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Old 05-23-2009, 08:31 PM
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T-400 & 700r4 swap

I recently blew up my th400 and im thinking about replacing it with the 700r4 to get the low first gear and overdrive. However im not sure what has to be changed in order for the 700 to work. i know its 3 in longer and driveshaft will be shortened but what about the minor details i.e will flexplate need to be changed? is the speedometer hookup different? and how hard is it to adjust the tv cable?

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Old 05-28-2009, 05:24 PM
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well actually the th400 is the oddball automatic trans

since most variants of the 400 trans have the 32 spline output and is an odd length compared to the other auto trans models. some 400 trans variants such as the 3L80HD have the 27 spline output however.

also the th400 has a wider trans mount bolt spacing than other models, so it used its own mount.

so converting to one or away from a th400 requires modifications/changes that are not typically needed when converting to or from other trans models.

700r4 is a finicky trans... or can be...
the TV cable must be set properly or it can die early.

flexplates; most all of them have both bolt patterns in them=small and large= you shouldnt need to swap it,
but its not impossible that you have one that doesnt have the smaller pattern(compared to the larger th400 one) needed for the 7004r converter...

both the 700r4 and the th400 will have same mechanical cable driven style speedos, same threads and size on the speedo gear retainer bodies..

drivshaft length depends on which th400 you have exactly,.

an old car long tail one,,,

or an 80s short tail one,,


its easy to shorten the driveshaft an inch or two yourself if you have a good old arc stick welder.

vey simple really.

ive done many including the one under my 91 s-10 2wd to convert it from the original 2 piece shaft to a one piece from an 89 full size chevy van because my rear shaft was bent severely due to locked up frozen u joints

you can find out trans length details with some searching of the forums here or on google

good luck!

Last edited by fast68; 05-28-2009 at 05:29 PM.
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