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Old 08-25-2016, 02:34 PM
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Default TH400 Crossmember for 78 Trans Am

Many many years ago and the day before Pinks All Out, Greg Merrick and I were replacing the TH350 in my car with a TH400 Greg built for Steve Hodge. Long story short, I had to make due with the TH350 crossmember as I flipped it around and redrilled the frame rails to make it work.

I recently ordered a TH400 crossmember for a 72 Camaro and was going to install it today. It looks just the same as the backwards TH350 crossmember. Am I missing something? Are the mounting holes on the frame different for each tranny or do I have the wrong mount?
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1978 T/A 463 Pontiac, KRE 74cc 292CFM D-ports, Lunati VooDoo, V-max lifters, TKII, ATM 850 E85 carb, TCI TH-350 race tranny, 3600 converter 3.73 12 bolt 11.63@116.68mph
1981 T/A 4-speed 406 Pontiac, Merrick ported 6X heads, Comp 270S cam, Crosswind intake 750 Street Demon, 3.42 30 spline Eaton posi street car.
1980 Formula 350 Pontiac back burner project
1972 LeMans 350 Pontiac
  #2  
Old 08-25-2016, 04:05 PM
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The 70-74's had a different front sub frame, last time I checked it wouldn't work.

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Old 08-25-2016, 04:05 PM
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72 will probably not fit the frame rails I think earlier second gens are narrower. TH400 mounting pad should be about 6" further back. G Force/Summit make one 7lbs heavier than the factory crossmember. Also can cut the tab off a TH 350 for a 78 and weld it on the opposite side. I made a wooden jig to do that on the one for my 79 Camaro/IA2 project.

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1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
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Old 08-25-2016, 07:07 PM
'ol Pinion head 'ol Pinion head is offline
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The '70-74 crossmember is slightly narrower and its an absolute RiG is to oblong the holes and angle the bolts slightly. performed that 3 decades ago in my '80 T/A, & the crossmember wasn't bolted in that good. A much cleaner job is as Skip mentions, take the '75-81 F crossmember, lop the tab off it & weld it to the opposite side. Have ran across that performed several times in late '70's F bodys that I was installing built up rearends in.

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Old 08-26-2016, 02:47 PM
JC455 JC455 is offline
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Some Caddies (I think the Seville models) came w/ a TH400 in them through the late 70's. They used the same subframe (from what I've read through old Hot Rod tech articles) as the F-cars, so it'll be a bolt-in, if you can find one.
Otherwise, mod a TH350 (make sure to offset the mounting tab to the proper side), or order s Summit unit.

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Old 08-26-2016, 08:47 PM
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Some pictures of the jig and crossmember ready for welding. The TH 350 mount as well as the BowTie Overdrives both have the mount slightly angle up towards the front so I kept the same angle.
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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
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Old 08-27-2016, 03:29 AM
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That looks real good Skip

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1978 T/A 463 Pontiac, KRE 74cc 292CFM D-ports, Lunati VooDoo, V-max lifters, TKII, ATM 850 E85 carb, TCI TH-350 race tranny, 3600 converter 3.73 12 bolt 11.63@116.68mph
1981 T/A 4-speed 406 Pontiac, Merrick ported 6X heads, Comp 270S cam, Crosswind intake 750 Street Demon, 3.42 30 spline Eaton posi street car.
1980 Formula 350 Pontiac back burner project
1972 LeMans 350 Pontiac
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Old 08-27-2016, 03:31 AM
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Quote:
Originally Posted by JC455 View Post
Some Caddies (I think the Seville models) came w/ a TH400 in them through the late 70's. They used the same subframe (from what I've read through old Hot Rod tech articles) as the F-cars, so it'll be a bolt-in, if you can find one.
Otherwise, mod a TH350 (make sure to offset the mounting tab to the proper side), or order s Summit unit.
Cool to know, thanks for sharing that. We have a bone yard that is full of old cars like that so I just might need to take a trip.

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Johnny US Army Retired
1978 T/A 463 Pontiac, KRE 74cc 292CFM D-ports, Lunati VooDoo, V-max lifters, TKII, ATM 850 E85 carb, TCI TH-350 race tranny, 3600 converter 3.73 12 bolt 11.63@116.68mph
1981 T/A 4-speed 406 Pontiac, Merrick ported 6X heads, Comp 270S cam, Crosswind intake 750 Street Demon, 3.42 30 spline Eaton posi street car.
1980 Formula 350 Pontiac back burner project
1972 LeMans 350 Pontiac
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Old 08-27-2016, 04:16 AM
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That is nice Skip. Maybe we should send you our crossmembers! I did use the G-force unit as you might recall in my 75 Formula when swapping the TH-350 to the 400 and failing to get the cross member change to work out correctly. But your setup makes it look repeatable and proven. Maybe you should make some measurements and a plan for that.

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Old 08-27-2016, 03:47 PM
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or this aftermarket piece.

http://americanpowertrain.com/c-3145...body-t400.html

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Old 08-27-2016, 03:57 PM
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I also used the G force on the jig besides the Bow Tie. It is about 7lbs heavier than the factory(feels heavier than that). It does have nice arcs for the exhaust I think the Bow Tie does not-it is more straight. The G force also is almost level at the tranny mounting pad vs angled like the factory and Bow Tie are.
Plan is to use the welded factory on the TH400 going in the Camaro/IA and probably the G force for the 200R I'm planning for the 78 TA since 200R and TH400 are about the same.

Maybe use the jig to weld on a plate to raise a section on the BowTie for exhaust clearance.

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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
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Old 08-29-2016, 12:49 PM
Don 79 TA Don 79 TA is offline
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i think american power train makes a universal setup
and places like hurstdriveline have various x-members (gforce too)
there is a subframe difference
the universal kit i received from APT was for first gens
i think it fits early second too, not sure
last year i tried again, and they sent me 3 claiming they would work, they did not
hurstdriveline one fit right in (i wanted the two bolt trans mount in place of the single one i had been using)

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Old 08-29-2016, 12:59 PM
428goat 428goat is offline
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Johnny406
So what is Greg merrick up to now?
He bulit a engine back in 1999.

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