I just found myself a 4th gen aluminum driveshaft... was pretty excited and I went to bolt it in and it turns out the bearing caps on the rear u-joint are too big to fit into the pinion yoke on my 9-bolt.

I'm kind of at a loss about what to do about it. I mean I can only assume there are coversion u-joints out there, but I'd have no idea what to get or how much it costs... Not sure what the exact name of the parts I'm looking for are and what sizes I need. I'm thinking about just getting my money back for it because I just cant afford to chase parts for this thing.

Im pretty sure the other caps on the u-joint are the same size as the ones that dont fit... so I doubt I can just go buy a 3rd gen u-joint... and a 4th gen u-joint clearly doesn't fit... so woujld I have to look for some kind of custom piece?

And I thought these were all supposed to interchange anyway? I'm really confused about this.

The bearing caps on my old steel driveshaft's u-joint measure 1 and 1/8 inches, the new aluminum driveshaft measures 1 3/16's inches.

And what confuses me is this driveshaft I have fit perfectly into my 10-bolt and my 9-bolt. My driveshaft should fit every stock 4th gen rear from wht I understand. So why the hell doesn't this driveshaft fit my pinion yoke?

Does anyone know what the u-joint cap size is for a stock 4th gen? If I can get a stock replacement u-joint I'll do that... but Im assuming this IS stock which is what's confusing me.


Edit:

For anyone who has this problem in the future, I'm pretty sure I just figured it out. We'll see tomorrow. But 1350 series pinion yokes that most aftermarket rears use, use 1 and 3/16's diamter u-joint caps. Stock is 1 and 1/8. This thing apparently has a u-joint to mate to a 12-bolt or 9-inch that I dont have.

I wish I caught that before I pulled everything apart...

 

A stock 3rd gen rearend yoke takes a 1310 series u-joint. The other common series are 1330 and 1350, which both have bigger caps and a wider cross. Just about any shop that does driveshaft work should be able to easily tell you which u-joint you need. Or you can measure it each way yourself and go to neapco.com. They have all the common sizes listed. You'll need to measure the outside diameter of each cap, and the width of the cross in each direction. Conversion u-joints are very common and easy to get if you end up needing one. I had to use a 1310/1330 conversion u-joint when I put a Tremec TKO in my car since the slip yoke is a Ford 1330 style, and my driveshaft needed the 1310 size. Picked one up on ebay for like 20 bucks.

 

Yeah I went and got a stock replacement u-joint for an LS1 car and it fit perfectly.

 

Was there a conversion u-joint in it then, when you bought it? Was that what the problem was?

 

From the research I've done, it seems that GM is about the only company that commonly uses the weenie 1310 u-joint size. Even my 12 bolt in the 69 Camaro still uses a 1310. Never broke one though, so apparently they're strong enough unless you're running a nasty motor that puts out 500 HP or better.

 

Yeah... it was, as far as I can tell, a 1350 U-joint. the DS side bearing caps were 1 and 1/8, while the diff yoke side caps were 1 and 3/16's, which is supposed to be the size of the 1350 conversion caps.

So I'm pretty sure that's all it was. Threw me through a hell of a loop though.