Fabbing advice for tunnel hoop for 518 install

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mbaird

mbaird

mbaird
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I am installing a 46rh in my Barracuda so I had to cut out the portion of the tunnel portion of the torsion bar crossmember and wanted some input on my idea for replacing it. It has to be slim for clearance yet structurally sound.
I have shaped a piece of 3” x 3/16 flat bar that I will weld to the ends of the cross members but want the reinforce it. I was thinking I could shape and weld another piece of 2.5” 1/4” flat stock on top of it . My theory is tat this will give it more torsional rigidity . And since it will be welded to the tunnel it will be strong enough to.

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It's gonna need to be shaped similarly to what was cut out. In other words, it will need the 90 degree angles following the curvature on both the front and rear sides, like the original, or it will flex. .......and it will flex.
 
mbaird said:
I am installing a 46rh in my Barracuda so I had to cut out the portion of the tunnel portion of the torsion bar crossmember and wanted some input on my idea for replacing it. It has to be slim for clearance yet structurally sound.
I have shaped a piece of 3” x 3/16 flat bar that I will weld to the ends of the cross members but want the reinforce it. I was thinking I could shape and weld another piece of 2.5” 1/4” flat stock on top of it . My theory is tat this will give it more torsional rigidity . And since it will be welded to the tunnel it will be strong enough to.

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Welded in subframe connectors can help support that area.
 
mbaird said:
I am installing a 46rh in my Barracuda so I had to cut out the portion of the tunnel portion of the torsion bar crossmember and wanted some input on my idea for replacing it. It has to be slim for clearance yet structurally sound.
I have shaped a piece of 3” x 3/16 flat bar that I will weld to the ends of the cross members but want the reinforce it. I was thinking I could shape and weld another piece of 2.5” 1/4” flat stock on top of it . My theory is tat this will give it more torsional rigidity . And since it will be welded to the tunnel it will be strong enough to.

View attachment 1716355593

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That's pretty much how it was handled in this thread:
42RH / A-500 in a Duster with pictures.
I'd say you're on the right track.
 
I will be installing frame connectors using 3”x2” rectangle tubing and plan teeing into them with sections of the same where the trany mount is. Then using a section of 3”x1/4” flat stock as trany mount support bolted to them . Might have to shape the flat stock for exhaust clearance.
 
I'm not seeing this working. Your replacing boxed structure with flat stock. I believe it would be best to fabricate a box structure that has clearance. While this may be a smaller cross section than the original, it will have more structural integrity than flat stock. If you fully welding all the box seams it may be stronger than the original.
 
While it was a good idea to do what you can to replace the structure that you cut away, it does not do the whole job IMHO. You didn't say what you were doing with a crossmember, but I would recommend the US Car Tool cross member. It is made of very heavy metal and is well boxed and can really do a lot to replace the structural integrity of what you cut out. You torsion bars are always pulling down to the center and will distort the floor pans. Chrysler C-bodies in the early 70s had no hoop over the transmission tunnel. Instead they have a substantial cross member that takes the load - you just have to remember to take the load off the torsion bars BEFORE removing the cross member, especially if the car will sit like that for any real period of time. Here are the finished changes in my frame for my A500. For my RH42 the case is wider on the drivers side so I had to move the inner trans bolt on that side (along with the tube inside the frame), but the 727 based trans are different and may not require that.
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I did back off the torsion bars prior to cutting . As for the tunnel section my thoughts are that 5/16 off stacked flat stock will equal or exceed the 12 gauge stamped hoop that it is replacing . I do understand the 3D shape of a thinner metal will outperform that of a thicker one . Plus the welds should add to rigidity….
 

If you weld that big ol honkin piece of 3/16 that you did an awesome job of shaping in, to where you have the minimum amount of clearance needed, I don't see how it will twist. You will need the transmission in place to get everything right. If it were mine, I would want the engine to be at the same level as factory, or just a wee bit up if that helps get the tip of the tailshaft up in the right place and not be pointing too far down because the trans is slightly longer. With you adding frame connectors, there should be no twist from the torsion bars. If it still worries you when you get all that done, you could possibly add a short triangulation brace in front of the torsion bar crossmember, over to the frame rail on each side. If your floor pan wasn't too nice to hack up, you could cut it open all the way across the top of the torsion bar crossmember and add a piece of 1/4" flat bar fitted tightly and welded all the way across, just inside of it from rocker to rocker. You're doing a good job, so I'm looking forward to seeing how it goes.

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When I finally start the T5 swap on Vixen, my plan is to get an extra upper hoop and make one larger one from that ans the original I cut out. Hopefully that way, it will look factory and maybe be easier to fabricate.
 
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