Small, Medium, or Large?

Driveshaft yokes and universal joints, that is…

The other night, my mind was messing with me (again – this is a regular occurrence) and wouldn’t let me go back to sleep. In search of random things to worry about, it picked the question of whether all the driveshaft yokes on the Jeep were the same size. At one time in its life, the answer was Yes – and they were “small.” [By way of a somewhat oversimplified explanation, universal joints, which are held in yokes, are what connect a driveshaft to both the transmission and the axle. They allow the axle to move up and down relative to the transmission while still keeping them connected and transferring power. The smaller and lighter the vehicle, and the less power produced by the engine, the smaller the universal joints typically are. They have a numbering system that I don’t understand, but basically it boils down to this: Lightweight vehicles like my Jeep use “small,” midsize vehicles like a half- or three-quarter-ton pickup use “medium,” and one-ton pickups and SUVs use “large.”]

When I set about ordering the components of my drivetrain, I knew I wanted to step up from “small” to either “medium” or “large” – mainly because the stuff I was ordering wasn’t available with the “small” version. The question then became whether “medium” or “large” would be my answer.

I talked with my transfer-case expert, the “magic gearbox” expert, and the axle experts. The magic gearbox guy said the boxes typically come with “large” but that he could build it with “medium” if that’s what I wanted. The axle guys didn’t care either way. The transfer-case guy said it would depend on what was available to fit a 50-year-old transfer case’s output shaft, but he thought that “medium” was available. So I decided to go with “medium” and ordered the first two axle center-sections, delivered the transfer case to the expert, and started working with the magic-box guy about that piece.

Time passed. The two center-sections arrived with “medium” yokes as requested. The magic box dragged on. The transfer case finally got finished, but with “large” yokes instead of “medium.” So I asked the magic-box guy to switch to “large” so things would match. I knew one of the axle center-section yokes would go away (it was replaced by the magic box), so I wasn’t worried about that one. And I knew that I could easily get a “large” yoke for the other center-section once I had all the parts in hand and set about having them assembled.

More time passed. I ended up delivering one of the center sections to the magic-box guy last summer on my big trip, and it eventually arrived back here in Tucson last fall. Then I ordered the third center section, this time with a “large” yoke, to go in the front axle. That took more time to arrive than I had anticipated, so axle assembly didn’t happen until after Christmas. I picked everything up shortly after New Year’s Day and brought it home, but as I recounted in my last post, I took the middle axle (the one with the magic box) back to the assembler for some adjustments so that two burly pieces of steel wouldn’t try to occupy the same place in space at the same time. (That’s never a good thing.)

This brings me back to the other night. I knew that the assembler and I had talked about replacing the “medium” yoke with a “large” one for the back axle, but I didn’t remember ever checking to see that it had been done. So I wrote a note to myself – I’ve FINALLY learned to keep pen and paper by my bed for midnight notes – and ultimately went back to sleep. When I got up, I made sure I transferred my note to my “to-do” list, and this morning I got down to that entry.

I went out to the garage and pulled out the sample universal joint I had bought for size comparisons, and checked the back axle. The U-joint didn’t fit. Crap – the assembler had replaced a “medium” with another “medium”! I checked the spare yoke that had come back in the box of parts, and, yup – it was a “medium” too. So I thought I should make sure about the others. The front axle has a “large” yoke, and both ends of the transfer case are also size “large.” So only the back axle needs to be changed.

So I took the tires off the back axle, loaded it in the truck, and trundled off to see my friendly assembler. When I got there, I asked him if he had replaced the yoke in the back axle and he answered that he hadn’t, because while he remembered we had *talked* about it, he couldn’t remember what the decision had been, and he didn’t have any notes about it in his computer or on the work order. That was actually a relief, because it absolved him of replacing like for like. I felt better that he hadn’t made a dumb mistake like that.

I left the axle with him today, so that he can get a size “large” yoke for the back axle and put it on. He says he should have both axles done for me sometime next week.

Although I *really* wish my mind wouldn’t mess with my sleep, in this case it worked out well because I caught a potential problem early, before any major assembly took place. It would have been harder to change the “medium” yoke for a “large” one after the axles had been attached to the frame – and I hate to think about trying to get the whole (non-running) Jeep down there if it had gotten that far before I figured this out!

So when all is said and done, all the universal joints in the drivetrain will be “large.” Since I’m not running a super-duper, gobs-of-horsepower-and-torque mega-engine, the drivetrain should be plenty stout and hard to break. I hope. 😉

Categories: axle, drivetrain, Jeep | Leave a comment

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