Smooth driveability and reliability depend on having a driveshaft and universal joints in good repair. A Mustang's driveline is something you want to know is there without being able to feel it. When the driveshaft and universal joints, or U-joints, are felt and heard, it's usually bad news.
Driveshaft vibration and noise are usually the result of improper dynamic balance, poor runout (shaft not perfectly straight), excessive slip-yoke and bushing wear, defective or worn universal joints, or excessively worn rear-axle pinion bearings.
Wikipedia, a free online encyclopedia that's editable by the public, defines a universal joint as something that allows a rigid rod (shaft) to articulate or bend in any direction. U-joints have a tough task because they transfer an engine's power from the transmission to the rear axle. So they must be strong and capable of articulating with the driveshaft and rear axle, which move up and down with the suspension system as the car rolls down the road.
According to Wikipedia, the universal joint dates back to ancient Greek times. In 1545, an Italian mathematician, Gerolamo Cardano, advanced the idea further, suggesting they be used for transmitting motive power. In the 1600s, Robert Hooke created the first working universal joint, also known as Hooke's joint. Our own Henry Ford coined the name "universal joint." Other names include Cardan joints, Hardy-Spicer joints, and Polhem knots.
Universal joints are chosen for their applications based on vehicle weight, expected duty, and the amount of power the engine will produce. For example, you wouldn't run a six-cylinder universal joint and driveshaft package with a 428 Cobra Jet engine; there wouldn't be enough shaft or U-joint to handle the torque. So we have to size universal joints, driveshafts, and yokes to power, vehicle weight, and anticipated duty for safe, reliable operation.
These are typical Spicer universal joints from Inland Empire Driveline. On the left is a 1310 with a grease zerk and internal C-clip bearing caps. On the right is a 1330 sized for a big-block Mustang and designed for external clips. Note the absence of a grease zerk on the right because the 1330 is lifetime lubricated. Lifetime-lubricated universal joints are stronger because they're solid steel and void of internal grease passages. | 
Basic universal joint size is apparent here. This is a 1330 with 111/48-inch caps. Cap size depends on application. You can select universal joints sized for two different yoke sizes by going with two bearing cap sizes. Inland Empire Driveline can help you with the details. As a rule, bearing cap size is the same for 360 degrees in most Ford applications. | 
There are several universal joint and yoke manufacturers. Spicer has the greatest amount of driveline experience in the industry. Inland Empire Driveline uses Spicer in all its driveshaft applications. |
There are five basic universal joint sizes common to Mustangs. From smallest to largest, they are the Spicer 1100, 1260, 1310, 1330, and 1350. The numbers identify basic dimensions without regard to bearing cap size, which depends on driveshaft and yoke size. For six-cylinder applications, expect to see a Spicer 1100 or 1260. For light-duty V-8 applications, you'll see the Spicer 1310. For heavy-duty applications, such as big-blocks, expect to see the 1330. A popular upgrade from that is the Spicer 1350, which is a race-ready universal joint sized for horrific amounts of torque.
Universal joints are rated based on continuous, endurance, and peak torque. Continuous torque determines how much torque the universal joint can stand for an extended period of time. This is based on the life rating of bearing caps-5,000 hours of operation with the driveshaft at 3 feet misalignment at 100 rpm, which is torture for a U-joint. This is the worst-case scenario, known as B-10 bearing life, for a universal joint. A Mustang will never see this kind of abuse.
Endurance torque is similar to continuous torque, but it's based on reverse torque, with the shaft operating both ways in normal operation. Because we rarely back up the same way we drive forward, this isn't a concern in normal operation. Peak torque is the maximum amount of torque a universal joint can stand before it fails. Ratings are rooted in material strength, which is rooted in material type and thickness. The larger a universal joint, the stronger it is. You must also have a shaft large enough to back up that strength. As a rule, it's impossible to get one without the other thanks to manufacturers' yoke sizing.
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When ordering universal joints, these are the dimensions you will need. "E" is yoke saddle length from bearing cap end to bearing cap end. "D" is bearing cap outside diameter. "A" is from bearing cap to bearing cap. The basic Spicer number will be 1260, 1310, 1330, or 1350. In Mustang V-8 applications, 1310 is most common. Big-blocks use 1330. If you're racing, get the 1350. | 
Universal joints should be replaced by a professional driveline shop. Beating on the caps with a hammer is foolish because it may cause damage and bearing failure. Hitting them with a hammer can knock out needle bearings, wedging them between the cap and joint or losing them entirely. Play it safe and always use a hydraulic press to replace universal joints. Any neighborhood machine shop or repair facility will have a press. |
When it's time to replace universal joints and internal C-clips are involved, we suggest a new driveshaft with external retaining clips. When installed properly, the clips remain secure and safe. Early Mustangs had internal C-clips for easy installation during manufacture, but they can sometimes fly out, yielding an unsafe condition. | 
Basic driveshaft elements include, from left, a slip-yoke, a universal joint, and a driveshaft. The slip-yoke rides smoothly in and out of the transmission as the rear axle articulates with road conditions. Universal joints allow articulation and power transfer. As a rule, they're sized the same fore and aft in stock applications. Slip and pinion yokes vary according to application. There were a lot of running changes during the first nine years of Mustang production. Expect to see variations throughout a given model year. | 
Shown here are identically sized universal joints for different applications. On the left is a Spicer 1310 designed for a steel driveshaft. On the right is a 1310 intended for an aluminum shaft with a protective coating on the bearing caps. Because aluminum and steel are dissimilar metals, corrosion can set in quickly, causing bearing cap failure. |
Here are the basic elements of a driveshaft assembly. Your Mustang's driveshaft is a steel or aluminum tube fitted with cast-iron or aluminum yokes pressed in and welded to each end. Assembly is a precision process that must be carefully followed for smooth operation. Inline Empire Driveline builds driveshafts to order using Spicer components. Unless you have an odd combination of driveline components, Mustangs Plus can ship an Inland Empire Driveline shaft designed for your application from inventory | 
Here's a two-piece, rubber-insulated Ford driveshaft, common to Mustangs with automatic transmissions. While Ford took this approach to reduce vibration, two-piece shafts actually develop vibration and strength issues over time and use. We suggest replacing it with a steel or aluminum one-piece shaft for safety and reliability. Unless you're building a concours-restored show car, there's no reason to use a two-piece shaft. | 
Here's a comparison of slip-yokes. A Spicer 1350/28-spline is on the left and a more common 1310/25-spline is on the right. |