3sp to 4sp swap

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

W

Wheelytech

Joined
Aug 31, 2026
Threads
6
Messages
51
Location
Washington
First, I’ve tried searching. Some of the old links in the tech section don't work

Hey all, once again I’m getting ahead of myself in my new but what will be very long build.

My rig new to me.
9/73 74 FST fj40 2f engine, 3sp trans.

What would it take to swap in a 4 speed in place of the current 3sp. Can I retain the transfer case?

I’ve heard the 3 and 4 take different bell housings? I dont know what housing was used when they swapped in the 2f.

I know that 30years ago when I was in high-school and I melted down the transfer case (apparently they need oil) in my 78 4sd fj40 we used a conversion gear and a 3 speed transfer case.

So I’m guessing this is a simple question for you all.

If I’m looking for the parts to swap in a 4speed. What am I looking for?

Feel free to point me to an old thread.

Thx.
 
Bell Housing is different - 4 speed one is dowelled to the tranny. 3'sp transfer case will work on a 4 speed tranny with an adapter shaft Mark's off road had/have it. You will get about a half gear lower in low range low, high high range is the same 1 to 1. A five speed with over drive is a far better choice.

4 sp tranny is longer than the 3 sp. Tranny hump and drive line length mods needed.
 
charliemeyer007 said:
Bell Housing is different - 4 speed one is dowelled to the tranny. 3'sp transfer case will work on a 4 speed tranny with an adapter shaft Mark's off road had/have it. You will get about a half gear lower in low range low, high high range is the same 1 to 1. A five speed with over drive is a far better choice.

4 sp tranny is longer than the 3 sp. Tranny hump and drive line length mods needed.
Click to expand...
That’s great info I didn’t even think about I assumed the top end ratio would be a little over drive. I guess that’s what i get.
I’m going with an aqualu tub which has its own tunnel.

I understand that a 5 speed is much more in mods is that right.

Thanks!
 
Wheelytech said:
That’s great info I didn’t even think about I assumed the top end ratio would be a little over drive. I guess that’s what i get.
I’m going with an aqualu tub which has its own tunnel.

I understand that a 5 speed is much more in mods is that right.

Thanks!
Click to expand...
33s with 3:70 rears is a far cheaper and easier mod, that nets nearly the same results.
 
I don't think its a lot more to do a 5 speed. I bought an Advance Adapter Chevy V8 bell housing with attached Toy 4 sp and its transfer case. I wanted to build a Corvette 283 for it.
 
brian said:
Shouldn't need changed in a 74.
Click to expand...
I definitely had to get a new transmission hump when I went from 3sp to 4sp on my 10/73. Found a 1975 hump that worked.

But I did the J30 to H42 using the 4sp bell housing, kept the 3sp clutch (I had it. It works. Didn't see the need to change it), and 3sp tcase with the adapter gear from Marks Off Road (call him)

I did need to remove a cross member tube due to the extra length of the transmission to clear the tcase rear output. I was going to make the cross member tube a bolt on by welding plates on the frame and ends of the tube. But ended up just recycling the cross member tube. Not the only FJ40 that I know of with that cross member missing after a transmission swap. It's fine.

Very happy with the swap. Almost as life changing as front disc brakes. Almost
 
Thanks everyone. My questions have been answered. At this time I have no reason to swap. In my pee brain I was hoping for. Little longer legs on the highway as I start from scratch on this build.
I was educated that a 4speed will not do that for me.
thx
 
Biscuit said:
I definitely had to get a new transmission hump when I went from 3sp to 4sp on my 10/73. Found a 1975 hump that worked.

But I did the J30 to H42 using the 4sp bell housing, kept the 3sp clutch (I had it. It works. Didn't see the need to change it), and 3sp tcase with the adapter gear from Marks Off Road (call him)

I did need to remove a cross member tube due to the extra length of the transmission to clear the tcase rear output. I was going to make the cross member tube a bolt on by welding plates on the frame and ends of the tube. But ended up just recycling the cross member tube. Not the only FJ40 that I know of with that cross member missing after a transmission swap. It's fine.

Very happy with the swap. Almost as life changing as front disc brakes. Almost
Click to expand...

I plan a four for my 2/73 FST. I wouldn't need to change tunnel since 9/72-1/79 the outline is the same. 9/72 the shape of the torque tube behind the transfer case changed. It was a little further back and arched where the drum for the hand brake is on the back of the transfer case. Transmission tunnel and torque tube changed for a four speed which started 9/72-9/73 with a four speed used in the HJ45. 9/73 a redesigned four speed came out. That included two versions. The H41 which was closer gearing to the earlier four speed. Plus the H42 which is more street friendly gearing. For some reason out 74 model did not change until 1/74.
 
Thats where the fun is with the late 1973 build dates for US FJ40s. We get all the mishmash of stuff.
 
On my ex-three-speed '73 I hit the perfect sweet spot for my needs: an H41 transmission with split transfer case and Sumo low range gears, plus a swap from the stock 4.10 diffs to 3.70. Now I turn a comfortable 2,500 rpm on the highway, yet have a 54:1 first low compared to the factory 20-something:1
 
You must log in or register to reply here.

Share this page

IH8MUD Forum | Toyota, Lexus & Off-Road Community
Community platform by XenForo® © 2010-2026 XenForo Ltd.

© 2000-2026 Tie Rod Media LLC

Back
Top Bottom