CJ750 toolbox
A final drive takes a dump — Mark Fisher

Had a bad day last Saturday. I had just installed my new vintage white-faced Russian speedo and decided to head about 15 miles out of town for a swap meet. I was on the freeway for about two minutes when I heard that SPWEEE sound a speedo makes when it winds up and is about to implode. Sure enough the needle was pegged at 120 and the spring broke before I could slow down enough to pull over. I pulled off the freeway and disconnected the cable and decided to press on. We made it to the swap meet just fine, but while there I noticed I had forgotten to re-install the rear diff filler plug when I checked it that morning. I had set the plug on the sidecar frame and driven off, so now it's happily living in the bushes somewhere between Minneapolis and Delano. The fluid level was still okay so I plugged the hole and we loaded up and headed back into town. About halfway back the diff started pounding like someone was hitting it from the inside with a rawhide mallet. It only happened when I accelerated and we were in the middle of nowhere so I had to limp it back into town until we got close enough to find a rib joint with beer.

After a relaxing drink and some great ribs I put the bike on the center stand and noticed that the two halves of the drive housing were moving in opposite directions. All those original locknuts and washers I had torqued last year were now anywhere from 1 to 1½ turns loose. I tightened everything up and we limped back home. The noise decreased in volume but I knew I had broken a tooth or had some other semi-catastrophic failure.


Tonight I took the bike apart and found just what I had thought... and more. I broke a tooth on the ring gear, the outer diff housing is completely broken out around one of the bolts and the inner housing is hogged out around the pinion bearing. (The pinion and bearing dropped out into my hand.) What I think happened is the snap ring holding the pinion tight on the bearing broke and it started hammering forward and back with every acceleration until the bearing broke loose and started doing the same. Either way a lot of metal was floating around and chewed things up pretty well.