T90 Transmission Variations


Bruce inquired on the Bulletin Board about the different versions of the T-90 transmission. "What is the difference between these types of T-90 Gearbox: T-90, T-90A, T-90C and T-90J. I know that there are different ratios for first gear and also that some have selectors from the side not the top and I suspect that the length of the main drive gears are different?"

Ruiz posted: "CJ-2A, CJ-3A and M38 used the T-90A. The M38A1 used T-90A1 while the CJ-3B used either the T-90A1 or T-90C. CJ-5 and CJ-6 used T-90A or T-90C. Utility wagons and trucks used T-90A or T-90J.

"The T-90A1 and T-90C use the same case marked T-90A and are floor shift. The T-90A1 has a 15/16-inch input shaft and 18 teeth on the input gear, while the T-90C has the same size input shaft, but 16 teeth on the input gear. The T-90J has 1-1/8th input shaft with 18 teeth on the input gear and is either a column or floor shift."

Maxx added: "This is from Novak Adapters great website: 'All T-90 transmissions have a 1-3/8 inch, six-spline output shaft for mounting the transfer case input gear. There are three major versions of the T90 to differentiate:


'The T-90 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 seven inch input shaft (stick-out length) with 15/16ths inch, 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 2.98:1 first gear and a 1.66:1 second gear.


'The T-90 used from used from 1963 to 1971 with the F-head four-cylinder engine is designated as the T-90C. It features top shifters, a seven inch input shaft (stick-out length) with 15/16ths inch, 10 splines and a felt-sealed front bearing retainer assembly. Since the T-90C 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 T-90C has a 16-tooth, helically cut input gear. The cluster gear has a 35-tooth driven gear, resulting in a 3.44:1 first gear and a 1.85:1 second gear.


'The T-90J 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 inch input shaft (stick-out length) with a 10 x 1-1/8" splines and a neoprene-sealed front bearing retainer assembly. The T-90J 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.'"

Bill Coelho asked: "Some of the information that I have gathered from various books says that there is a 2.79:1 first gear in early T-90A models and a 3.34 or 3.39:1 first in the T-90C models. Can anyone verify this as fact? If so, are the first gears interchangeable?"

Dave answered: "The T-90 did have an optional first gear of 3.31:1 in the '60s with the 4.27 axle ratio. You can modify your T-90 if you can find the gears. What you want is the 16 tooth input shaft, and the 35 tooth cluster gear. All of the other parts are the same."

While collecting parts to rebuild his '63 3B, Doug came across a T-90 case marked T-90E-1. "I have been searching and searching but have not come up with any references for this T-90E-1. I have seen T-90A, C, and J, but that’s it. This one is a floor shift and does not mate to the Spicer 18 transfer case without an adapter. Does anyone have any information on these things?"

Andy Stock replied: "I think that the main difference between the A, C, J, E, etc. was the gear ratios. I think that third was always 1.00:1 but first and second varied, depending on the application. Rick Grover lists on his speed calculator page an ASI-T90E with a 3.34:1 first, 1.85:1 second, and 4.53:1 reverse."

Frank Kotyz from the Czech Republic inquired: "What differences are there between the T-90 and T-90A1? I have two CJ-3Bs from '53 and '57 and both have the T-90A1. My manual only describes the T-90."

Bill said: "I think that there was a gear change from 26-tooth to 29-tooth."

Oldtime explained: "The correct transmission for the '53 and '57 CJ-3B is the T-90A1. That number should be easily noticed on the outer left side of the case. As Bill mentioned, you will notice a difference between these years with respect to the main drive gear. This gear mounts on the transmission main shaft, yet operates inside of the transfer case. This is accurately a transfer case part as indicated in the parts lists. The main drive gear powers the transfer case intermediate gear. The '53 should have a 26 tooth count while your '57 should have a 29 tooth count output drive gear. I believe that your service manual is designating the transmission as a model T-90 for general reference to the many possible variations. These variables are insignificant for most service applications. Not likely that the transmissions were changed as they are quite durable and easy to repair."

Synchronized First Gear?

Chuck inquired: "Can a T-90's first gear be made to synchronize? A Jeep buddy insists that it can be done using the T-98's second gear! He further states that the T-98 can be used as an overdrive unit by putting T-90 gears in for first through third and then having a 1:1 ratio gear put in for the T-98's fourth gear. I was under the assumption that the two were not compatible in respect to swapping gears."

Vern replied: "I've heard that there was a late version of the T-90 produced in South America that had a synchronized first gear. There were many other similar 3-speeds with a synchronized first, so the concept is not too difficult. In practice, many of the parts are different, such as the cluster gear, etc., making a home swap pretty tricky. Don't quote me on the existence of the South American T-90, but am pretty sure it was true.

"As far as using T-98 4-speed parts for such a swap, I just so happen to have both a T-98 and T-90 in my shop. I went out and compared the two for the swap like your friend suggested. While the differences are quite involved, here are two of the major reasons why it wouldn't work:

1) The T-90 cluster gear has straight teeth where it meshes with first gear. The second gear from a T-98 has angled teeth. There is no way that they would mesh.

2) The T-90 R-1 slider gear (1st gear) is about one inch smaller in diameter than the T-98 second gear. Even if the teeth were the same, the difference in diameter precludes doing the swap. First and third gears in the T-98 were substantially different sizes and wouldn't swap in place, either.

"I'm not well versed in later Jeep transmissions, but is it possible to swap some parts between something like a T-150 and the T-90? Maybe that is what he was thinking, if that is even possible."

Chuck added: "Apparently someone has rigged up a T-98 much the same way, but not exactly, as a T-90 and then used fourth gear as an overdrive. To prove his point, my buddy had a type of Scout which he claims is a four-speed with modified Jeep running gear, and darned if the thing didn't seem as if it had an overdrive! I think that he stated that the synchronized first gear was from a Scout. He wasn't able to give manufacturer of the fourth gear since he was not the builder, but the concept really has me wondering if an overdrive/ fourth gear is viable in a CJ-3B or even worth the obvious trouble."

Vern then posted: "Hmm, this is getting interesting. I have one idea that might explain the mystery transmission, but it requires some fairly heavy-duty modifications. International Harvester offered a 4-speed with a synchronized first gear. Many IH and Jeep drive train parts were interchangeable. I'm no expert on which model it was exactly, but think it was called the T-19A.

"The case for the T-19A was identical or nearly so to the T-18 and T-98 four-speeds. All of these 4-speeds have a case that is 11.875" long, with adapters needed on both ends. The stock T-90 is only eight inches, so the 4-speed conversion is over 5" longer with the adapters. The 4-speeds are also taller, so floorboard modifications or a body lift are required. Of course, the driveshaft lengths have to be reworked, too.

"The T-98 I have, which came from a Ford truck, has the 6.32 first gear which is definitely not synchronized. I believe that the T-19A was the only 4-speed offered in that era that had a synchronized first gear, not including car transmissions. Note that the T-19A had a first gear ratio of 4.02, a concession to the synchromesh design limits.

"Now what if the mystery transmission was really a T-19A? And what if the axles had been re-geared to something like 4.27 instead of the stock 5.38? With the T-19A and re-geared axles, gears 1, 2, and 3 would have been very similar to 1,2 and 3 with a T-90 3-speed and 5.38 axles. And then when the T-19A was shifted into 4th gear, it would have felt like an overdrive because of the re-geared axles.

"That is the only plausible explanation that I can think of for a synchronized first and 'overdrive’ in fourth gear. On the T-90 3-speed, the input and output shafts are locked together in high gear. Likewise, both shafts are locked together in fourth gear on the T-98, T-18, T-19 and T-19A 4-speeds. Without a radical reworking of the transmission internals, I can't see how else you could get an 'overdrive' ratio from a 4-speed gearbox.

"On a slightly different note, if you are looking for an overdrive function, the Warn/Saturn unit is the easiest way to go. It literally bolts up with no other modifications needed. For what you'd spend on a T-19A, reworked drive shafts and re-geared axles, a good used Warn/Saturn OD would be much cheaper."

Ruiz Sebastian said: "I am from Argentina and I have never heard about T-90 complete synchronized. The 3-speed transmissions that were available here were:

1) Ford 3.03: first gear - 2.99:1; second - 1.75:1; third - 1:1, fully synchronized.

2) Chevrolet: first gear - 2.85:1; second - 1.68:1; third - 1:1, fully synchronized.

3) Dodge: first gear - 3.02:1; second - 1.76:1; third - 1:1, fully synchronized.

4) T-90."

Thanks to all the contributors, and Doug Hoffman for editing. -- Derek Redmond

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Last updated 6 March 2009 by Derek Redmond redmond@cj3b.info
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