The definitive guide to plugging your flat tire

Q: How do I know if a tire with a nail hole can be repaired? What is the proper repair method? Prices seem to vary a lot.

  • Choosing a car at dealership. Thoughtful grey hair man in formalwear leaning at the car and looking away

Q: How do I know if a tire with a nail hole can be repaired? What is the proper repair method? Prices seem to vary a lot.

A: There are two ways a tire can be repaired: with either a patch or a plug.

And there are two ways to repair a tire, from the outside or the inside.

The repair technician must use all of the above.

On inspecting the flat tire, measure the size of the puncture. The general consensus among tire companies is that the hole cannot exceed 6 mm in diameter. If it does, the tire is scrap. The puncture cannot be in the sidewall area, if it is, it is not safe to repair and it must be scrapped.

So if the hole in the tread area is smaller than 6 mm, the tire must be dismounted from the wheel and inspected internally. If the nail/screw/whatever has thrashed around and ripped the steel belts inside the tire and there are many broken bits of steel showing, the tire must go to the scrap heap, too.

If it is a clean wound, it can be repaired with a patch and plug system.

A patch plug is a piece of rubber that has a mushroom-like cap on the end of a stem. There are some variable diameter plugs without the cap, but these are not recommended.

The repair process goes like this: On the inside, the area of the puncture is cleaned with rubber cleaner, lightly sanded and cleaned again. The repair area should cover about a 5 cm diameter around the hole.

Then a powerful adhesive is applied and the stem of the patch plug is inserted through the hole from the inside of the tire. The cap of the plug sticks to the inside of the tire. From the outside of the tire, the excess length of the stem is cut off to match the tire tread depth.

So now the tire has a large patch on the inside covering the puncture and the holed area of the tread has been replaced by the stem. This is a proper tire repair.

Some shops are not quite so fussy about doing it right. They do not dismount the tire from the wheel. Mistake number 1. If there are steel wires exposed inside the tire, they can rust. The rust travels along the steel belt from where it started and this can lead to belt failure and a wobbly tire six months later.

Mistake number 2 comes when the technician reams out the hole, fills it with strong rubber glue and jams in a domeless plug. After it dries, the excess stem is cut off and $20 later, you are out the door.

The problem with this type of repair is that the plug can come flying out, usually at high speeds. If the glue was stale, not enough was used, or the plug not inserted deeply enough, your tire may become unplugged.

The guidelines for Michelin, BFGoodrich and Uniroyal tires allow up to three patch-plug repairs per tire in the tread area. However, punctures must be at least 90 degrees around the tire from each other.

Adhering to these Michelin rules will allow the tire to retain its speed capabilities.

Quoting from Michelin’s repair guide: “If the above repair limits are followed, the tire will retain its original speed rating. Plug-only repairs done on the wheel are considered improper and therefore, not recommended. Such repairs are not reliable and may cause further damage to the tire. Tires with such repairs should be scrapped.”

For Michelin Zero Pressure (ZP) self-supporting run-flat tires, the repair procedure is exactly the same with one exception: the tire can only be repaired once. A second puncture consigns the tire to the recycling heap.

Bridgestone also insists on the same patch-plug procedures to keep the tire’s warranty in effect. The tire maker also only allows repairs to the tread area. Another issue is that there must be at least 1.6 mm of tread depth left.

However, any repair to a speed-rated tire means “the rating is void if the tire is repaired, retreaded, damaged, abused, or otherwise altered from its original condition. Thereafter, it should be treated as a non-speed-rated tire. Improper repair voids the tire limited warranty.”

So it is time for a new tire if you have a performance car. To again quote the Bridgestone repair brochure, “To avoid reducing the speed capability of the vehicle, replace a speed-rated tire only with another tire having at least the same speed rating. It is the `top speed’ of the `slowest’ tire on the vehicle which limits the vehicle’s top speed without tire failure.”

Run-flat tire repairs require going to Bridgestone Firestone Run-Flat Certified Retailers. These retailers “have the necessary equipment and are specially trained to properly mount and demount RFT tires and to handle TPMS (Tire Pressure Monitoring Systems) devices. Conventional mounting equipment may irreparably damage RFT tires and an improper repair is unsafe and will void the limited warranty.”

“Bridgestone RFT tires are not repairable in any of the following situations,” according to the repair manual: “If the tire was operated with inflation pressure less than 15 p.s.i. (100 kPa); abrasion or other damage is present on the exterior tread, sidewall or bead areas; abrasion, wrinkling, or separation is present on the tire interior and finally any condition or damage is present that disqualifies repair of a conventional tire.”

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