25 Transmission's GM roots

[Cruiser_Nerd]

I can't think of an original application that would have been coupled direct to a transfer. But I wonder what a torque tube output looks like, have to check the doodlebug this weekend. This is cool stuff. Oooo, maybe the original had an under or overdrive on it......

 

[Cruiser_Nerd]

41-47 chevy with a torque tube pictured is about the longest output. G506 military was shorter.

 

[FJnotCJ]

OK, so another part of the mystery is solved....I disassembled the GM transmission to compare it's main shaft with the 25's. Upon close examination it looks like a PO had a short GM shaft mated and welded to the rear (or transfer case) portion of a 25 shaft. All the PO had to do then was expand the four top mounting holes in the 25 tcase and mount it to the GM transmission. Pretty crafty...but what this means to me is that a set of GM gears of this vintage (possibly newer?) will fit in a 25 case. No mod to the tcase needed

. Add a GM shift cover and only the very few in the know will notice. Or do this to a 25 tcase...

 

[cult45]

So you reckon you could use one of these in a non-txfer 25 if you couldn't find a stock box?

 

[FJnotCJ]

This particular GM transmission, the pre-SM 420 or M194, from the 1940's is a direct bolt on item to the FJ-25 bell housing only. The top the trans will pop right out of the original 25 transmission cover by only an extra inch or two. What you would do aft of the GM transmission without transfer is up to you, I believe they did come with a parking brake similar to the 25s...

Of course, with an FJ40 3- or 4-speed bell housing paired to the F135 in a 25 the choices of transmissions are much greater...

 

[FJnotCJ]

I had to do it. I took a bare 25 transmission case and installed the GM gears in it along with the modified mainshaft. Had to use the input shaft bearing for the 25 on the GM input shaft, no groove on the GM bearing, but all else fit well. I removed the shift forks and rods from the GM trans cover and they fit in the 25 trans cover no problem. A GM shift stick can be modified easily to replicate a 25 shift stick, the GM is just a bit longer and can be ground down. So, if you have a lump of rust inside a 25 trans, this may be an option if you care to cut up your 25 mainshaft and don't want to bother with new-fangled "synchromesh". If you have a lump of rust in your 25 tcase this will still allow you to run around in 2wd. Bonus 7:1 or so ratio in first...

 

[Cruiser_Nerd]

Phil, the only thing that amazes me more that you spent the time figuring out how this was put together is.............................someone spent the time figuring out how to put it together in the first place. Had to been someone who was really familiar with the GM box and was saying all the same things.

Maybe I'll play with the box in the Chevy, that spliced shaft looks pretty scary though.

 

[Splangy]

I got my transmission back together after finally sourcing some new synchros, springs, and mainshaft needle bearings. Everything now shifts and sounds as well as an FJ25 transmission is going to.

One thing that donned on me while putting it back together is that you do not want to drive with free wheel hubs on an FJ25. If you don't keep the front driveshaft turning, the transfer bearings will never get lubricated. It might not be the end of the world for the idler and front output bearings since they wouldn't be moving, but the rear output would never see oil if the front drive gears aren't moving and throwing oil around. I don't remember seeing this discussed before and I can't believe it didn't don on me the first time I had it apart. Or maybe I'm crazy and I'm missing someone here.

 

[AD Adel]

The countrshafts are of similar construction, and I think the GM second gear could be used in the 25 with minor machining, same tooth count and diameter, just a little too thick...shaft diameter is the same, bearings and thrust washers are the same.
View attachment 63458View attachment 63459

The countrshafts are of similar construction, and I think the GM second gear could be used in the 25 with minor machining, same tooth count and diameter, just a little too thick...shaft diameter is the same, bearings and thrust washers are the same.
View attachment 63458View attachment 63459

I had hopes of using the GM innards in the 25 case, not only for parts availability, but a possibility of a lower 1st ratio....twas not to be....you can see that Toyota kept the same basic design and construction but refined the gears with synchros in 3rd and 4th

25 innards below, GM belower
View attachment 63454View attachment 63455

Dear Sir, since i saw this, im trying to understand the chevy tranny comes out of which vehicle exact, i kept searching but no luck, i have fj25 and most of parts are damaged and stuck with parys, i need some help to rebuild the tranny.. pls assist me with some refernces ie matching gear model number or any reference to make search easy.. appreciate it..