Nutsert strength for transmission cross member

[bradpac]

I have a 72 Dodge Dart that I'm rebuilding with an overdrive transmission. In this process we put .120 wall 2x3 tubing subframe connectors in it. The overdrive transmission mount is a little further back than the original. I don't have any experience with nutserts, threaded inserts, whatever you want to call them. Are they strong enough to hold a transmission cross member in place? My original idea was to take a piece of 3/16" angle, weld nuts onto it, drill some holes in the sub frame connectors to recess the nut and weld the angle to the connector. I realize this is a lot stronger than drilling holes in the thinner material and using inserts, but if that is strong enough it would be a lot quicker, cleaner, and easier.

 

[astroracer]

If the nutserts are in tension (on the bottom of the frame rail) I wouldn't trust them. They will last for a long time but the issue is vibration and the hanging load pulling down on them. If they were in shear (in the side of the frame rail) I would trust them to work very well. if you can put them in the side of the rail do that.
The other thing to consider is, you still need to drill a good sized hole into the rail for the nutserts. At this point you are almost to the clearance hole size for your nut plate...
Mark

 

[Stooge]

If im understanding the new set up correctly, would weld nuts be a better option for this? im set up for smaller nutserts, but assuming the transmission crossmember would use bolts in the 3/8" range, the tooling for nutserts that size could be pretty spendy, especially compared to the few dollars for a package of weld nuts. https://www.mcmaster.com/weld-nuts/

 

[MoonRise]

0.120 wall steel tube?

Put some 3/8-16 riv-nuts in there. Steel, not aluminum.

Put some ears/flanges on your cross member, mount that to the sides or bottom or top of your 0.120 wall subframe tubes with four 3/8-16 bolts and it should not be going anywhere.

The installation hole for some 3/8-16 rivnuts should be about 13mm (17/32") per Astro Tools.

https://www.astrotools.com/100pc-3-8-16-steel-rivet-nuts.html

 

[bradpac]

If im understanding the new set up correctly, would weld nuts be a better option for this? im set up for smaller nutserts, but assuming the transmission crossmember would use bolts in the 3/8" range, the tooling for nutserts that size could be pretty spendy, especially compared to the few dollars for a package of weld nuts. https://www.mcmaster.com/weld-nuts/

Weld nuts would certainly be a cheaper option. I've been wanting a nutsert/rivnut tool for a while so cost of that isn't a big concern though. This would just be another excuse to get one.

 

[bradpac]

If they were in shear (in the side of the frame rail) I would trust them to work very well. if you can put them in the side of the rail do that.
Mark

The plan was to have them on the side of the tube. 3/8" bolts, two per side. Basically a piece of square tubing with plates on the end for the cross member, with the bolts front and back. Simple.

 

[Graham08]

If it was mine, I would either use your original solution with the angle, which would be super strong, or weld threaded bungs or nuts into the wall of the subframe connector tube. I would have a hard time trusting rivnuts in that application.

 

[zmotorsports]

While the nutserts are great, I personally wouldn't trust them in that application.

I would weld a threaded bung into the tubing, use a tab and then a captured nut inside of the frame or something along those lines. That would be much stronger and more reliable in my opinion. For cleanliness, I would use a threaded bung drilled and welded into the tubing, as long as the weld holding the bung in is solid.

 

[MoonRise]

Random web pics (not mine ) of a transmission cross member bolted with flanges/ears to the top of the frame rails.

Put a rivnut in that type of situation, and I really don't think it is going anywhere.

http://www.v8beetleresource.com/chassis17trans3.html

(of course, welding things in can work too. Use weld nuts, cross-tube reinforcement welded in place through the subframe and then through bolted, weld the cross member to the subframe but then changing things involves cutting the cross member off , multiple ways to skin this cat. )

 

[bradpac]

Random web pics (not mine ) of a transmission cross member bolted with flanges/ears to the top of the frame rails.

I agree, having a flange on top of the rails where the bolts are merely holding it in from shifting would be the best way, but these connectors on a unibody car are welded to the floor and actually go through it in the back seat area, so that isn't an option.

 

[bradpac]

The consensus seems to be that rivnuts would work, but safety factor says go with something burned in. I think in the interest of making be feel the best I'm going to weld bungs or nuts in the tubing.

Thanks everyone for the replies, once I get going on it again I'll get some pictures posted up.

 

[theoldwizard1]

Third, threaded bung.

 

[MoonRise]

Though holes going horizontally through the subframe tubes, weld nut on the outboard face of the subframe tubes, cross member with flanges/ears inboard of the subframe, bolts go through the cross member flanges/ears then through the subframe tubes and finally thread into the welded nuts on the outboard faces of the subframe tubes.

Reinforcing tubes welded to/through the holes in the subframe rails/tubes to transfer the bolt tensile loading from the nut to the reinforcing tube to the cross member flanges/ears and then to the bolt head would be good. Instead of transferring the bolt tensile load to just the sides of the subframe rails/tubes.

Multiple ways to get there.

 

[mikepelchy]

On the 97-06 Jeep Wranglers there were 6 1/2" nutserts installed on the bottom of the frame rail that held up the transmission cross member. I had a couple of bolts get rusted to the nutserts from mud getting into the frame while wheel so I ended up having to replace them. The original ones were 9 years old when I replaced them.

 

[joe49]

Drill thru holes 5/8'' slip in 2'' pieces of 5/8'' by 1/8'' wall tube and weld in, then use thru bolts with lock nuts.

 

[DTE]

A couple thoughts for me are you still using the stock front suspension ? if so I would want to try to keep the stock crossmember in place to keep the torsion bar mounts tied together.

 

[bradpac]

A couple thoughts for me are you still using the stock front suspension ? if so I would want to try to keep the stock cross member in place to keep the torsion bar mounts tied together.

Yes, stock torsion bars. That cross member will remain tied together in some capacity for strength. To clear the overdrive transmission the upper part had to be cut away. I will probably fab a removable piece of channel on the bottom side that replaces the old trans mount piece.
I have considered just extending a piece of plate off of that to the new trans mount further back, but having another tube at the right spot would add even more strength.

 

[matt_i]

My preference when strength is an issue is to drill & tap solid round stock for the depth needed. Then drill completely thru the tube and tig-weld to both walls.

On the tube ends I'd drill & tap heavy metal blocks and tig weld both the face and plug weld a little further back. Heavy metal is generic for something more than 1/4" thick. 2 diameters of thread engagement is ideal although down to 1 will work in ferrous metals.