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Warn Overdrive Rebuild—Making Jeeps Go Fast Again

Posted in How To: Transmission Drivetrain on December 11, 2017
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Back in the day, old iron didn’t need to move very fast. A 50mph top speed flatfender Jeep did its job on the battlefield and on the farm in civilian life. It was a time when torque and grunt were more important than keeping up with traffic or how good your fuel economy was. It didn’t take long for the world to speed up, and before we knew it, factory 5.38:1 geared Jeeps with 1:1 final drive transmissions were being left in the dust.

It was then that the Thompson brothers decided it was about time for some highway gearing options. Using some inspiration from an old Studebaker overdrive design, they created the first auxiliary bolt-on overdrive unit for the Dana 18 Jeep transfer case. Eventually, Warn industries ended up with the rights to the design and started mass production in 1962. For more history on the evolution of the overdrive, check out Jp article “Speeding up the Early Jeeps—Old School” (

Here we see all of the guts of a Warn overdrive unit for an older Jeep. Thousands of overdrives have been worked over on this very spot in Herm’s shop. These cleaned and polished parts are about to be assembled into a like-new condition unit with all-new internal parts. Here’s how the job goes down.

Fortunately, in today’s modern Jeep world there is a guy keeping these old overdrives alive. Herm The Overdrive Guy ( is based out of Washington State and is the go-to guy for everything related to Warn, Husky, Dualmatic, and modern Saturn overdrives. Herm sells complete 25-percent overdrives or his signature 30-percent overdrive unit, in addition to offering rebuild services for all of the variations of overdrives for early Jeeps. When our Warn overdrive became jammed in OD and wouldn’t shift out, we limped it home and knew it was time to call Herm.

Since we had plenty of experience rebuilding motors, transmissions, transfer cases, and axle gear setups, it seemed that rebuilding a small little overdrive unit would be no big deal. A quick call to Herm to order a rebuild kit changed our thought process. Despite our experience, Herm still steered us away from digging into an overdrive rebuild ourselves. Half of the battle is knowing how badly worn the reusable components are in your unit, and the other half is knowing the tricks to getting them apart and back together. Herm charges a super-affordable $45 flat labor rate (plus parts) that is designed to be the offer you can’t refuse. It is cheap insurance to know that your overdrive was rebuilt correctly and received all of the attention to detail it deserves. Herm let Jp get a sneak peek inside his shop to see what goes into a rebuild. If your overdrive is on the fritz, give Herm a call.

The leading cause of failure in an overdrive is lack of oil. Unfortunately for most Jeep owners, the Dana 18 transfer cases are typically leakers. In all of Herm’s extensive experience with overdrives, overpowering and breaking them has never been an issue. It all comes down to severe wear on components due to lack of lubrication. Keep a close eye on the T-case fluid level.
To achieve the 25-percent gear reduction, there are four planetary gears that run on the inside of this bowl gear. The amount of fine metal debris clogging the gear teeth is an indication that the bearings have seen better days. Digging out the heavy grit with a mechanics pick is the first step in the cleanup process.
Even though this flat needle bearing may look okay, there is no way to tell a good one from a bad one. All of the bearings are replaced with new ones. You can see the four-gear planetary assembly that resides in the bowl gear sitting on the workbench.
After removing the center pins, the four planetary gears drop out of the spyder blank. Each gear contains two rows of 16 needle bearings that ride on the center pins. That makes for a total of 128 needle bearings that keep the planetary gears spinning.
The planetary gear center pins exhibited signs of mature wear. It is expected to see the path of the needle bearings on the center pins, but in this case there was some pitting on the bearing surface. This is evidence of lack of lubrication and/or heavy use. In any case, the pins are always replaced.
The spyder blank that came out of our Warn overdrive is in the back. The wear path of the bearings is noticeable, but the surfaces are smooth and have plenty of life. On the spyder blank in the front we see an example of heavy wear and pitting on the bearing surface—this is a worn-out unit.
If a worn spyder blank comes into the shop it is not thrown away. Herm has developed a bronze bushing that is precision machined and pressed onto the shaft of the spyder blank. It replaces the cage bearings that interface between the spyder blank shaft and the bowl gear. The bronze bushing is just as strong and reliable as the roller bearings it replaces. It also saves the customer from having to buy a brand-new spyder blank.
Sometimes the overdrive is so neglected that the planetary gears are corroded in place. When it becomes impossible to remove the gear in the conventional manner, extreme measures must be taken. Herm showed us some precise torch work to melt and smash the gears out of the spyder blank. Luckily, our overdrive didn’t need this kind of witchcraft.
Now we move on to tearing down the rear case assembly. The rear bearing cap and nut had already been removed for this photograph, and now the shifter detent ball is being removed. This will allow the shift rail and fork to be removed.
The detent ball spring will be replaced in the rebuild. Notice how the original spring (top) is permanently compressed from all of the years it was in service. The new spring (bottom) is the originally intended length and spring rate. This will help keep the overdrive in gear and deliver a nice solid-feeling shift.
A couple of snap rings later, the rest of the rear drive assembly comes apart. Most of the rear case guts are on the table. Herm removed the old O-ring seal for the shift rail.
This is the shifter slider collar that engages either direct drive or overdrive. You can clearly see on the left that the original slider has the tips of the teeth worn down from all of the years of use. This wear will promote gear grinding when shifting. It will be replaced with a new slide collar on the right with nice, sharp gear tips to allow for smooth gear selection.
There are three shift dogs riding on the inside of the shifter slider ring to lock the slider into place and prevent the unit from popping out of gear. The worn shift dog on the right has rounded edges that reduce the locking ability and may allow the unit to pop out of gear. Comparing the new shift dog on the left, the crisp edges will provide more reliable gear retention.
Herm uses an automatic hot parts washer to clean all of the components of the overdrive after it has been disassembled. This removes all of the dirt and grime with little effort before the overdrive is reassembled with new parts.
The squeaky-clean parts come out of the hot parts washer and get a final inspection before reassembly.
The shift dogs that we talked about earlier ride in three slots on the aluminum drive member. If the edges of the slots have any burrs or nicks it will cause shifting problems. A bench grinder with a wire wheel and polisher are used to clean up any abnormalities on the components. Details like this make the difference between a quality rebuild and just putting new parts in place.
Out with the old and in with the new. A hydraulic press helps remove the worn out bearings and installs the new bearings.
Do you remember the tarnished bowl gear that we started with? It was clogged with metal shavings that had to be dug out, and it looked to be in poor shape. This is that same bowl gear after Herm finished working his magic to make it like new again. Big difference.
All of the parts are laid out here on the workbench. We like shiny things, but we like them better all assembled. Herm starts the rebuild by assembling the aluminum drive member, shift dogs and springs, shifter slider, shift fork and shift rail into the rear case cover. A new O-ring is installed in the shift rail housing as well as a new paper gasket for the case cover.
The outer housing is slid onto the case cover, and then the splined shaft is inserted into the aluminum drive member.
The rear bearing is carefully tapped into place and fully seated followed by a spacer, castle nut, and cotter pin. The bearing cap and new gasket are finally fitted over top of the rear bearing assembly.
The spyder gears and spyder blank were all in good shape and were reused on this rebuild. Naturally, the needle bearings and thrust washers were all replaced with new parts. POP QUIZ! How many total needle bearings do the four spyder gears roll about? Answer: 128—two rows of 16 needle bearings per gear.
The center pins are tapped into place very cautiously so that none of the needle bearings are dislodged. As you can tell by most of the assembly pictures, there is a healthy amount of grease smothered on all moving parts. This primes the unit with lubricant until gear oil can properly work its way into all of the crevasses. It is important to note that NO synthetic oil can be used in your Dana 18 or overdrive. Always use standard 80-90W gear oil.
After the planetary assembly is wiggled into the bowl gear with all new bearings, the spacer washer is secured with the Allen head screws, completing the rebuild.
A freshly rebuilt overdrive unit is a thing of beauty, and as such requires a little attention during installation. At a minimum, the transfer case gear oil needs to be drained, the T-case filled with diesel fuel, and then spun for 60 seconds (with the Jeep on jackstands) to clean out any contamination, then drained. Finish off the job with new gear oil. You do not want to spin a bunch of old wear and metal shavings into your fresh overdrive. We will be completely rebuilding our Dana 18 before reinstalling this overdrive, but that story is for another day.

Overdrive FAQ

Here are some quick answers to some common questions regarding these overdrives:
• For overdrive removal and installation techniques, we suggest consulting the instructions that can be found at You should never need to cut, torch, hammer, or use excessive force when removing or installing your overdrive. Permanent damage will be incurred.
• Warn, Husky, Dualmatic, and Saturn are all equally strong and reliable.
• It is okay to use your OD in low range; the input speed and torque to the OD from the transmission is the same as high range. The low range gear reduction occurs after the OD.
• Big horsepower is not an issue. One of Herm’s customers is using an overdrive behind a Cummins 4BT pushing over 800 lb-ft of torque.
• Lack of oil will damage an overdrive. Make sure the oil catch trough is properly installed and that the oil level in the transfer case is checked often.

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