Swapping TH400 to 700R4

[Stepperado]

I am at a complete loss.

I thought since our trucks came with both the TH400 and the 700R4 transmissions swapping from one to the other would be no big deal.

Well, to make a long story short, I have been told by the good folks at Advance Adapters I need an adapter to the tune of $500 to make this happen. Is that really the case?

Certainly there are factory parts to mate a 700R4 to a 208 transfer case. Any help would be greatly appreciated.

Thanks,
Stepperado

 

[CSFJ]

I may be mistaken, but I believe you'll need the 700r, a transfer case with the 27 spline in put, and the drive shafts (?) Not sure on the shafts, it'll depend on if there's a significant length difference on the trans.

 

[1987 GMC Jimmy]

The early models had 27 spline input, but the later ones were 30 spline, right?

 

[CSFJ]

The early models had 27 spline input, but the later ones were 30 spline, right?

I thought all the 700r had 27 spline output. The th400 had 32.

 

[1987 GMC Jimmy]

I thought all the 700r had 27 spline output. The th400 had 32.

I've read from a couple different sources that they swapped to 30 spline in mid-1984 because the 27 spline setup was too weak.

 

[CSFJ]

I've never heard that before. The 700r and the th350 both had the 27 spline output. That's why so many people opted to put the 350 in if they had trouble keeping the 700 alive. It didn't require a different t-case.

 

[CSFJ]

I just went back through the transfer case ID thread, and the only t-case input spline options were 10 spline (sm465) 27 spline (th350, 700r4, 4l60), and 32 spline (th400, later sm465)

 

[1987 GMC Jimmy]

Well, here's one source. I don't know, though.

 

[CSFJ]

I don't know if they did that for the cars or two wheel drive trucks, but for our old 4x4s there is no 30 spline t-case.

 

[1987 GMC Jimmy]

Haha, okay. I believe you.

 

[Jrgunn5150]

I may be mistaken, but I believe you'll need the 700r, a transfer case with the 27 spline in put, and the drive shafts (?) Not sure on the shafts, it'll depend on if there's a significant length difference on the trans.

This is correct.

Swap to your 27 spline input (incredibly common), or grab a 241 pass drop case as an upgrade.

 

[bucket]

I've read from a couple different sources that they swapped to 30 spline in mid-1984 because the 27 spline setup was too weak.

That's the trans input shaft. Only makes a difference on torque converter selection.

The easiest solution is a professionally rebuilt 700r4 with an adapter and np208 from a donor truck. Other than doing something to enable the lock-up feature and adding a T.V. cable to the carb/tbi, it's a bolt in swap. Same shafts and everything.

However, I personally would not do it. I've run a few 700r4's now and several th400's, the 700r4's are the only ones that die.

 

[Jrgunn5150]

I agree, i would rather have the TH400 and drive slower, IMO.

 

[bucket]

I agree, i would rather have the TH400 and drive slower, IMO.

I can't run any faster with my 700r4. You try to push it too fast down the highway, the TC unlocks and the trans temp starts climbing quickly. If I didn't have the gauge to let me know when to back off, this 700 would have died just like the rest of them.

 

[Jrgunn5150]

I just mean 55-60 instead of 75-80 lol. Not 120.