Universally Speaking
Universal joints are installed at each end of the driveshaft to allow the shaft to articulate with the rear axle. These little guys have a big job to do. They have to keep power going to the rear axle under some very adverse conditions. The more power we throw at the driveshaft and universal joints, the larger and beefier these items have to be.
A driveshaft consists of six basic components: the tube, ends, universal joints, and the slip-yoke. The ends are precision fitted and welded to the tube. The shaft build begins with the universal joints installed in the ends along with the slip-yoke. This enables us to set the shaft up in the jig. | 
Driveshafts are measured lengthwise from center-to-center, meaning from the center of each universal joint cap. It's simple to make a driveshaft if you are going on measurements from an existing shaft. |
Our original shaft is a one-piece unit with the external C-clip universal joints. It becomes more challenging when you custom order a driveshaft for an unusual application. | |
Most Ford applications call for a Spicer 1310 universal joint at each end of the shaft. The 1310 vary according to cap size, meaning being able to tie the driveshaft to different applications. Most original Ford applications call for an inside clip universal joint that is numbered differently from the 1310. Your local Spicer dealer or Inland Empire Driveline will have the necessary information if you choose to rebuild your original driveshaft. If you're stepping up to more power, you will want to consider the 1330 or 1350, which are larger universal joints for more heavy-duty applications.
When we speak of 1310, 1330, and 1350 universal joints, we are talking about the basic joint body with each number without the caps. A basic 1310, for example, has the same size cap on all four sides: 1.063 inches. The same is true for the 1330: a larger universal joint, also with 1.063-inch caps in on all four sides. The 1350 joint is a super heavy-duty piece with 1.188-inch caps.
When we go with different cap sizes on the basic 1310, 1330, or 1350, we are then dealing with a conversion universal joint. For example, you want to install a 9-inch rearend in your Mustang with an 8-inch rearend. If driveshaft length works out, or you are able to go with a different yoke, all you need is a conversion 1310 with the correct 1.125- or 1.075-inch caps opposite the 1310's native 1.063-inch caps. With an 8-inch rearend, we would have the smaller 1.063-inch caps on all four sides.