Transmission Crossmembers
By Tom Stahman
Most people want to believe that First Generation Camaro's only have one transmission crossmember that works for all the engine combinations that are used. This statement is definately not true.
The first crossmember is the most common. This crossmember is used for all small block engines, including the 6 cylinder engines. See picture 1 , Crossmember A. This crossmember has the bolt pattern (2 slots) centered on the crossmember, which is also the centerline of the car. This crossmember is used for the Automatic Powerglide, the TH350 transmissions, as well as the 3 and 4 speed manual transmissions behind the 6 cylinder and small block Chevy engines.
The second crossmember is for manual transmissions behind Big Block engines. See picture 1, Crossmember B. Big Block engines are offset 1/2" to the passenger side. The transmission mount is also moved forward 3/4" from the small block transmission's mount location.
The often misstaken crossmember is the third crossmember. (Picture 1, Crossmember C) This crossmember is actually the exact same bolt pattern as the small block Chevy's, with the cutout of a Big Block ! This crossmember is actually for the Pontiac Firebird.
The 4th crossmemer Picture 3, Crossmember D, is the one used for the Automatic TH-400 transmission used ONLY behind Big Block first generation Camaro's. This means that the transmission mounting pad is offset to the passenger side 1/2" and forward 3/4's of an inch from the location of a small block with a TH-400. I thought you said that small blocks did not come with TH-400's ? They didn't. However, some owners of first generation Camaro's, in the last 40 years, have decided that they wanted such an animal. The solution, you ask ? Again we turn to the first generation Pontiac Firebird. They came with the TH-400 transmission, and it was not offset. This crossmember will work perfectly for a small block Camaro with the TH-400.
The one problem is that most people, including aftermarket parts houses, do not know that the big block was offset. They also do not know, that the Firebid's is a different crossmember than that of the Chevrolet piece.
What has happened is that we have big bocks with the small block's crossmember and small blocks with the big block crossmember's. This also happens with the other transmissions too. The only way to be certain that you have the correct transmission crossmember, is to measure it from each end to the bolt pattern. If it is from a small block the measurement will be the same (on center). If it is for a big block the pattern will be offset to the passenger side by a 1/2" This means your measurement will be 1" longer than the other.
Many people think that since they have everything bolted in (even if the crossmember is wrong) that it won't cause any problems. This simply is not true. In pictures 4 and 5, it is shown what can happen if you are using the incorrect crossmember. These drawings are to scale and they are not exagerated. If you are running headers, you can see how they would be pinched against the steering box. This is a very common problem, and people do not understand how 1/2" can cause problems !
The other problem is that the transmission out put shaft must be parallel with the rear ends pinion (input) shaft. This can be offset, but it must remain parallel. If it is not parallel you may experiance vibrations and premature failure of parts such as u-joints or pinion bearings.
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