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NP435 Swap (1966-1977)

Swapping a NP435 tranny into a Early Bronco

This conversion is aimed primarily at those Bronco enthusiasts who have the stock 3 speed manual transmission in their Broncos. Owners of automatics would have to figure in the cost and effort of installing a stick shift bellhousing, flywheel, clutch and related linkages.

Most Bronco owners, I think, have at times found the 3 speed transmission less than adequate. At the very least the column shift mechanism that comes standard lacks the proper macho image a good many 4 wheelers want. Personally, I often wished I had a lower first gear. I can think of other times, but the situation that finally made me decide on the 4 speed was driving the Rubicon trail in central California. Low gear in the 3 speed and low range in the transfer case got me over the rocks and obstacles, but it required powering over them in a manner that, as my rocker panels can attest, often resulted in sheet metal damage or worse. Steep down hilling is also helped considerably with the lower first gear of this 4 speed. With the 3 speed, low gear low range is still not low enough to keep the vehicle from over speeding the engine on steep descents. This results in white knuckles on the steering wheel and the need for a very light foot on the brakes. If you lock up the brakes, you lose steerage and then you have real trouble. With this 4 speed in low gear low range, you will never need to touch the brakes no matter how steep the down hill. Getting started on an up hill grade is also made much easier because the vehicle can be started in gear without using the clutch.

If your only problem is image, then buy the commercially available floor shift conversion; this 4 speed conversion is not for you. If, however, you've ever wondered if there wasn't something better, read on.

The transmission I chose to use is the New Process 435 truck 4 speed found in Ford 1/2 ton 4x4 pickups from 1966 to 1975. This is the pickup that came with the single speed transfer case. (Others will work also, but this one is usually the least expensive.) The reason for choosing this transmission is no great mystery, the front of it already matches the bellhousing and clutch on the Bronco. In addition, the splines on the output shaft match the splines on the input shaft of the Bronco transfer case. Finally, and not to be overlooked, there is a commercially available adapter designed to put this transmission into an early Ford Bronco. The commercially available, though, is expensive. Make that EXPENSIVE to the tune of almost $500.00. In my area of the country and I expect most areas outside of California, the adapter is significantly more expensive than the transmission itself. After some investigation I decided I could design and build a considerably cheaper adapter that the one I could buy.

Actually, because the 435 transmission rarely fails and there is relatively little demand for this version, the prices I found on it were quite reasonable. I suspect that wrecking yard operators might even be willing to haggle some since the ones I found seemed well covered with dust.

As stated before, what makes this a reasonably easy conversion is that the 435 transmission already mates to the Bronco bellhousing. Also, its output shaft matches the transfer case input shaft. The only problem left is to somehow join the transmission housing to the transfer case housing. Ford does this in the stock situation by marrying the 3 speed to the transfer case with what is referred to as the intermediate housing. What the commercially available adapter does is replace the stock intermediate housing with one that matches the Bronco transfer case on one end and the 435 transmission on the other end. I approached the problem form a slightly different angle.

Bronco enthusiasts are probably aware of the fact that a small percentage of Broncos were sold with six cylinder engines. What is probably not such common knowledge is that because the six is longer than the V8, the transmission sits approximately 2.5" farther back in the chassis that in the V8 version. The transfer case and the rear crossmember, however, sit exactly in the same location in both versions. This is accomplished by having a shorter intermediate housing in the six cylinder version. This shorter six cylinder intermediate housing is the key to my inexpensive conversion. By matching the shorter six cylinder intermediate housing to the 435 transmission with a properly machined adapter plate a package is created that is the same length as the V8 3 speed and the V8 intermediate housing. It bolts directly to the Bronco transfer case and no modifications are necessary to either the crossmember, the driveshafts, or the transfer case shift mechanisms.

The six cylinder intermediate housing, however, is only half the key. The other half is the plate that matches the intermediate six cylinder intermediate housing to the back of the 435 transmission.

As stated, no driveline modification are necessary. However, a good sized hole does have to be cut in the floor and some means of covering it fashioned. I created a cover out of 20 Ga. Galvanized sheet metal and attached it to the floor with sheet metal screws. As an additional advantage to this conversion, I now have a removable tunnel cover similar to most pickup trucks that will make future clutch work a lot easier.

Finding the shorter six cylinder intermediate housing may or may not be an easy job. Any wrecking yard that has wrecked out a number of Broncos has probably had them at one time or another, but due to the fact that the demand is probably zero they may have tossed them in the scrap metal pile. If you are lucky enough to find one in the same yard you find the 435 transmission, you can probably haggle the yard into throwing it in for nothing. I personally got three of them that way. Don't expect to find them on the wrecking yard's inventory system. They will either still be attached to six cylinder transmissions or transfer cases, or they will be thrown in a general pile of 4x4 intermediate (adapter) housings. Familiarize yourself with what your V8 housing looks like and then look for one that looks nearly the same, but measures only 6.5" long from mounting flange to mounting flange.

Even though this conversion was carefully planned before any actual work was done I was still surprised by how easily it went together and how well it works. The New Process 435 transmission is a short easy shift transmission and is gorilla strong. The gear ratios in this version seem excellent for the application. I believe, with the possible exception of power steering, this is the most significant modification that can be performed on an early model Bronco.

Ratio Comparisons Bronco 3 speed to 435 4 speed

1st gear 2nd gear 3rd gear
3 speed 2.99 1.77 1.00
1st gear 2nd gear 3rd gear 4th gear
4 speed 6.69 3.34 1.66* 1.00

Final drive ratio with transmission in first gear and transfer case in low range, 72 or earlier 2.46 transfer case. (73 or later 2.3 transfer case will result in a slightly higher gear ratio, "lower numerically".)

3.50 axle gears 4.11 axle gears
3 speed 25.6 to 1 30.1 to 1
4 speed 57.4 to 1 67.4 to 1

  • 3rd gear ratio in 435 4 speed changed to 1.79 sometime in 1977. My preference is to stay with '76 or earlier transmissions.
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