Not every punctured tyre can be repaired, and the rules are not a matter of opinion. In the UK, tyre repairs for cars and light vehicles are governed by BS AU 159, the British Standard that defines where and how a tyre may be repaired. A competent fitter will apply it, and will refuse a repair that falls outside it — which is exactly what you want them to do.
The repairable area
A tyre may only be repaired within the central three-quarters of the tread width. Imagine the tread divided across its width into four equal strips: the outer strip on each shoulder is off limits. Damage there flexes far too much in use for any repair to hold.
Within that central area, the injury must be no larger than 6 mm in diameter for a car tyre.
What always means replacement
- Sidewall damage. Any cut, puncture or gouge in the sidewall. The sidewall does the flexing; a repair there will fail.
- Shoulder damage. Anything in the outer quarter of the tread on either side.
- A bulge or egg. That is internal structural damage — the casing cords have broken. The tyre is finished.
- A tyre that has been run flat. Even a short distance at speed destroys the inner liner. The damage is often invisible from outside, which is why the fitter checks the inside.
- Cords visible anywhere. Worn through, kerbed through, or cut through.
- Existing repairs too close by, or more than the permitted number of repairs already in the tyre.
- Below 1.6 mm tread. There is no point repairing a tyre that is already illegal.
- Most run-flats that have been driven on while deflated. Many manufacturers prohibit repair outright once the tyre has run in deflated mode.
How a proper repair is done
This is the part that separates a real repair from a roadside bodge:
- The wheel comes off the car and the tyre comes off the rim. There is no way to inspect a tyre properly with it still fitted.
- The inside of the tyre is inspected for run-flat damage, liner damage and previous repairs.
- The injury channel is cleaned and prepared with a carbide cutter.
- A combination unit — a stem that fills the hole plus a patch that seals the inner liner — is fitted from inside and rolled down.
- The tyre is refitted, the wheel is rebalanced (the repair adds mass at one point), and it is torqued to the manufacturer's figure.
An external "string" or "rope" plug pushed in from the outside is a temporary get-you-home measure only. It does not seal the liner and it is not a BS AU 159 repair.
What about the sealant in my boot?
Aerosol sealant kits are a genuine emergency tool — they will often get you off a dangerous spot. They are not a repair. Tell the fitter you have used one, because the tyre and the TPMS sensor will need cleaning out, and some fitters charge a little extra for that.
What happens if it cannot be repaired
You will be shown why, and quoted for a replacement in the correct size, load index and speed rating. You are free to say no — in which case you pay only the agreed call-out or inspection charge. Nobody should be replacing a tyre without telling you first.