3spd Vs 4spd Trans And Is It Adaptable?

[Wacko]

I Just Bought A 67 Fj40 And Have Lots Of Questions, And You Guys Look Like You Have The Answers......

The 40 I Bought Has A 3spd Trans In It And Seems To Only Get To 65mph. I Heard This Is Pretty Normal For These Vehicles, But I Would Like To Get To 75mph Do To I Drive Some Freeways And Am Likely To Get Ran Over.

Is It Possible To Switch Over To A 4spd Without Changing The Transfer Case And Would The 4spd Make A Difference?

Any Help Would Be Appreciated...

Thanks!

 

[CruisinTiger]

Yes you can switch to a 4spd tranny with a 3 spd transfer case...you have to replace the spline between the tranny and case..very easy to do. You have a 67 so you have an f engine...I know people have posted up here how fast they can drive there 40...but I would be cautious about drivin 75 mph with an f engine...had a buddy blow his engine this way...although his circumstances may of been different than yours. He has a 4spd in his...I have a 67 also and I'm about to put in a 4spd trranny and case, because I have them both and don't wanna use a 3spd tcase. The 4 spd will give you a higher gear to drive on the interstate...definately get a tune up done on your engine so you don't blow your engine. I've only had mine out a few times...runs about 60 or so..but that's all I want it to...mostly a trail rig. Where are you located? Good Luck Chuck

 

[cruiser_guy]

Changing to a 4 speed tranny will make absolutely NO difference to your top speed as both have a 1:1 ratio in the highest gear.
Your options are, larger tires, different diff gears or a Fairey or similar overdrive (very rare).

 

[Trollhole]

Maybe I'm wrong Chuck but a 4spd conversion will only need the tranny, flywheel, clutch from a 4spd to do the swap. Along with slavecylinder and new clutch line.

Im not aware of any splines that need to be changed and if you did for the front shaft on the clutch you are probably going to come out cheaper buying 4spd parts than getting someone to change out the input shaft on a 3spd. And I also thought output shafts on both trans were the same. I thought they changed on split cases. And changing splines IMO wouldn't be easy. It's taken me 3 months just to get the balls to start ripping into a 4spd to learn how to rebuild it.

I also thought that both final drives for a 3spd and 4spd were 1 to 1.


Either way the 3pd transfer case should work with whatever option you go with. IMO

You are better off getting bigger tires. A lot cheaper way to increasing hwy drive ability. Or you could change rig and pinions.

 

[Living in the Past]

The three speed and the four speed both are 1:1 top gear. The four H42 Is 3.55:1 first gear or close, the three speed is 2.87:1 or close. The four speed is a little lower first gear better slpit between gears and fully synchronize, but not higher highway speeds.

Quote cruiser_guy Fairey or similar overdrive (very rare).

Not for some people

John

 

[CruisinTiger]

opps my bad on the speed thing....not sure where I heard it at....maybe from a 4spd to a 5 spd..not sure. I thought I read on here that you had to change the shaft that went from the tranny to the t case cause the 3 spd tcase is different from the 4spd tcase. But you may be right marshall it may be on a split tcase that I am thinking about. I'll have to do some searching on here and see if I can't find out what I've read.

Am I thinking....even though the 3spd and 4spd are 1:1 the 4 spd is better because you have that 4th gear...therefore you are putting less strain on the engine when you drive at higher speeds...lower rpm's with the 4spd? Or am I totally talkin out of my ass....I'll do some research and hopefully straighten myself out...lol.

 

[Trollhole]

Or am I totally talkin out of my ass....I'll do some research and hopefully straighten myself out...lol.

ass

 

[CruisinTiger]

Here I found this on the 4spd to 3spd tcase hope this helps.

https://forum.ih8mud.com/showthread.php?t=33881&highlight=4spd+tranny+3spd+tcase

And here is one on how fast others drive there 40's


https://forum.ih8mud.com/showthread.php?t=36502&highlight=fast+40

 

[nuthinbutcruisers]

I'm in the middile of this (in my spare time) swap on a 55 right now. To do this swap, F engine, 3 speed, 3 speed T case to F engine, 4 speed, 3 speed T case; you will need;

A 4 speed bell housing

a "transition" Tcase input gear (16 spline)

PTO spacer (PTO gaer has 10 splines)

a "transition" rear Tcase bearing (same OD, larger ID)

a rear spacer (to accommodate the larger diameter end of the 4 speed output shaft)

a stake nut for the 4speed output shaft ( the output shafts threads are different)

new or used 4 speed bolts to mount the Tcase (the 4 Speed rear bearing retainer is thick plate that covers the entire back of the tranny so the bolts need to be longer)

gaskets and seals.

Drive shaft lengths change

Note: The transmission output shafts changed from 10 splines (3 speed) to 16 splines (4 speed) they also changed shape at the tail end, and changed spline count again to 19 splines (4speed w/split Tcase)

other considerations:

Tcase linkage; I'm using the 3speed Tcase linkage. Modifications are required.because the lever mount is in a different location on the 4 speed.
Modifications:

1) the shift lever guide top mounting ears were cut off. A 3/16 piece of flat stock was drilled to match 2 of the tranny top cover bolt locations and welded to the shift guide

2) swapped the 2 wheel drive/4 wheel drive selector shaft on the 3 speed
T case with one from a 4 speed T case. This places the shift lever in a correct horizontal location. this leaves the lever slightly too high and too far out by approximately 1/2" in both directions. (i'm still working on this)

3) The high range/low range linkage bar (has an "S" shape) was replaced with a 5/16 rod with a bolt welded to each end. (tacked in place)

That's where I am in this swap so far. other Items of note....

Several comapanies have the Tcase input (transition) gear in a kit form

I'm not sure if the motor mounts are the same beteween bell housings/frames

I've also heard that the clutch slave cylinder is mounted different but again I'm not positive.

Flywheel/Clutch... you can use either flywheel F or 2F but you need to use the clutch that mated with that flywheel. (I went with the 4 speed flywheel/clutch.

That's all I can think of so far. There are other options for linkages but this is what I'm doing. YMMV

Paul

 

[Living in the Past]

Quote: nuthinbutcruisers
I'm not sure if the motor mounts are the same beteween bell housings/frames

They are same


I've also heard that the clutch slave cylinder is mounted different but again I'm not positive.

Nope their the same too.

The biggest pain to me is that the three speed is three and a half inches shorter than the four speed. It just about hits the crossmember behind the transfercase. Not very easy to change the clutch after it's installed. Easiest way to work on it is to pull the whole engine otherwise you have to support the rear of the engine and remove the rear motor mounts and drop the back down to clear the crossmember.


John

 

[cruiser_guy]

Am I thinking....even though the 3spd and 4spd are 1:1 the 4 spd is better because you have that 4th gear...therefore you are putting less strain on the engine when you drive at higher speeds...lower rpm's with the 4spd? Or am I totally talkin out of my ass....I'll do some research and hopefully straighten myself out...lol.

1:1 is 1:1 makes absolutely NO difference what gears it's going through or how many other gears are in the gearbox. The 3 speed in 3rd and the 4 speed in 4th are EXACTLY the same load on the engine.

 

[Trollhole]

Well now that you know what is involved. Those tires sound pretty good. And 75 for a 40 is pretty fast. You sure you bought the right vehicle?

I drive 65mph and thats the fastest I would ever want to go in a 40 and thats with 33" tires. And nobody is going to run me over.

 

[cardinal fang]

Common mistake made when putting the 3 spd TC on the 4 spd tranny: You must install a small spacer that goes over the 16 spline input shaft and between the front of the 74 year gear and the 4 spd from housing. If that spacer is not installed the 74 year gear will ride up against the inside of the tranny case and ruin the case and gear. Don't remember the size of the spacer, but it is not very thick, just enough to keep the 74 gear from rubing against the tranny.

 

[Wacko]

Thanks Guys..............I think Trollhole is right, Bigger Tires Sounds Like The Ticket!

 

[CruisinTiger]

Well now that you know what is involved. Those tires sound pretty good. And 75 for a 40 is pretty fast. You sure you bought the right vehicle?

I drive 65mph and thats the fastest I would ever want to go in a 40 and thats with 33" tires. And nobody is going to run me over.

Ditto on that one!