Th400 Housings

the GM turbo 400/TH400 was in production from 1964-1991. over the years there were many changes to the design to both improve and reduce cost’s in later years. this is a short guide to explain the TH400’s features, changes, and hopefully identify your transmission to see what you have in your shop so you can determine for yourself if it’s worth the money for a rebuild.


 first off let’s go through the different housings.

 -first we have the roundtop bell housing case. this was used in 64 cadillac, 64-66 buick nailhead engines, and with an adapter plate it was installed in jeeps in later years. this is an oddball bird and you usually find it still hooked to an engine. basically the bell housing looks like a half moon with a provision for the starter to clear on the driver’s side similar to a later model transmission.

-next we have the 65-67 cadillac th400 transmission which appears like it’s more octagonal and has no top ears to the tranny like the later model BOP cases we are familiar with. these were unique to the cadillac in only these years and were paired behind 429 power plants.

-next we have our all too familiar BOP transmissions. for the sake of this section i will divide this into two generations of bop castings. first we have the 68-7heavy casting BOP housing6 heavy casting trannys used in buick,olds,pontiac, and cadillac. also heavy cases appear in 77-79 cadillacs in a short shaft version as well. these were among the heaviest castings as far as bell housings ever cast for the th400. the easiest way to tell what you have  is to take a gander .notice the extra material under the top ears. the extra material is carried all through the casting with extra material cast into the valve body area and the side ribbing . i figure about 5-6lbs extra aluminum over a chevy case.light BOP case housing


 then we have the lighter BOP cases used in 77-80 in metric gm full size. it is basically a lighter BOP case notice there is no extra material under the top ears. these housings are about as strong as a chevy case.

-then we have the chevy th400 case. used 65-91, and 73-later checker cabs. although there were many different castings i have seen over the years i have not seen any of them not crack in a demo derby. i have had “HD” housings that actually mic out thinner than late 60’s chevy car tranny housings.

 starting in the early 80’s the chevy th400 were primarily only in heavy duty applications and chevy/gmc trucks.  some of these were cast “HD” in the bell housing which was in referrence to a heavier duty rear planetary set internally.

  later model 88-91 th400 all had a slightly heavier casting(sorta) and had heavy duty internals, but did not have the “HD ” cast in the bell.  i mean as far as bell housing it is less that 1/16th of an inch. these 88-91 trannys also had different plumbing at the valve body than all other years in the casting, so the gasket used on these housings is different than the rest. some also had 4L80 parts incorporated toward the end of the run of the TH400 as believe it or not quite a few parts interchange between the two trannys.

in any and all situations an option such as a full girdle or a JW ultrabell which bolts to your front pump in place of the housing is a much stronger option. if you run a slider shaft it is not a matter of  if but an issue of when you will break a transmission housing.  heavy re-enforced oil pans aid in strength of a housing as well but not enough to make it worth the cost usually. food for thought guys!!

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