The most common reasons for belt failure are improper belt tension (too loose or too tight), poor alignment, pulley damage or distortion, environment (excessive heat and ozone), and using an improper belt for the application. Another issue is belt selection. Belt width and pulley width must match in every respect for proper function. Width match is important because there should be full contact (traction) between the belt and pulley.
Unless you've performed an upgrade to a serpentine beltdrive, your classic Ford is fitted with a V beltdrive common on all cars and trucks prior to the '80s. The V-belt design lends itself to strength and power because a V-belt is triangular. What's more, its shape increases contact area, giving increased traction on two sides instead of one.
Like tires, V-belts consist of rubber composition and cords wrapped inside a flexible extruded cover. Synthetic fiber cord is what gives the belt strength. Both rubber and cord work together in composite fashion to transfer power and deliver strength. In one sense, they work like bicycle chains-there's a tension (tight) side and a slack (loose) side. The tension side is the torque side, where pulling power is transmitted. The slack side feeds the tension side. This cycle of tension and slack is what challenges a belt most.
Cog belts provide improved...
Cog belts provide improved traction and cooling due to increased surface area. Improved belt traction means less slippage and heat. Today, manufacturers put cogs/ribs on the outside for even better performance. This belt style has proven itself quite well in regular use.
When selecting a belt, you want a perfect fit in the pulley sheath. Belt and sheath walls should mate perfectly and without gaps. Think of belt-wall contact as you would tire contact with the road. The more contact between belt and sheath, the more traction your belt will have.
Belt Tension
Belt tension has everything to do with performance and reliability. Belt slippage hampers performance because accessories don't function properly. Belts that are too tight put an excessive load on pulleys, shafts, and bearings. Excessive belt tension will also wear out a belt quickly. Proper belt tension is 1/2 inch of sideplay both ways. Press down on the belt midway between pulleys and observe the travel. Belt tension should be checked on the tension side, not the slack side; expect more play on the slack side. Never use the alternator's main case as a pry point for adjustment. Use the front case (or better yet, an alternator-case holding tool to which the breaker bar will be attached), and don't overdo it.
Pulley Condition and Sizing
Pulley warpage, nicks, and normal wear take their toll on belts as quickly as poor alignment and improper tension. Pulleys should have a smooth wear pattern for 360 degrees and be free of any irregularities. Even a small nick or rise in the contact area will go through a belt in short order. A pulley with runout (warpage) has the same effect as improper alignment because it puts unacceptable side-loads on the belt.
If you use a water-pump pulley that's too small and a crank pulley that's too large, you risk blowing cooling-system hoses because pump speed becomes excessive. The same can be said for the air-conditioning compressor and power-steering pump. When choosing pulley size, look to Ford for answers. Always opt for the same pulley sizes Ford used.
This is a typical belt/pulley...
This is a typical belt/pulley setup. Belt sidewall contact should be solid with each side of the sheath and at the same geometry. There should be an air gap below belt contact for heat dissipation. Each element of a V-belt performs a specific function: Tensile members (rayon cords) provide strength, while flexibility comes from the fabric and rubber composition.
For example, high-performance engines such as the 289 High Performance, the Boss 302, and the 427, were fitted with larger pulleys to keep rotational speeds safe. We see this most with alternators, generators, and power-steering pumps.
Pulley Alignment
Improper alignment forces belts to operate in ways they weren't designed to function. Pulley misalignment, distortion, and misfit can go through a belt in a couple of hundred miles.
When belts track normally through pulley sheaves, friction between the sheave and belt is reduced. Heat dissipates more quickly as the belt moves through pulleys when there's proper alignment. What's more, accessories such as alternators, water pumps, power-steering pumps, and air-conditioning compressors turn at normal operating speeds. When pulleys are misaligned, friction increases substantially, putting excessive side-loads on belts.
The best indicator of pulley alignment is a straight edge along with temporary belt installation. The belt and straight edge should be parallel with the pulleys.
 Here is a perfect pulley/belt...  Here is a perfect pulley/belt fit. The belt is flush at the perimeter, with perfect surface contact into the groove. |  Here's a remanufactured Autolite...  Here's a remanufactured Autolite alternator with a reproduction belt. Note the improper fit; the belt is a pinch too wide for the pulley. Your belt should fit into a pulley groove without gaps or overflow. |  Through the years, there has...  Through the years, there has been much speculation about whether to use one belt or two. Two belts are solid security if you're concerned about belt failure. When going with a two-belt package, specify matched-pair belts. |
 Larger alternator and power-steering-pump...  Larger alternator and power-steering-pump pulley sizes keep accessory speed conservative at 7,500 rpm. |  The easiest way to check for...  The easiest way to check for proper belt tension is through the use of a belt tension gauge. It works like a torque wrench. Lean on it to determine how much effort it takes to get a specific reading. |  Have you ever walked into...  Have you ever walked into an auto parts store thinking you know the proper belt size, only to come home and discover your replacement is too short or too long? We ran a string around all of the pulleys-with accessories at proper adjustment. This way, we established belt length adjusted to the 1/2-inch rule. We also supplied the parts store with the pulley width dimensions. |