25 Transmission's GM roots

[FJnotCJ]

Most have noted the likeness between the early F and GM I-6s, I guess I shouldn't have been so suprised when I found the 25 trans to be closely related to an early (pre-47? or Father of SM420) GM four speed

The upper box is a 25, lower is GM...cases are nearly identical except threading and drain/fill plugs, the covers are close. This GM trans last saw service outside of a truck (I'm guessing), that's why the shifter is bent backwards....

 

[FJnotCJ]

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

 

[FJnotCJ]

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.

 

[FJnotCJ]

Other interchange possibilities: reverse shift fork, reverse idler shaft and bushings, and that's about it...the GM bolts right up to the 25 bellhousing, has the mounting lugs for the parking brake, but no option for a transfer case as far as I can tell, not a concern for one PO whose work I "undid"

 

[Cruiser_Nerd]

...shaft diameter is the same, bearings and thrust washers are the same.

Thank you Phil!

I believe the Chevy truck tranny was the M194. I'll find out for sure now.

 

[Cruiser_Nerd]

Oh, and thanks for the great writeup and photos.

Like you said, I knew they were close, but now it's here for everyone to see. Also saves me from pulling the tranny from the doodlebug and getting my wife all excited thinking I'm working on it.

 

[FJnotCJ]

Tried to post numbers in a readable format above, no worky right.....

25 countershaft diameter, front (4th) 38.05mm
GM " " " 37.9 mm (worn bad)

25 Cshaft dia rear (1st) 43.97-44.0mm
GM " " 43.95mm (worn)

25 cshaft bearing F NSK R238 od 62mm
GM " Hyatt Orb 1206 (maybe 1208?) od 62.02mm

25 cshaft bearing R NSK R244A od 72mm
GM " Hyatt Orb 1207 od 72mm

25 cshaft thrust washer 3.4mm thick
GM " 3.2mm thick (again, worn)

The input shaft to mainshaft roller bearings are way different between the 25 and GM box

 

[Cruiser_Nerd]

Missed the GM rear cshaft bearing OD.
After you get that, call it a day, hit the hottub.

 

[FJnotCJ]

Fixed.....since I have two of these GM trannies, does this make me a doodlebug parts hoarder also? If yes, what is a doodlebug and where can I get more?

 

[Cruiser_Nerd]

Just to post this somewhere for record:

Needle bearings in input shaft measure:
.746" long x .220" diameter (good) /.217" (well worn)

Transmission input bearing: SKF 6209NRJEM (6209 w/ a snap ring)
Transmission output and transfer front output bearings: Nachi 6307ZZE (6307 double shielded, remove one shield)

I'll put the doodlebug info in the chat thread.

 

[Cruiser_Nerd]

I tried these guys for the countershaft bearings, let you know how it works out:

http://www.obschevy.com/

Part # 142260 is a C1206 interchanges w/ Hyatt Orbital 1206
Part # 121856 is a C1207 interchanges w/ Hyatt Orbital 1207

 

[Cruiser_Nerd]

Well it appears I cleaned Obsolete Chevy out of Part #142660 the 1206 Hyatt Orbital front countershaft bearing. They did scrounge one up and I got my package with a front and rear NOS Hyatt countershaft bearings today...........................which just so happens to match the FJ25 bearings exactly.

No pictures, I was so excited I ran to the garage and assembled a tranny.

I ordered a couple front countershaft bearings from Drivetrain Specialists let you know how that works out.

 

[Cruiser_Nerd]

Well, that worked out excellent. The front countershaft bearing is a Bower M1206EL. I quick look at the Bower catalog shows the rear countershaft bearing would be a Bower M1207EL. Why the bearing shop couldn't figure this out I have no idea.

Drivetrain Specialists www.drivetrain.com was great, knew their stuff, no B.S., and cheapest by far. I dealed with Bart...........gave him the number and dimensions and he had someone pull the part and check it out. If I had to do it again I would just give them a ring with all the numbers.........first.

Now to get this thing off my bench.

 

[FJnotCJ]

How did the pressing off then on of gears go? Did you re-use those four pins or were you able to find new ones?

 

[Cruiser_Nerd]

I reused the keys. Mine were fine. Yours were beat up a bit, there was a little slop where the big spacer was. The countershaft is on the way back to you, I was thinking with a some machining and spacer work you could interchange reverse (which was good) for second because they have same tooth count, at least getting you a good 2,3,4 and marginal 1 and R ............. not bad for the street.

 

[Cruiser_Nerd]

Something to look out for if you have one apart:

Two of the three trannies had this problem with the 1-2 sliding gear on the output shaft. First gear was loose where it is riveted onto the hub. This one was real bad and I didn't use it, it was slopping back and forth about 1/8", the other just had a little wiggle, so I just repeened the rivets.

 

[pardion]

The gear to hub slop is also very common with the fj40 early transfercase lo out put gear, and peening or presing the rivets gives the gear new life. John#3

 

[FJnotCJ]

Thought I'd dredge this thread up for a little update...

A while back I got ahold of a GM trans/25 tcase combo and just now got around to poking at it. Seems that this particular trans probably had been part of a fwd application, it has a long output shaft, longer than the ones I took apart (above). This output shaft mates up to the 25 tcase input gear and has the length and spline count to receive the 25 drive shaft flange. This opens up a few options, such as GM innards in a 25 trans, GM trans/25 tcase, or maybe GM trans/GM tcase (if Toyota copied the GM tcase also)

This GM trans was nearly a drop in solution for the prior owner, he had to modify the in-out lever to emerge through the trans cover just to the right of the gear shift lever and wallowed out a few existing holes in the tcase to mount it up to the trans, allowing for different fasteners...View attachment 883517View attachment 883517

 

[FJnotCJ]

Innards, no synchros, but bone simple....

 

[FJnotCJ]

stock vs modified in-out lever, this could probably be done better...