hey everyone I'm new this fourm it seems pretty cool.
I need help finding out what rear i have in my T/A?
it's a 87 with a 700R4 soon to be TH400. I need to know what size it is so i can buy a set of 4.56 gears for it is it a 7.5",8.0" or 8.5"?????? btw does anyone have a longer drive shaft I think i need one cause according the specs i read on here the 400 is a little shorter than my 700R4. well buys thanks for your help and info

 

you have a 7.5

 

Thanks Ede now i can order my gears

 

Why are you going to need a TH400? Just curious. A TH350 is an easier swap.

You'll also need a TH400 slip yoke. The smaller 700R4/TH350/TH200 etc driveshaft yoke won't fit in the TH400.

You'll need to have a driveshaft custom made. If the TH400 yoke sits in the same position as where the 700R4's yoke was, you need a driveshaft 2 1/2" longer. There are no factory driveshafts that are the exact length you need. You could find the 3" longer driveshaft from a 82-83 f-body that came with a TH200 tranny and get it shortened 1/2" but it wouldn't really be worth it. If you think you need a TH400 because of how much power you're going to have then the tiny factory driveshaft won't last long. A 3 x .083" driveshaft should be used.

Install the TH400. Slide the TH400's slip yoke into the transmission until it stops. Pull it back out 1". Measure from the center of the u-joints on the yoke and the diff. That's how long the driveshaft will be. It will all depend on what yoke you have in the tranny. Not all are splined the same. I have 2 different ones. One is splined all the way to the end. The other is only splined half way. I could use 2 different length driveshafts.

 

the reason i'm putting in a TH400 is cause its a freshly built trans with a shift kit with about 450 miles on it and i'm paying $00000000000000000.00
It was my dads he had it in his 61 Vette then he bought a 85 vette motor and trans cause he wanted TPI for it so he got a good deal on the motor and trans so he put that in along with 85 front and rear suspension. he asked me if i wanted the TH400 so i couldn't say no!