71 c20 transmission swap

[Daverde]

I have a 1971 c20 4wd with the stock 4 speed standard trans
I would like to change it over to a th400 transmission
does any one have any information on this?
what parts I need ect ect?
thanks Dave

[JimKshortstep4x4]

My friend has a TH 400 in his truck and the transmission/transfer setup came out of a 78 1 ton truck.

The following was needed to install the combo:

Auto cross member was moved rearward and the drive shaft shortened.

Shift lever was rebent to allow it to come through the floor hump in the right location.

Auto column with linkage was installed.

Radiator with internal transmission cooler was swapped in and lines made to run to the transmission.

Vacuum line for the modulator was made up and installed.

Kickdown switch and wiring was installed.

A longer speedometer cable was needed.

This post should probably be moved to the 4x4 tech forum for more detailed information.

Jim

[mrein3]

Originally Posted by Daverde

I have a 1971 c20 4wd with the stock 4 speed standard trans
I would like to change it over to a th400 transmission
does any one have any information on this?
what parts I need ect ect?
thanks Dave

th400s were never offered in 67-72 trucks. However I have seen a th400/NP205 in a 1972 pick-up so I know it is possible.

What I do know is that if you find a th350/NP205 in a 67-72 pick-up, you can use the driveshafts, speedo cable, and transmission crossmember that are already in your truck.

In addition to the different tranny/adapter/t-case you will then just need to find an auto steering column, the linkage from the column to the transmission, transmission cooler lines and a radiator with a tank for the colling lines to run through.

Back when these trucks were made GM made all the transmission/transfer case combinations the same over-all length. Since the transmissions are all different lengths, the difference was made up in the transmission to transfer case adapter. All the adapters only go with the transmission they came with and they are all different lengths.

Another tidbit you should know is that your transfercase input shaft has 10 splines on it because of your 4-speed. 3-speed and 4-speed manual transfercases had the 10 spline input. The transfercase behind a factory th350 equipped truck (same transfercase) has a 28 spline input shaft. Your best bet is to find a complete tranny/adapter/t-case from a running truck to begin your swap.

The speedo cable screws in to the same spot on your new transfercase so unless you change tire sizes at the same time, if your speedo is right on now, it will be right on with the new transmission. On trucks with different rear end gears GM screwed a reducer or speeder upper on the transfercase then the speedo cable screwed into that. So if you look at your case and see a device hanging off the t-case with your speedo screwed in to it, keep that for your new combo. Similarily if your new combo has the device and current doesn't, don't put that device in line.

If you can manage a couple hundred pounds of iron and aluminum this swap isn't bad. It is pretty straight forward. The only hassle is the weight of the components you are attempting to move around.

[Sinister]

I have a TH-400 in my '69, original equipment.

[mrein3]

Originally Posted by Sinister

I have a TH-400 in my '69, original equipment.

Four wheel drive?

[Sinister]

Doh, apologies, 2WD. I just woke up.

[Daverde]

Thanks to all who did reply to me on this....
I just discovered more rust in the frame......(Bad under front of cab by firewall.)
ITs terminal.....Bummed....the truck needs rockers and the usual cab corners
And the Bed is junk...its too bad I am the 3rd owner it has a great running 350
4bbl (original engine +drive train untouched but RUSTED OUT..otherwise.