A Smaller Shaft

A life long problem, too long driveshafts, is plaguing this swap project.

First, some teaching. A "U-Joint," short for universal joint, is a small piece of hardware that has two bearings on it. One set of the bearings runs up and down and the other set runs left to right. This forms a "+"-sign shape that allows things like a driveshaft to move both vertically and horizontally. If you've owned a busted old front-wheel drive car you may have needed to have a "CV" or "Constant Velocity" joint replaced. The universal joint is the grandfather of the CV joint and they both perform the same function.

U-Joints are sized by their cap size and their width. Common U-Joint sizes are 1310, 1330, and 1350. Sizes any larger than that can be found on heavy duty trucks but for cars, those three sizes are what can usually be found holding things together. All of the documentation I collected before performing my swap into the M3 indicated the stock Fox-body Mustang driveshaft uses 1310 U-Joints. This was going to work nicely since i also happened to purchase a JagsThatRun (JTR) BMW differential flange that uses 1310 U-Joints.

Unfortunately, I was fouled. When trying to assemble everything I noticed the Mustang U-Joint was noticeably larger than what the JTR flange would accommodate. A few Google searches and some head scratching revealed that Ford changed the U Joints used in the Mustang in 1987. On Manual-V8 cars, the driveshaft uses a 1330 U-Joint. Off to the local parts store I went to get the adapter U-Joint. There was a point in time that I lavished when the parts-counter guys would inquire as to what I was doing with my cars. I've grown to really appreciate counter staff that will simply do what I ask when I say, "I don't have an application, but here is a list of part numbers, one of them should cross to something you have on the shelf." And, "No, I'll pay the extra $3 for the one that will actually fit without grinding and welding."

On the return from the parts store, I swapped on the flange and attempted to fit the driveshaft into the car. I discovered that the driveshaft is about 1.5" too long. Given that I'm putting a weird Ford engine into an even stranger BMW (parts wise) this is amazingly close. All appearances indicate the driveshaft would fit if I assembled things differently but for safety and servicing I'm going to get the driveshaft shortened a bit.