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The Novak Guide to the
Borg-Warner T90 Transmission

The Jeep T-90 TransmissionThe Borg-Warner T90 transmission was one of the most common transmissions found in 1946 to 1971 Jeeps. Because of its long production span, it is nearly legendary. It is a good transmission when well maintained and has often been adapted to V8, V6 and I4 power.

The T90 was the improved progeny of the T84, of military MB & GPW fame. The T90 was the standard three-speed as found in CJ Universals, Utility Trucks & Wagons, J Series and Forward Control Trucks.

Features
The T90 is a top loaded, top shifting or side shifting transmission.

The T90 is fully synchronized in the second and third gears. All gears are helically cut except first and reverse, which are spur gears.

Identification
The T90 transmission is 9" long and features a cast iron top cover that is retained by six bolts and a main case of cast iron. The case itself has a prominent bulge on the driver's side with two protruding bosses as provisions for side-shifting actuators.

T90 Side-Shift version
The side-shift or column-shift version of the T90.

All 4wd T90 transmissions have a 1-3/8" x 6 spline output shaft for mounting the transfer case input gear. A 2wd version of the T90 does exist and was installed in some Jeeps but is a rarity.

Jeeps also had a T86 transmission that appears similar to the T90. Many parts interchange, however first & reverse gears are helically cut in the T86 in lieu of the spur cut gears in the T90.

There are three major versions of the T90 to differentiate.

T90A1
The T90 used from 1946 to 1962 with the L and F-head four-cylinder engines is designated as the T90A1. These versions all feature top shifters, a 7" input shaft (stick-out length) with 15/16" x 10 splines and a felt-sealed front bearing retainer assembly. The input shaft has an 18 tooth, helically cut input gear. The cluster gear has a 33 tooth driven gear, resulting in a 2.79:1 first gear and a 1.55:1 second gear.

Jeep T90 Specifications

T90C
The T90 used from 1963 to 1971 with the F-head four-cylinder engine is designated as the T90C. It features top shifters, a 7" input shaft (stick-out length) with 15/16" x 10 splines and a felt-sealed front bearing retainer assembly. Since the T90C uses a case marked "T90A", you will need assurance that the transmission in question came from a post-'62 Jeep, or you will need to pull the top cover and count teeth. The T90C has a 16 tooth, helically cut input gear. The cluster gear has a 35 tooth driven gear, resulting in a 3.34:1 first gear and a 1.85:1 second gear.

T90J
The T90J was used with the six-cylinder trucks from 1952 to 1965. These were either column or floor shift models. They had an ~9-1/4" input shaft (stick-out length) with a 10 x 1-1/8" splines and a neoprene-sealed front bearing retainer assembly. The T90J has an 18 tooth, helically cut input gear. The cluster gear has a 33 tooth driven gear, resulting in a 2.79:1 first gear and a 1.66:1 second gear.

T90 Shifter & Top Covers
The column shift T90J can be easily converted from column (or "transmission side shift") to floor shift ("top shift") by installing a floor shift top cover assembly.

The T90 Reverse Idler

T90's were produced with 16 tooth reverse idler gears up to 1965. Models afterwards were produced with 15 teeth reverse idler gears. These gears do interchange perfectly well.

The T90 features a conventional "H" shift pattern, with reverse being towards the driver and up.

The M38 and M38A1 Jeeps have basically the same T90 3 speed transmission as CJ's but they have two plugs that screw into the front of the shifter cover rail bores that protrude far enough to hit the back face of some conversion applications. They may have to be modifed and plugged as appropriate.

Transfer Case Compatability
The Jeep T90 was factory-married to the Jeep Dana Spicer Model 18 transfer case, and only the small input bore versions.

T90 was available attached to the Dana 20 in the 1962-1965 J Series truck.

It is possible to join a T90 to the Dana Spicer Model 20 transfer case by using a bearing retainer adapter, bearing and snap ring assembly #90-20, provided by Novak.

A T90 ready for a GM engineEngine Compatibility and Adaptability
GM
Buick V6, Chevrolet Small Block V6 and V8 conversions to the T90 are done with steady frequency and success.

Ford 2000 & 2300 CC I4
A cult classic of swaps, the Ford Pinto, Mustang II, Ranger and other similar Ford I4 engines are adaptable to the Jeep T90A and T90C.

AMC Jeep
The T90 and AMC eras are anachronistic and the transmission was never joined to an AMC engine. There are neither factory nor aftermarket provisions nor compelling reasons to do so.

Rebuilding the T90
The T90 is simple and enjoyable to rebuild. Many shadetree mechanics do very outstanding rebuilds if they have access to a press, snap ring pliers and bearing pullers. Many choose to do a full rebuild during the adaptation process, and our instruction guides feature all the details, diagrams, pics and tricks required to do professional level work.

Rebuild and repair components for the T90 Transmission
When filling your T90 with gear oil, we recommend that you select a conventional mineral oil or a para-synthetic in lieu of a full synthetic oil. Properly assembled manual gearboxes do not have the thermal strains seen by combustion engines or hypoid gears. Synthetic fluid in these gearboxes, while not harmful, is probably an economic waste.

Hypoid gear oil is sulphurized higher than transmission oil and can be mildly corrosive to the non-ferrous alloys used for synchros, bushings and thrust washers in these transmissions.

An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated unless you operate your vehicle in a very warm environment.

 

A top view of a T-90 gear box
A top view of a T90, meticulously rebuilt and ready for a GM bellhousing adapter. The longer input serves the purpose of leaving the transmission in the factory location while spacing the engine ahead enough to clear the firewall.