double hump crossmember 73-77 GM de Verschillen

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kloenie
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double hump crossmember 73-77 GM de Verschillen

Post by kloenie » Fri Aug 03, 2007 12:18 pm

OK, hopefully with some pics and short descriptions we can clear up the confusion with double-hump (DH) transmission crossmembers on 4th gen El Caminos. Of course this all comes to light because of the desire to install a true dual exhaust system on our '77 project. (Many thanks go to Caveman49 (Lee) for his help in solving this puzzle...)

A little history first - from what I've been able to find, all '73 - '75 Elkys were equipped with the provisions for DH crossmembers, whether they had dual exhaust or not (all 454-equipped cars had dual exhaust). Then Uncle Sam stuck his nose in the works and along came the catalytic convertor in late '75. With this was a shift to single exhaust systems with an inline cat, and a redesigned crossmember. Dual exhaust systems were never offered again.

So... why can't those of us with a '76 - '77 Elky who want dual exhaust just take a DH crossmember from a '73 - '75 and bolt it up and run with it..?? Simple answer - GM changed the frames (at least in the area where the transmission crossmember bolts up) in '76 to coincide with the new, wider single-hump crossemember. Let's take a look....

(Note - you can click on any image for a larger, detailed pic)

Below are the two crossmembers laid out - top one is the DH, bottom one is out of our '77. Visually they are quite different. Measurements are very different too.

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DH crossmember measures 46-3/4" end-to-end, while the '77's SH version measures 51" end-to-end...

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So what's up with the difference in widths? Why, the frame attachment points, of course....

'73 - '75 frames had extra brackets - or "tabs" as I like to call them - welded to the frame rails that served as the attachment points for the DH crossmember. Starting in '76, these tabs were eliminated....

A couple of '74 frames....

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And here's a pic of ProStreet Garry's DH setup on his project '75
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Here's a '76 frame showing NO TABS...

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For the '76-'77 models, the frames were changed to eliminate the mount tabs, therefore a DH crossmember cannot just be "bolted up." The later models' crossmembers were longer and bolted thru holes in the actual frame rails themselves, as you can see in the above pic.

So... how do those of us with either a '76 or '77 get dual exhaust? There are no aftermarket alternatives available, like in the 5th gens. Caveman49 suggested welding-on tabs that'll allow the use of an earlier DH crossmember. Great idea, but this would most easily be accomplished during a frame-off restoration. To me, it would seem like modifying, or "notching," the stock single-hump crossmember would be the simpliest solution, and one that we're probably gonna try.

If anyone has more they can add to this discussion, or if my info is a little off, then chime in here...

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