Buick Big Block Transmission X-mbr
by George Nenadovich
Here is a side by side comparison of the small block transmission
cross member (lower) and a big block version (upper). The upper one is from
a 70 GS455 coupe and has part number 9796240 stamped on the area indicated
by the white arrow.
There are four cross members:
1)small block coupe
2)big block coupe
3)small block convertible
4)big block convertible
Convertible versions are shorter because they have a boxed
frame while the coupe versions use a C-channel frame.
Buick uses the rubber isolators in metal brackets which are
attached to the frame rails as pictured above. Oldsmobile does not use isolators
and the cross member is bolted directly to the frame. A common problem with
Buick members is the ends will rust away since moisture can collect inside
the isolator slot. You can either replace the cross member with a rust free
version or fix your existing one. A repair kit is offered to solve this
problem by Mike Garrison at Mr
The arrow indicates where the emergency brake cable passes
through the Buick big block cross member. The opening measures 1/2"
wide and 1.5" long. I believe this hole could be made in a small block
cross member by drilling two 1/2" holes then connect them with a plasma
cutter or using a milling machine.
Here is an Olds Cutlass x-mbr below. Notice the bolt holes
on the end and the slot for TH400 applications. This was removed from a
70 Cutlass 4dr with 350/TH350 set-up.
You can use the Oldsmobile cross members as an alternative
if you can not find a Buick version. For convertibles, all GM A-body wagons
have boxed frames which use the same cross member as convertibles. Chevrolet
versions will not be correct since those are made from cylindrical steel(tubes)
instead of flat bar stock before being stamped into the form as pictured
Also, all 69-72 Pontiac Grand Prix models have boxed frames
and have TH400 transmissions as standard equipment. These cars will have
the same cross members as convertibles.