I figured i would post this since it might be useful for someone with the same or similar problem as i had. My car mysteriously had 3 different size bolts that held the stock transmission cross member in place which seemed fine but when i got around to installing my UMI cross member i ran into some problems with the UMI holes being to small to accommodate the larger bolts someone had put in. Im pretty sure one of the owners before me forced some different bolts in place after they lost them or some damn thing. At any rate I know a few people that have dealt with the same or similar thing, either the treads where completely stripped, someone drilled and trap to larger bolts. iv heard of one guy who at the time didn't know that someone welded in the cross member to the frame (probably couldn't get the bolts in after striping all the threads).

I drilled out pilot holes from the bottom using a 3/16th 12" drill bit that i cut down to 8" and then used a 32mm hole saw from the top.









There is a ridge that needs to be cut down so the nut can sit flat. I used a 1" wood spade bit it was much cheaper then buying a 3/4 step down drill bit. The spade will work for cutting 2 ridge at a time then it will need to be file sharpened again the plus side is it cleans the surrounding metal to be welded.

 

I got some M10 x 1.5 (10.9 grade) hex nuts and M10 x 1.5 x 40mm (10.9 grade) bolts. I bolted them in then welded the nuts in place.

 

Now back to factory m10x1.5 and appearance .

 

this would make a good "how to" thread since this is a pretty common problem.

 

Yea i figured it would because this does seem like a very common problem

 

Wow, I'm not sure I would have gone to all the work (the part you didn't show, pulling the interior to get to it), why not just find the biggest one and tap them all to that size and then if you have to just open up the holes in the cross-member to accommodate?

I don't know what you mean by "step down bit" but if you mean step bits, well, you can get a set of 3 from HF for $14, and I've gotten them on sale for $9 ($3 per is cheap). If they're not long enough to get in there, well just use a spade bit extension or most now have a hex shaft in it that will fit in a magnetic screwdriver bit holder.

Otherwise just use a 1/2" or so normal twist bit, it will cut out that lip no problem and you won't abuse/break the spade bit (which is dangerous also). A carbide burr or chamfering bit will also work well. The point is that there are plenty of cheap bits that could be used instead that people probably already have, and if they don't they're cheap and you'll have them afterwards for the next project and won't destroy them or possibly hurt yourself like you will the spade bit.

Oh, and you probably want to use as little weld as possible, since the heat of welding will soften the hardened nut making it easy to strip. You probably also want to add fender washers under your bolt heads, since those small washers will eventually distort the holes and pull through.

 

The interiors been out of my car for years so no worries there. I'm sure one could do this by moving the seats as far back as possible pulling the center console and rolling the carpet back, but its not all that hard to pull the interior either and i feel it would be safer. As far as drilling and taping gos you can pretty much drill and tap as large as you wanted but you'll be left with 1/4th to 1/8th of available threads for the bolts depending on what size you drill the factory weld nuts since the ridge adds probably 1/8" more of threads. I personally wouldn't bother trying to modify the cross member slots on the 3/8" thick UMI peace. i mean one could if they decided to drill and tap to lager bolts but you would almost need a mill vise and drill press once that larger bit touches down on the already slotted hole its going to shake that thing like a unbalanced washing machine not only that but you would have to drill an estimated 3 times per slot there are 6 slots total then file. im not sure if the slotted holes are big enough to fit a carbide burr to rotter them out more either. As i recall the stock one already has holes lager enough to accommodate a 12mm bolt.

What i meant by step down is 3/4" drill Bit with 1/2" shank. Im just used to calling them that. It actually was easier to use the spade since it had a 3/8th shrink and i can move it around better since the holes that where cut with the hole saw weren't relatively center to the the weld nuts/bolt holes. the spade bit was something like $2.89. it only took about 30sec for the spade to clean the ridge flush to the surface of the weld nut.

I jumped from nut to nut when i was welding to lessen the chance of distortion and as you mentioned softening the harden nut. The 10mm washers where temporary i will be using the supplied lock washers that came with the UMI peace. Otherwise you would be right.

 

I know its an old thread, but do you have pics of the floor board? I have to do this as my factory tabs broke loose so Im thinking your method is the best for repair.

 

I don't have any off hand, but drilling a pilot hole is probably going to be the best method for locating.

 

For those looking for another simple option for this fix using knurled inserts (sometimes known as nutserts) that will work to add another bolt hole, or repair stripped ones. See post here:

https://www.thirdgen.org/forums/susp...ml#post6108373