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