Transmission Case Identification and Interchange
The TH400 has found its way into various vehicle, chassis and engine combinations over the years. This means the transmission cases were cast with numerous transmission to engine mounting flange configurations. The following information will center around the most commonly encountered engine mounting flange configurations which are separated into two groups. The first group is comprised of transmission cases designed to fit Buick, Cadillac, Oldsmobile and Pontiac engines and is referred to as the BCOP casting. The second group is comprised of transmission cases designed to fit Chevrolet engines and is referred to as the Chevrolet casting. There are design features which are both common and unique to each casting group. Because of this, similarities between both groups will be covered first, follow by the features unique to each group.
All 1965 thru 1974 transmission cases were cast and machined to accept the eight-bolt oil pump assembly. The casting received 18 center support/intermediate clutch backing plate lugs, and steel vent tube. See Figure 13-. Note the extra material at the 7:00 position of the pump mounting flange with the eight-bolt case in Figure compared to the six-bolt case in Figure.
For the 1975 model year, two of the oil pump mounting bolts previously fit to the transmission case at the 1:00 and 7:00 positions were deleted, along with their thru holes in the oil pump body casting. With some cases, the two unused bolt hole bosses were completely removed from the casting. With others, either one or both bolt hole bosses may be present, and when present, may or may not be drilled and tapped. Note the presence of the mounting bolt hole boss at the 1:00 position with the 1976 Buick case shown in Figure 13-. The casting received 17 center support/intermediate clutch backing plate lugs, and plastic vent tube. See Figure 13-. The location of the missing lug varies between the 2:00 and 3:00 position.
The BCOP Transmission Case
Several light-duty and heavy-versions of the BCOP TH400 transmission case were produced from 1965 thru 1985. There were also many “blended” castings which resulted in a case with a combination of features found in both versions. When a BCOP case is referenced in this section as light-duty, the casting is said to include all traits outlined in this section used for comparison with what is said to be a heavy-duty casting. The same can be said for the heavy-duty casting. It is also interesting to note that none of the features of the heavy-duty BCOP case were ever incorporated into any of the Chevrolet castings. The heavy duty BCOP case include several casting benefits that significantly increase the structural rigidity of the component. Note the reinforced bellhousing area of the heavy-duty casting with its increased material in the bellhousing mounting area and cast reinforcement ribs.