TH350 TO TH400 swap

[baclay9]

I am going to be replaceing my TH350 with a TH400 since I am going to a BB. It is going onto a 81 SWB frame. I was wondering if I was going to have to move the crossmember or find a different crossmember for the TH400?

Thanks

[Seven9er]

You have to move the crossmember back. There are already holes in the frame for it. You will also need a new drive shaft, do you have the 2 piece or 1?

[LiveAnimals]

Correct me if I'm wrong but I think the shift linkage is different, and you can use your original drive shaft but the yoke is different and you'll have to get it shortened.

[72lb4x4]

Maybe the motor mount barackets can be moved forward for the extra length of the TH400. I think it's 1" longer than a TH350?

The TH400 has an electric kickdown so that's something else to think about.

[BLE 'BURBAN]

The TH400 came in various configurations and 3 different bolt patterns.
Configurations were 2wd short-shaft, 2wd long-shaft & 4wd.
The bolt patterns on the bellhousings were...
1. Chevy engines only

2. B-O-P & Cadillac engines only

3. The rarest of them all Universal bolt pattern that would bolt up to just about any G.M. engine in it's day.

As for the configurations go, in 2wd there were 2 lengths. the more common one is the long-shaft (Output shaft at the rear of the tranny) and the short-shaft.
If you get a longshaft, then you will need to move the crossmember back and have your drive shaft shortened. With a short-shaft, it's a direct swap out from the TH350 to the TH400.
Now a good tranny shop SHOULD be able to rebuild your long-shaft into a short-shaft for you.

Also, as mentioned by 72lb4x4 above, you will need an electric kick down switch for the TH400. These are getting harder and harder to find in junkyards. Thankfully LOKAR has come out with one finally which can be found through jegs, summit or even from LOKAR themselves. Here's a link I found for jegs just to show you what theirs looks like.
JEGS/LOKAR TH400 KICKDOWN SWITCH

As for the shift linkage, it's all the same no need to relocate any of it.
Enjoy and have fun!!!
Dave

[baclay9]

Awesome answer Dave! Can you tell me how to distinguish a long shaft from a short shaft.

Can you offer a brief explanation of how the electric kickdown works? I always thought the kickdowns on th400's were vacuum or something. Very interesting.
Posted via Mobile Device

[BLE 'BURBAN]

It's been awhile since I did the swap myself (1992) but if memory is right the longshaft is 8" longer than the shortshaft. basically the TH350 and the TH400 SS are both the same overall leangth from the front leading edge of bellhousing to the end of the output shaft.
Oh one thing just popped into my mind as well... The output shafts came in 2 different styles as well, one utilizes a slip on yolk and the other style has a threaded end to use with a retaining nut to hold on the yolk.

As for the kickdown switch goes, it engages at a preset throttle position. simple 2 wire hook up from switched power source(ignition on only) to kickdown switch then from switch to connector on the side of the tranny.
I've got an old switch around here somewheres I'll have to see if'n I can find it and get a pic of it for ya.

[baclay9]

Thank you sir

[KQQL IT]

You kinda dont need the electric kick down, the vacuum controls the line pressure. As ive been told and never had one on my crap.

[BLE 'BURBAN]

Originally Posted by KQQL IT

You kinda don't (spell check) need the electric kick down, the vacuum controls the line pressure. As I've(spell check) been told and never had one on my crap.

You do if you want a true passing gear. The vacuum line controlling the line pressure only works for up shifting during acceleration to get up to normal cruising speed and deceleration. Otherwise it's all go pedal unless you wanna manually shift into second gear for passing. But then what's the point of having an automatic tranny?

[LiveAnimals]

I don't know if this helps

[KQQL IT]

Thanks pal didnt know this was grammar school. buts you've showed your self well

[RandyP]

That spell check thing is a setting on his browser, not something he did on purpose. It just showed up on his quote or copy/paste. I have it as well, and I still have tons of grammar issues.

[baclay9]

I am beginning my search for a TH400.

What kinds of things should I look for on the Trans as I go to inspect them. First thing I am going to do is have it rebuilt no matter what the original owner says about its condition because I don't want to install twice. I know to look for cracks and stuff in the housing but what else? What kinds of things will I need on the trans?

Do all TH400's have trans coolers? Do I need one on a truck that wont be pulling anything? Should I make sure in has any kinds of modules or anything like that attached to it? Thanks for your help.

[BLE 'BURBAN]

Ok here's the switch as it's mounted to the go pedal bracket the repaired black-white wire plugs into an ignition on accessory power source on your fuse block and the faded orange wire goes through the firewall and down to the tranny.

As the go pedal is put to the metal, the upper swing arm where your go pedal cable is attached to pushes on the switch until it closes the circuit and sends power to the kickdown solenoid inside the tranny via the connection on the side of the transmission.

[baclay9]

So I would either need to find one of those brackets and switches or buy the aftermarket one from Lokar?

[nbpro]

This is off topic, but browswer addons slow your browsing down, just so you know.

[BLE 'BURBAN]

Originally Posted by baclay9

So I would either need to find one of those brackets and switches or buy the aftermarket one from Lokar?

You got it.
And if you were a paid subscriber I could offer to sell it to you for a wallet size picture of Andrew Jackson + freight. But as you aren't yet I can't.

See what $25.00 a year can get you on here and so much more as well!!!

[baclay9]

I have been meaning to rectify that and this seems like the perfect time. LOL Give me a sec...
Posted via Mobile Device

[baclay9]

Ta daaahhhhhh

[baclay9]

I found a TH400 on CL for $75. As I said I am going to have it rebuilt. It was behind a 82 Chevy 1 ton desiel. Should that matter.

[baclay9]

Here is another option. There is a shop in OKC that has a reputation for building really strong trannys. He told me he had one of his street/strip models on the shelf ready to go for $1375. That is with no core or anything.

Does that sound like a decent price?

I didn't think it was too bad considering it is going to cost me around $200'for a core off CL and $700 to $1000 to have it rebuilt by a shop I have never heard of, assuming it is in ok shape.
Posted via Mobile Device

[RandyP]

Dang, it costs that much to have a trans overhauled these days? I'll be in for sticker shock when I do mine it seems. All I remember is being able to get a TH350 built and installed for about $350, granted that was back in the 80's.

[baclay9]

Granted these prices are for a really strong trans. But, I had one shop quote me 700 for a rebuild if I brought in a good core and everyone I talked to said that sounded really cheap and I might should stay away from them. LOL
Posted via Mobile Device