From JeepTech and JeepTech Publishing LLC, the Jeep knowledge base.
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The T-18 is a heavy duty four speed with unsynchronized first gear. These transmissions were manufactured by Borg Warner and they have a cast iron case. The casting number on the drivers side of the T-18 is T-18 or 13-01. The T-18 has a 1 1/8" 10 spline input shaft and a 1 3/8" 6 spline output.
The T-98 has a very similar appearance to the T-18 except a casting number of T98. It is heavy duty like the T-18, but a bit more prone to wear since bearings and lubrication are not as good. Internals from a T-18 can be put in a T-98 case. The side picture above is actually a T-98 with granny T-18 guts. The T-98 has a 1 1/16" 10 spline input shaft and 1 3/8" 6 spline output.
T-18s used after 1978 have reverse down by fourth and earlier T-18s have reverse up by third. The shape of the case is also different with the '79+ cases for the different reverse configuration. On the right is a T-18 out of a '79 CJ-5. The blue T-18 in the picture above is out of a '78 CJ-5. Notice the later T-18 has a flat side rather than the bulging side of the earlier T-18. Both versions have a PTO port on the drivers side.
The T-18s used in full size Jeeps(FSJs) such as the Chorokee, Wagoneer, and J-series pickups use a transmission to bell housing adapter. Three basic variations exist with different adapters.
- Early 70s T-18s use a 1" cast iron adapter with a deep AMC bell housing. The bell housing is the same as the bell housing used with the T-15.
- Later 70s FSJs with V8 engines use a 5" aluminum adapter and a AMC T-15 bell housing. There are pictures of this version below.
- Later 70s FSJs with I6 engines use a shoter aluminum adapter and a AMC T-15 bell housing.
The different adapter means a different length input shaft is used. The input shaft size and spline count is consistent for all the Jeep T-18s, but the pilot bushing size varies. The different adapters used with the later V8 and I6 engine where presumably used to keep the drive shaft lengths consistent regardless of engine.
All Jeep T-18s come with a 1" adapter for the Dana 20 transfer case except the T-18 used in 1980 and 1981 J-20 pickups. These trucks used the NP208 transfer case and the adapter for the NP208 is around 9" long and it uses a round bolt pattern. The output shaft of this version of the T-18 is 23 spline. The Dana 20 version of the T-18 has the big hole "Texas" bolt pattern and 6 spline output shaft.
The T-18 was optional on CJs from 1971-1979. The T-18A version used from 1977-1979 is the best transmission ever put in a CJ by the factory since it features a granny low first gear(6:32:1.) Earlier T-18s have the same great strength, but a 4.03:1 first. The T-98 is the predecessor to the T-18 that was an option for CJs from 1955-1970.
Other Jeep trucks also had the T-98 as an option from 1955-1970, the close ratio T-18 optional from 1971-1975, and the wide ratio T-18 from 1976-1979. After 1979, the wide ratio T-18 was only avilable as an option in the J-20 pickup truck from 1980-1981. This version of the T-18 has the round New Process bolt pattern and 23 spline output, but the adapter is long for a short wheel base Jeep. Earlier T-18s and T-98s used the "Texas" bolt pattern and 6 spline output. The truck version of the T-18 uses a long input shaft compared to the T-98 or CJ T-18s.