On my 84 berlinetta I noticed i only have 2 threaded inserts on each side for the transmission crossmember. But there's a 3rd insert but no threads. And the umi transmission crossmember has 3 bolt holes on each side. Do I only use 2 bolts? Do I tap threads in the third insert?

 

It's up to you whether you use the extra holes.

It's not difficult to just run a tap through them, and it's added structure.

 

That's what i was wondering. If it was those holes that were used or if it was the hollow ones in rear. I don't mind tapping them.

 

The "hollow ones in the rear" are gauge holes for the dimensions of the "frame".

Yes you can tap the 3rd hole on each side for the crossmember. That was used for the crossmember in 82 & some 83 cars for the 200C and 4-speed transmissions, which are shorter than the 700 or the T-5. The rear-most hole on each side was added in 83 for those. The T-56 crossmember, being even longer, puts considerable TORQUE on the bolts, if you use only 2; it will almost certainly crack the "frame" around them if you use only 2. It's bad enough even if you use all 3.

Be aware that those aren't real "nuts" exactly; they're chinesium CRAP, captured in a little cage. Designed for one-time use ONLY and a frequent failure point later in the car's life. Be REAL CAREFUL with them. They strip out REAL EEEEEZZZZY, and the cage is real fragile too.

 

Ideally, tapping the two and using all 6 is ideal. I actually removed all the "nuts" in the frame. Then made to 2- 2(ish) inch wide pieces/strips of 1/8 steel w three holes the same as the frame/x-member. I fully welded gr8 nuts to the metal strips. I welded a wire to the strips and fished them in the oval access hole in the frame. Then, once in place I drilled 2 small holes in the frame and welded the strips to the stock frame. This gave me 6 real nuts actually welded to thicker metal to distribute the load of the x member. Now, the metal around the plate is only so strong and will it crack w 5-600hp? We'll see.

 

That's some god info I was looking for. You guys happen to know thread size for the tap? That's what led to me noticing was inspecting the threads.

 

I didn't use a stock bolt size when I did it all. I used a slightly larger size.

 

Stock bolt size is 10mm std (not "fine") thread; 1.5mm I think butt don't hold me to that.

I like Fire's reinforcement idea. That whole part of the car is kinda weeeeek. Another tactic that would work, is to cut the floor above that, put in some kind of backup plate in there from above, and patch it back.

Although, the problem isn't "horsepower", or really anything else to do with the drive train; it's the weight of the transmission trying to make the crossmember flop up and down, and the torque that those itty bitty plate parts of it that bolt to the "frame" exert on the "frame", and the very high leverage that the extended crossmember has on it. Remember, the original crossmember is centered on the bolts, meaning, there's NO torque whatsoever on it; all the forces that it exerts on the bolt holes, are straight up and down. Not so when the trans mount is 8" or whatever it is, behind the bolts. The force on the rearmost bolt especially, trying to pull it out of the "frame" when the car bottoms out, is probably 4 - 5 times what the dead weight of the transmission would put on the "frame" around that bolt hole, compared to a stock crossmember. No matter how much reinforcement you can put there, it's still trying to flex and twist that section of the "frame", rather than just pulling and pushing on it.

 

On the later cars, it's a plate with punch formed holes welded in place in the rail, and thread forming bolts are run up in them.
they strip out easily

 

ouch, that sucks.

and yeah, the t56 x member hangs out really far.

 

that is a good idea goin through the floor.
But also I'm looking at makeing less the 400 hp.and be doing more auto cross , road course raceing and not so much hard launches. But it's definitely something to think about.

 

As said, the problem isn't horsepower, or launches; it's the fact that the torque arm's trans mount location forces it to put force on the crossmember in a direction the "frame" was never intended to endure. Bumps, especially bottoming out, is the worst-case scenario. Like those places where you have to cross a wide high-crown road with rain gutters on both sides, at speed.

 

I think either way would work well but they both have their limitations. Sofas point on the rest of the material around the holes is very important. And if the torque arm is on the x member (mine is) it's that much more up and down force.

The material that makes up the frame rails is pure crap. My Alston SFCs are mounted to the same area and the x member had to be clearanced for them. We added a little metal to the area in the process to further buttress the x member mounting area. I think that an auto x car w less than 400hp should be fine but at the same time how many flex cycles will 40 year old metal handle? It's a tough call w where to start and stop w support.

 

I did similar to what firechicken described. Made backing plates with nuts welded. Used every available hole.