Monthly Archives: July 2013

Simple TH400’s are not so simple

when it comes to a turbo 400, as transmissions go, it is a fairly straight-forward 3 speed beast of an automatic transmission, with a fairly simple design as automatics go. produced from 1964-1991, it is largely obsolete for street use by modern standards, leaving the th400 basically either for classic cars or performance use. now there is this myth that all th400 were basically the same and the parts interchange. this is not exactly true, although if you look in any parts catalog or on-line source you would get that impression. for the most part anything from 71-87 will interchange, and thats a large chunk.

to start at the drive clutches: there is somewhere around 18 different pistons that could have been used in the forward and direct clutch packs, 4 different direct drums, three different styles of clutchs produced by 3 different manufacturers, and around 3 different types of steels in both kolene and regular steel(6).

now to keep going, there are two types of center supports, two different pressure regulators, up to 9 different ways you can mix and match up different pump halves for the front pump(some aren’t correct), 3 different case designs at the hydraulic circuitry,two different forward hubs,two different mainshafts, 2 different valve bodies, two different int pistons, 3 different accumulator pistons, 3 different pick up tubes to the filter just for the shallow pan, and two different pump gear designs. these are all the differences that you cannot see from the outside.

doing a quick math check, this leaves us with the possibility of somewhere around 20,150,000 different possible combinations of how a th400 can go together internally-not including aftermarket parts.

now, for the most part if you tear down and rebuild a th400 as an individual unit and take your time using all the same parts that came out of that particular unit, replacing stuff as necessary, there is hardly ever a problem. when you start tearing down multiple trannys at the same time and build parts ahead of time, this is where the fun begins. the thing you really got to watch above all else is how you pack your forward and direct clutches. too tight and you burn up the trans prematurely- too loose of a tolerance and parts will have a tendency to smack together and break the backing plates…..and worse yet certain clutch to piston combinations can snag the steels and keep it from applying all together.

for arguement sake, lets leave the clutches and steels out of the equation: there is still nearly 72 different ways to screw up mixing and matching up how you put together your pistons and drums. ok lets say you pack your foward and direct clutches ahead of time to be ready for a customer. you take his chevy 400 truck trans apart, simply grab a ready to go packed forward clutch off the shelf and put it back together. it went together fine right. but by mistake you grabbed a forward drum out of a 65 caddy trans with a large drive hub and you have a late model gearset. oops. you will be destin for an angry phone call.

Transmissions cannot and should not ever be mass produced. you have to treat them as you would treat a performance engine build: as an individual beast. even though you can go fast you have to take your time and be purposeful in how you build. it is a mistake that i made early on myself, and i watch shops do it all around me these days. when i started building newer transmissions i slowed down and was more meticulous in how i build. i then went back and took the same methodology to a th400 again and i tell ya what, i think it really made all the difference in the world. moral to the story: Don’t ever sacrifice craftsmanship for profit, it will bite you in the ass in the long run.

now a th400 is relatively a simple design, they get worse/more complicated from there.