Which flat tires can be repaired

Picked up a screw? Whether it can be fixed comes down to three things: where the hole is, how big it is, and what happened to the tire after it went in. The tire industry publishes the rules. The U.S. Tire Manufacturers Association (USTMA) puts them in a wall chart called Puncture Repair Procedures for Passenger and Light Truck Tires. Here's what they say, in plain language.

The three rules for a repairable puncture

It has to be in the tread

Repairs are limited to the tread area, the part of the tire that meets the road. Not the sidewall. Not the shoulder, where the tread rounds over into the sidewall. Damage that runs into the shoulder or the belt edge, or angles into it, can't be repaired. If it isn't clear whether the hole reaches the shoulder, the tire comes out of service.

Why so strict? The sidewall and shoulder flex with every revolution, and a 225/65R17 turns roughly 700 times every mile. A patch in that zone gets worked back and forth constantly and won't hold.

It has to be 1/4 inch or smaller

The injury can't be more than 1/4 inch (6 mm) in diameter. Most nails and screws come in under that. A bolt, a large piece of metal or a cut usually doesn't.

The inside has to be intact

This is the one people don't expect. A tire driven even a short distance while badly underinflated can be damaged beyond repair. The sidewall flexes far more than it was built to, and the inner liner and body plies get ground up inside. From the outside the tire looks fine. Inside, it's finished. USTMA compares it to running an engine without enough oil: adding air back doesn't undo the damage, and the tire can still fail later.

That's why the tire has to come off the wheel for a full inspection of the inside before anyone decides it's repairable.

How a proper repair is done

1. The tire comes off the wheel. No exceptions.
2. The inside is inspected for liner damage, other punctures and old repairs.
3. The injury is cleaned out and prepared.
4. A rubber stem fills the hole, and a patch seals the inner liner. Often it's a single combination unit with both.
5. The tire goes back on the wheel, gets balanced and is set to placard pressure.

USTMA is direct about what doesn't count. A plug alone isn't a proper repair. A patch alone isn't either. Never repair a tire while it's still on the wheel, and never do an outside-in repair. A tire that already has an improper repair in it should be scrapped, not repaired again.

A string plug from a roadside kit can get you to a shop, but it isn't a proper repair. Drive gently, get the tire looked at soon, and know that once it's off the wheel it may not qualify for a permanent repair.

When it can't be fixed

• A puncture in the sidewall or shoulder
• A hole larger than 1/4 inch, or a cut or tear
• An inner liner damaged by driving low or flat
• A repair that would overlap an existing repair
• An existing improper repair
• Bulges, blisters or exposed cord anywhere on the tire

Individual tire makers can add their own limits on top of USTMA's, based on things like speed rating, load index or run-flat construction. A shop should check the maker's repair policy for anything unusual.

Run-flat tires

Run-flats can keep rolling with no air for a limited distance. That's the point of them, and it complicates repair. Some tire makers set repair limits or restrictions on run-flats, so the maker's policy has to be checked before anyone repairs one. Whether the tire was driven while flat matters a lot, since that kind of internal damage can't be seen from outside. Run-flats also require a working TPMS, because they can look normal with little or no air. If you have run-flats, tell us when you book.

Sealant cans and mobility kits

Some vehicles come with a temporary mobility kit instead of a spare: a bottle of sealant and a small compressor. Use it if you need it, but know what it is. The tire industry doesn't consider it a repair. It's a way to reach a shop, within the speed and distance limits in your owner's manual.

Aftermarket aerosol sealants are a different story. USTMA doesn't endorse them. Some contain flammable propellants. They can damage the tire, they can interfere with or damage the TPMS sensor inside the wheel, and they aren't considered a proper repair. If you've used one, tell the shop before they open the tire.

If you're on the spare

Compact spares, the skinny T-type ones, are temporary. They take 60 psi, and your owner's manual lists limits on speed, distance and where the spare can go on the car. Don't put the real repair off for weeks. Check the spare's pressure once a month too, so it's ready when you need it.

Slow leaks count

A tire that needs air every week is leaking for a reason: a puncture, road hazard damage, a leaking valve, a corroded or damaged wheel, or a bead that isn't sealing. Every day it runs low it builds heat and internal damage, and a repairable nail hole can turn into a replacement tire. Tires naturally lose 1 to 2 psi a month. If one keeps losing more than 2 psi a month, have it looked at. The TPMS guide explains why the dash light may not catch it in time.

Got a nail or a slow leak? Book a flat tire repair at our Montclair shop, or call (909) 321-2191 if you're not sure the car should be driven in. We take the tire off the wheel, inspect the inside, and repair it from the inside if it qualifies. If it doesn't, we'll show you why and help you match it, new from our tire lineup or a used take-off if the other tires are partly worn. Not sure it's a puncture at all? A tire inspection finds the leak.