It is the largest internal diameter the drum may reach, cast into the part, and past it the drum has too little material to absorb heat or resist the shoes.

Drums wear from the inside outwards, so the diameter grows through their life.

A thin drum overheats faster, fades sooner, and can distort or crack under the outward force of the shoes.

The limit accounts for both wear and any machining, so a drum machined close to the limit has no service life left.

Measure with a drum gauge at several points and at several depths, because wear is rarely even.

The figure is often accompanied by a separate machining limit slightly below the maximum, which leaves material for wear after the cut.

Measure at several points around and along the bore - an oval drum varies around, a bell-mouthed one varies along.

An oval drum gives a braking force that rises and falls with each wheel revolution, felt as a pulsing that tracks road speed.

A bell-mouthed drum has worn wider at its open end, so the shoe contacts only a narrow band and braking is weak despite plenty of lining.

Both are invisible without measurement, which is why gauging is part of the job rather than an optional check.

Both can usually be machined out if the resulting diameter stays within limit.

A ridge at the closed end of the bore is normal wear and confirms that the friction surface has receded - it also has to be removed before a new, full-width shoe will seat.

Routinely, on heavy vehicles - provided the finished diameter stays within the limit and both drums on an axle are cut to match.

Machining restores a true, concentric, correctly finished surface and cures ovality, bell-mouthing and scoring.

The finished surface texture matters: too smooth and the shoes glaze, too rough and they wear rapidly.

Both drums on an axle should finish at the same diameter, or braking will be unequal side to side.

After machining, the shoes must be adjusted out to suit the larger bore, and automatic adjusters may not have the range to do it themselves.

On light vehicles, new drums are often cheap enough that machining is not worth the handling.

Repeated heat cycling, usually accelerated by dragging shoes, prolonged descents, or a drum already near its wear limit.

Each heavy stop heats the friction surface far faster than the outer surface, and the resulting stress is relieved by fine cracks that grow.

Fine heat-check crazing on the surface is common and not in itself a scrap condition; a crack that runs through the section is.

Dragging shoes - from weak return springs, a seized adjuster or a sticking wheel cylinder - keep the drum hot continuously and are a frequent underlying cause.

Any crack that reaches the open edge or runs radially through the friction surface means the drum is scrap, with no repair available.

Inspect for cracks whenever the drum is off, and investigate the cause rather than just fitting a new drum.

Rarely, and only where the vehicle manufacturer offers it. The two systems differ in actuation force, parking brake and load sensing, not just in the friction arrangement.

A drum brake's self-servo effect means the actuation system supplies less force than a disc setup of the same capacity needs.

Converting therefore involves the master cylinder or air chamber sizing, the load sensing valve, and often the axle itself.

The parking brake is the other obstacle: it is usually integrated into the drum and needs a separate solution on a disc.

There are also type-approval implications in most markets for altering a braking system, which can affect the vehicle's roadworthiness certification.

Where the manufacturer offers both, choosing disc at purchase is straightforward. Converting afterwards very seldom is.

Back the adjuster off, then use the extractor threads in the drum face - not a hammer on the friction surface.

Two things hold a drum on: corrosion between the drum and the hub, and the wear ridge catching on the shoes.

Backing off the adjuster through the backplate slot retracts the shoes and removes the second problem, and it should always be the first step.

Most drums have threaded holes in the face for forcing bolts. Tightening them evenly pushes the drum off the hub without shock loading anything.

Penetrating fluid at the hub spigot, time, and even heat are all legitimate. Hammering the friction surface is not - it distorts the drum and it is the surface the shoes must run against.

On a vehicle that has stood, expect this to take longer than the rest of the job and plan the bay time accordingly.

Yes - new linings contact only a fraction of the drum surface until they have worn to match its curvature.

A new shoe is ground to a nominal radius and a machined drum is a slightly different one, so initial contact is a narrow band.

Until that band widens, braking force is well below what the same brake will give a few hundred kilometres later.

The bedding process is a series of moderate applications with cooling between, exactly as for a disc brake, and it should be part of the road test.

Some heavy vehicle shops arc-grind the linings to the measured drum diameter, which gives full contact immediately - though dust extraction is essential when doing it.

Re-check the adjustment after bedding in. The clearance set on assembly will have changed as the linings seated.