which tells me that the 80mm is engaging just about all the threads in the stud that it can for maximum strength. It was easily at least 20 threads (I sort-of lost count around there), but that corresponds to at least 30mm of engaged threading which should be more than sufficient.
Now we'll see how it handles the driving loads placed on it...
Anyhow, in the end, after cleaning up the end of the stud, the rear subframe and differential seemed to just slide into place as if nothing was wrong, and the 80mm bolt threaded in with no issues. I had also gotten a 90mm bolt, just in case, but it bottomed out in the threading before tightening onto the subframe, which tells me that the 80mm is engaging just about all the threads in the stud that it can for maximum strength. It was easily at least 20 threads (I sort-of lost count around there), but that corresponds to at least 30mm of engaged threading which should be more than sufficient.
Now we'll see how it handles the driving loads placed on it...
08-19-18, 04:06 PM
#4
Rotary Enthusiast
Triplets!??? Lord have mercy! Better stock up on the brewskies while beer money is still available!
On another note, I recall a couple other similar treads were these bolts snapped upon removal. That would be a scary experience! Maybe folks should take precautions and liberally soak these bolts in penetrating oil before attempting removal.
On another note, I recall a couple other similar treads were these bolts snapped upon removal. That would be a scary experience! Maybe folks should take precautions and liberally soak these bolts in penetrating oil before attempting removal.
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