TH400 2wd to 4wd Conversion

[BigRix]

After many sleepless nights searching the internet I think I have found out what it takes to convert a 2wd TH400 trans for use in a 4wd behind a 6.2 diesel.

I will document the process that I am using over the next few days.

The best 2wd case to use is one that has the bosses for the bolt on torque convertor cover but they were never drilled and tapped. This is for all intents and purposes a HD or "K" case. Note, I can find no evidence that any TH400 HD 4wd cases ever had a K cast into them like the TH350's did. If this case is not available a standard 2wd case can be used but no provision for engine strut bars will be available.

Several extra parts will need to be sourced, an output shaft, a governor, and a low stall convertor.

First the output shaft. I have found many posts that indicate that the "4wd" output shaft is required but none that include and description of said shaft. If you are like me you have to be able to tell your local tranny guy what to look for when he is searching through a pile of output shafts. The proper shaft will have 32 splines and measure 6 3/8" from the face of the thrust washer surface.

Second the governor. The diesel governor should have solid weights with no lightening holes. This allows centrifugal force to make the weights extend at a lower speed thereby causing lower shift points.

Third is the torque convertor. I was not able to find the exact matching convertor but I had two that would most likely work. Cheap and expensive. I went cheap with the GM3 HD low stall with three mounting pads. My other option was an Allison part with the same stall speed and six mounting pads and a price tag that was triple the cost of the GM3.

The transmission must be disassembled to change out the output shaft and I am using this opportunity to freshen up the internals. If you are not comfortable with process then you need to find a friendly transmission builder in your area. The governor and converter are the easy part.

Drilling and tapping the case will be covered in a later post.

Pictures

Output Shaft
Un-drilled Case
Reinforced case near extension housing
Reinforced low/reverse band pins
Diesel governor on right

 

[Csm Davis]

Another case to look at is the multipattern bellhousing case they are very strong because of extra material.

 

[BigRix]

Finishing up the last few bits on the conversion.

Starting with the drilling and tapping. Only one hole lined up so I drilled and tapped that one first. 10x1.5mm Tightened that one finger tight and lined up the cover then clamped it in place. I then used a small socket to center my pilot holes. I then drilled to size and tapped. Looks factory.

Next was the freeze plug to block off the speedo hole. I searched the Internet till I was cross eyed and could not find the proper size. Here ya go. Hole measured 1.950" got a Dorman freeze plug 555-041.

 

[Csm Davis]

Didn't think about it till now but the only hd 400 two wheeldrive transmission I know of is the dumptruck or motorhome transmission and it has a straight cut Sprag gear

 

[BigRix]

That is actually a TH475

 

[Csm Davis]

Okay what's the difference? My transmission guy said I could put the straight cut gears in any of my other 400's

 

[BigRix]

That's about the only difference, straight cut gears to reduce thrust loading in heavy vehicle applications. The cases are usually beefed up in all the same places that 4x4 cases are.

 

[Csm Davis]

So that would be THE 400 to get if you can.

 

[ODdave]

What are the chances of the th400's from behind 4bt"s having straight cut gears??? any way to tell from the outside?

 

[Csm Davis]

Bump! Is there a way to tell from outside of case?

 

[BigRix]

My research indicates no visual differences between th400 and th475 other than the tag. They are used mostly in big box trucks and step vans, same as the 4bt, so its possible they already have th475's behind them.