Patch or Plug?

As a general rule, we here never plug a speed rated tire for any reason. It gets a patch though, depending on the damage. We do patch service vehicles that run at 25 mph or less. There are many opinions about whether and how it should be done. The general consensus seems to be a 1/4-inch or less hole may be plugged to keep moisture from the steel belts, but must also be patched. Makes good sense, but so do other opinions. Do you have information on this subject?

The general consensus is to install a one- or two-piece repair unit, which includes filling the injury and sealing the liner of the tire. The maximum size for a nail hole repair in a radial truck tire is 3/8-inch. The maximum for passenger and light truck tires is ¼-inch. To be considered a nail hole repair, the injury must be in the 'crown' area of the tire, from the centerline of the tread extending out to one inch from both shoulders.

The tire must be removed from the vehicle and wheel so that a complete inspection of the outside and inside of the tire can be performed. This inspection should be done at a well-lighted tire inspection stand, or spreader. The injuring object should be removed and the injury probed to determine the angle of the injury.

One-piece repair units are suitable for punctures that are straight in, or at less than a 25 degree angle. Most tire repair material companies provide guidelines for determining the angle of the puncture. They also provide guidance and tools, which are used to measure the diameter of the hole. If the hole is already larger than 3/8-inch for a truck tire, it will not be acceptable for just a nail hole repair after the damaged cables have been removed from the punctured area.

The tire must be inspected for other injuries and run-flat damage.

Inner liners of tubeless tires are designed to partially seal around puncturing objects. If a tire is under-inflated by 80 percent or more, it should be treated as a tire with the potential of exploding from a zipper rupture. Inspection of the outer sidewall using a light may show a rippled sidewall. This rippling indicates the body plies in the sidewall of the tire are damaged. Such a tire is unsuitable for repair.

Another sign of run-flat damage is discoloration of the liner. Radial truck tires may show less liner discoloration than passenger and light truck tires. Over-deflection can be due to some combination of under-inflation or overloading. The sidewall should be carefully inspected for signs of a potential zipper rupture.

If the tire has passed inspection, as repairable, the next step is to clean out the injury with the proper size carbide cutter. The Tire Industry Association (TIA) recommends making three passes from the inside, then three more from the outside, to remove all frayed belt cables. The steel cable should all be encased in rubber to prevent trapped air or damage to the repair unit. After drilling, the injury should be re-inspected to ensure the damage or rust does not extend further than expected.

Tread punctures often expose the belts of the tire to air and moisture, which can cause rust. All of this rust must be removed before the tire can be properly repaired.

A puncture in a tire acts like a siphon when it is driven on wet roads. As the tire rolls out of a puddle, the tread rubber decompresses. This decompression is much like a bulb siphon, and water is sucked into the casing. If this water has road salt, it is very corrosive to steel tire belts.

Most tire repair material manufacturers recommend cleaning the tire liner with a pre-buff solution, then scraping it off the buffing area. Prior to curing, new tires are treated with a mold release material, so they do not stick in the curing presses. This mold release lube will reduce the adhesion of the repair materials if it is not removed before buffing the liner.

After the liner is buffed to the appropriate Rubber Manufacturers' Association (RMA) texture, the injury and repair unit will be cemented. The cement provides lubrication for pulling the stem through the hole and vulcanizes the repair unit to the injury and liner.

Once the tire has been successfully repaired, it can be mounted on the wheel and aired up according to OSHA regulations in a safety cage. Even though the tire was carefully inspected, there is still a possibility of a zipper rupture.

A final note on just patching tires: in addition to the damage from air and moisture, which cause belts to rust, the patched tire will be prone to being re-punctured in the same location.

These types of on-the-wheel repairs are not approved by tire manufacturers, makers of tire repair materials or TIA. With an on-the-wheel plug only repair, there is no way to determine if there is significant damage to the liner of the tire or sidewall body plies. The damage could cause catastrophic results if the tire is put back into service at 100 to 130 psi, depending on application.

Jim Green is Field Service Supervisor for Yokohama Tire Corporation.