Overdrives are cool, and while we all would like to have one, tossing a couple grand into a new transmission with an overdrive is not usually an option. What if you could bolt one on in just a few minutes and cruise at highway speeds without redlining your motor? You can if you own an old Jeep equipped with a Spicer 18-and for a lot less than an NV4500. This overdrive of which we speak was originally called the Warn All-range Overdrive, and now, Advance Adapters' Saturn Overdrive. This miracle of modern (well, not so modern) machinery enables the transfer case and transmission to have twice the amount of gears that the stock setup has, and way more than you will ever need. That's right-16 forward and four reverse gears if you are using a four-speed. By multiplying the transmission gears by 75 percent when engaged, the overdrive effectively reduces the ring-and-pinion ratio from 5.38:1 to 4.03:1 in two and four high. While Advance Adapters has offered completely new overdrives since the early '90s under the Saturn Overdrive namesake, many of the original Warn units can still be located in junkyards and Jeeps around the country. The Warn Overdrives were first sold in the '50s, so there should be a barrel full of them at many a scrap yard. If you happen upon one, don't pass it up because chances are you can get it cheap, especially if them junkyard boys ain't real sure what the heck that thang is. And if it doesn't work, don't worry-the Saturn parts can be used on the earlier Warn overdrives to get them back into running order. These overdrives are fairly easy to work on and make an old Jeep drive like a really old Cadillac instead of a really old tractor. We found a Saturn overdrive from '95 in need of a new shifter fork and synchro parts, so check out the captions to help you find and rebuild your overdrive. Part numbers, diagrams, and some disassembly instructions can be found in the Advance Adapters Overdrive instruction manual, so it's a good idea to get ahold of one before working on your overdrive.
When walking through your local junkyard, keep your eyes peeled for a Spicer 18 with an aluminum thing bolted onto it. That's the Saturn or Warn overdrive. If you're feelin' like dealin', try to convince the junk man that the overdrive is actually part of the T-case. Two for one, baby! | 
If you are buying a used Saturn/Warn overdrive, make sure you get all of the parts shown here. This is what you should have if you just took the unit out of the T-case. An uninstalled Warn/Saturn has two main parts to it (A) the gear and planetary, and (B) the back half, which houses the synchro and shift mechanisms. | 
We started with the gear and planetary. Use a screwdriver to remove this snap-ring and then flip the gear over and remove the spacer washer and hex nuts that hold it in place. Now you can remove the planetary and take a look at the needle bearings and washers. |
Next, turn all the planetary gears to make sure they are spinning freely. If one or more feel gritty or won't turn, grab an awl and drive out the locating pin. Be ready to catch the three washers and 16 little needle bearings in there. |  |  |
Tips And Tricks
*Get those synchro rings lined up properly with the shift dogs, and use some grease to hold them in place.
*Make sure you reinstall the oil scoop in the Saturn/Warn before you reinstall it or else it will turn itself into overdrive blender.
*If the units pops out of gear, you may need a new shift detent spring and ball.
*Make sure you use the special sealing washers on the bolts and add some silicon sealant to the bolt closest to the top of the shift rod to keep it from peeing oil.
*The Saturn will have a date stamped into it on the back, outer rim around the back cover, while the Warn unit has a Warn tag on the back cover.
*Hold onto the cover plate, drive gear, and nut in case the overdrive goes south while out on the trail or road.