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Swap 3 speed for a four speed

onpier55

Full Member
Joined
Oct 19, 2017
Messages
339
Loc.
Roseville Ca
I just picked up my 1967 bronco. No motor in it. It has a 3speed manual. I would like a four speed. How many mods will I need to do? Drive shaft cutting? It came with two motors. 302 and 351w. Vin says 289 original. Not stock. Came with a new 3.5” suspension lift not installed yet and a 2” body liftbfor installed yet. I’m leaning towards the 351w. Any advice would be appreciated. Haven’t decided on which four speed yet. Still doing research and depends on what I find used.
 

Sporto

Sr. Member
Joined
Oct 10, 2005
Messages
904
Welcome to the addiction...Most of the vendors have complete 5spd conversion kits (including a new transmission) that is a very popular upgrade, but if you want the 4Spd there is also a very common conversion using the later model Bronco/Pickup 4spd, which is similar to the three speed (no OD), but has a really low granny gear. So I suppose it depends on what you intend to do with your Bronco. Street or off road.

Search around on this site and you will learn a lot about these conversions, each have their own pros and cons...

Good luck

Rich
 

norm02

Full Member
Joined
Feb 8, 2016
Messages
336
I would go 351W and a 5-speed.


Sent from my iPhone using Tapatalk
 

Apogee

Contributor
Bronco Guru
Joined
Nov 26, 2005
Messages
6,033
Will the five speed to have a low enough first gear for off-road hunting? In your opinion.

It depends on the 5-speed. If you go NV4500 (5.61 or 6.34 1st gear ratio) or ZF (5.08 or 5.72:1 f1st gear ratio), then first gears are considered a granny gear, and are not generally used on the street unless you're on a serious hill, pulling a trailer, or both.

The lighter-duty transmission options include the NV3550, AX-15, M5OD, etc, and those have much taller 1st gear ratios, quicker shifting and less torque capacity.

On the less expensive side of things, the NP435 is a popular transmission swap to gain a granny gear over the stock 3-speed. It's a heavy-duty truck transmission, virtually indestructable, but has a non-synchro 6.69:1 1st gear with basically a 3-speed after that. I believe the T-18 or T-19 could be a similar option, but with a synchronized 1st gear, but I only have experience with the NP435.

As noted above, what's the intended use for the rig and budget to get there?
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
So why do you want to change the transmission? Your comment about a low enough gear makes me think you just want to add a granny gear. In that case the NP435 is a pretty straight forward swap It drives just about like the stock 3-speed but you gain a granny gear.

But if you want to close up the gaps between gears, the 435 isn't going to do anything for you. That is where you start looking at the 5-speeds. But the 5-speeds have an overdrive. And now you are getting deeper into overall gearing and tire size as well. The Bronco is a brick wearing a parachute on the highway. You can't treat it like a modern muscle car and expect it to idle down the highway just off idle. General rule of thumb is with an overdrive transmission you should aim for about 2000 RPM at 60 MPH. The smaller the engine the higher the RPM, a big inch engine that has a lot of torque just off idle can run less RPM.

So you need to find your gear ratio and tire size you plan to run. It may be a 5-speed will be useless or (more typical) require axle regearing in order to be able to use it.
 

Nothing Special

Sr. Member
Joined
Nov 25, 2016
Messages
804
....I believe the T-18 or T-19 could be a similar option, but with a synchronized 1st gear, but I only have experience with the NP435....

Only the T-19 has a synchro'd 1st gear. The T-18 is functionally pretty indistinguishable from the NP-435, except that the T-18 came in a close-ratio version with a 4:1 1st gear as well as the wide ratio with a 6.31:1 1st gear

....The Bronco is a brick wearing a parachute on the highway. You can't treat it like a modern muscle car and expect it to idle down the highway just off idle. General rule of thumb is with an overdrive transmission you should aim for about 2000 RPM at 60 MPH. The smaller the engine the higher the RPM, a big inch engine that has a lot of torque just off idle can run less RPM.

So you need to find your gear ratio and tire size you plan to run. It may be a 5-speed will be useless or (more typical) require axle regearing in order to be able to use it.

Good advice. My Bronco is running 4.10 gears and 33" tires. With the NV3550's 0.78:1 overdrive it puts me at 1950 rpm at 60 mph, which works really nice. Even with the stockish 302 I start in 2nd gear and it'll pull OD from about 35 mph if I'm not in a hurry or towing a trailer. And if I am in a hurry or towing I can start in 1st and shift later.
 

El Kabong

Contributor
Driving stuff Henry built
Joined
Oct 8, 2009
Messages
1,487
I did the NP 435 swap with a gearbanger shifter. I wanted the granny gear for playing offroad. As others have said, knowing your intended usage will help to get recommendations that fit your needs.

The NP 435 is similar length to the stock 3 speed, so some of the swap is simpler. The same driveshafts work, unless you are adding enough lift to need longer d-shafts anyway. But the shifter comes thru up under the dash, so it needs to be bent back quite a bit, or replaced with a gearbanger type shifter to work. It also requires a good sized hole in the trans tunnel, but vendors that sell the gearbanger also offer sheet metal to cover that. A taller body lift helps too. Mine has a 1 inch BL & required a pretty good hole. It is also a heavy feeling truck-like shift, so if your'e looking for good road manners, you might want something smoother.

If you do choose the NP 435, be aware that there are different versions & not all will work. Dodge & GM are a no go. Ford married requires an output shaft swap, while Ford divorced or 2wd is easier. Novak's site has a lot of info regarding identifying the correct tranny.
 
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onpier55

onpier55

Full Member
Joined
Oct 19, 2017
Messages
339
Loc.
Roseville Ca
Thanks for all of your input. As far as budget, a five speed and the adapters are not in my budget, I don’t really care about overdrive at this point. From what I take from all of the input, I take it that I only need the granny gear for extreme 4 wheeling. Rock crawling is not my gig. Pretty much want to build a hunting rig for my son and myself. Thanks again.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
The granny makes a nice off-road rig. Crawling generally involves moving to an automatic and lower gears in the axles and transfer case.

Budget in mind, I would recomend the NP435. That was a pretty fun transmission back when I had one. With a set of 33" tires and 4.11 gears I did highway trips, hill climbs, trail rides, and a little bit of rocks. Lot more rocks once I went to automatic and geared everything else down.
 

Pa PITT

Contributor
Bronco Guru
Joined
Jul 15, 2005
Messages
11,252
Loc.
Stephenville TEXAS
I have the NP435 in my 72. I also have the 4BTCummins ... I don't take mine on the hwy but just a few times each year.
You mentioned wheeling Which is about what I use mine for most of the time. My wheeling is very light .. As in just checking cows in my pastures. Mine has 3:55 gears .. So I'm in 2nd gear to go to the pasture .. good quality speed .. Then when I get into the cows I'll drop it into LOW Range of the Transfer case ... Slows me down enough to get a good look . Then a lot of times while going across the back I like to be in 3rd gear
But also in low range on T/C .
... Using my T/C I split between the gears . But I would like to have a 5th gear.
My Cake & eat it too.
 
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