Application Information

To select the proper overdrive model, find your vehicle model and transmission type listed below.

To determine what tooth count you have in your D-18 transfer case, the rear cover plate that is held on with 5 bolts needs to be removed (Image). Once the cover plate is removed, count the heli-cut teeth on the output shaft gear (Image). Then count the number of splines on the output shaft (Image). The 15 teeth are pto engagement teeth.  If you have a pto installed on your transfer case it will need to be removed or you can remove the pan and count the high range gear on the lower shaft. It will be the same tooth count as the outputshaft gear. Please do not refer to tooth count associated with the stock tooth count as your transfer case may have been changed. Normally a 3/4 intermediate shaft will have a 27 tooth outputshaft gear, a 1 1/8″ intermediate shaft will have a 26 tooth gear and an 1 1/4″ intermediate shaft will have a 29 tooth gear.


All Overdrives use 80-90w weight oil. Do NOT use synthetic oil.


Description                                        Vehicle Models

26 Tooth-6 Spline                                M38,  M38-A1, CJ-2A,  CJ-3A,  some CJ-3B,                   4X4,  4-73  4X4                                     (Utility)  4-63

26 Tooth-6 Spline                                Tera-Low equipped  D-18 transfer cases

27 Tooth-6 Spline                                MB,  GPW,  WWll Jeep and Hotchkiss M201

29 Tooth-6 Spline                                 CJ-5,  CJ-5A,  CJ-6,  CJ-6A, (Utility) 4-74 4X4,             3B, (Utility) 6-266, 6-230,                  some CJ- Brazilian Jeeps with 3 speed transmission

29 tooth-10 Spline                                 CJ-5,  CJ-5A,  CJ-6,  CJ-6A   T-14 Transmission

29 Tooth-Metric 10 Spline                   Mitsubishi Jeeps until 1974

29 Tooth-Metric 15 Spline                   Mahindra Jeep Vehicles

29 Tooth-21 Spline                                Brazilian Jeep vehicles with 4 speed transmission

29 Tooth-23 Spline                                Torque-flite equipped Scouts with D-18

30 Tooth-Metric 10 Spline                   Mitsubishi  Jeeps  1974 through May 1977—limited production

31 Tooth-Metric 10 Spline                    Mitsubishi   Jeeps   from may 1977

Jack  O’brian built transfer case


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