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TH400 experts, please

Discussion in 'The Garage' started by elacruze, Dec 16, 2004.

  1. elacruze

    elacruze 1/2 ton status

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    I'm building a switch-pitch TH400 to go in front of a 203/205 doubler. I have three 400s-a '64.5-'67 Caddy, a '78-'79 w/203 tcase (unknown GM engine) and a '85 6.2 diesel w/205 tcase. What I want to know, is what are the best pieces of the three to use to get the strongest unit? The Caddy trans has 34 element sprags and 4 gear planetaries (Do they all?). Are there differences between the gas and diesel chevy cases, strengthwise? I've only disassembled the Caddy trans so far so I don't know what's in the other two yet. This is my first auto tranny adventure. I'd like to put in a 3:1 low planetary, but it's ~$700... any thoughts/advice?
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    I'm by no means an expert, but there might be some info on Oldsmobile sites about the switch pitch.

    I'm sure many parts interchange with "standard" TH400's, but I'd be pretty surprised if the case itself was one of them.

    On most GM trannies, the tag riveted to the passenger side rear(IIRC) with a two or three letter code, tells you the relative strength rating of the tranny. An AA code I believe would have been a very weak (or slushy shifting) tranny out of a Cadillac, while an OZ would be the strongest possible transmission for an Oldsmobile. (CZ for Chev, etc)

    I had a Cadillac TH400 for some time that I was running, and it was something like an AC or something. It held up good though, so I don't know as I'd worry TOO much about the strength rating on the tag, although any additional clutches, roller bearings, or metal vs. plastic thrust washers are all benefits of stronger transmission assemblies, and later on (80's) were a major difference (for instance) between an OA tranny and an OZ.

    If swapping parts between trannies, (or even rebuilding one) I can't emphasize enough that cutting corners will lead to ruin. Measure EVERYTHING that should be measured, from output shaft endplay to clearance on the planetary gears.
     
  3. elacruze

    elacruze 1/2 ton status

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    Thanks. I researched everything I could find on S/P trannies-they'll work in any case, surprisingly. It's just the pump, input shaft, and converter that are different. I actually found one perf. tranny shop making a lockup converter for th400 using S/P stuff. I'm a forklift tech, so I've seen lots of fluid transes, but not car stuff. I need specific TH400 hard parts info. I intend to do a full rebuild, as far as clutches, seals, gaskets, etc.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I *do* know the best planetaries you can get (and which are still available through the aftermarket last I heard) are from the TH475(think thats right) the ones from the motorhomes and such, with straight cut gears.

    Haven't really heard too much about the TH400's being "upgraded" like the later trannies. Not sure if it's physical size, or just a superior design from the factory, but with the 700 and 200, there are all sorts of upgrades for adding clutches, roller bearings, etc.

    When I rebuilt my 70's 400, I don't recall any plastic thrust washers, etc., that would have led me to believe it was a "weak" tranny, whereas that was obvious in the 80's 200's I've had apart.
     
  5. BranndonC

    BranndonC 3/4 ton status

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    anyone know if i can use parts from a th425 front wheel drive cadillac trans when i rebuild my th400 from an 84 chevy?
     
  6. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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