Organic for anything driven in traffic. Sintered, ceramic or paddle facings only where heat and torque genuinely exceed what organic will hold.

Organic facings engage progressively, which is what makes a vehicle easy to pull away in and gentle on the driveline.

They are also the most tolerant of the slipping that normal driving and manoeuvring involve, which is where clutch wear actually happens.

Sintered and ceramic facings hold far more torque and survive far more heat, and they engage sharply enough to make smooth pull-away difficult.

Paddle designs reduce the contact area to increase pressure, which raises capacity and makes engagement more abrupt still.

For fleet and commercial vehicles the answer is essentially always organic, in the grade the manufacturer specified.

It absorbs the torsional shock of engagement and damps engine firing pulses before they reach the gearbox.

Springs arranged around the hub allow a few degrees of rotation between the friction facing and the splined centre.

That takes the edge off take-up and stops engine pulses exciting resonances in the gearbox, which is what produces gearbox rattle at idle.

A rigid hub transmits everything, which is acceptable in competition use and unpleasant and damaging in a road vehicle.

Where a dual-mass flywheel has been replaced with a solid one, the disc must have a sprung hub - the damping the dual-mass provided has to come from somewhere.

Worn hub springs give a rattle that changes with clutch position and a slight harshness on take-up.

From the crankshaft rear main seal behind the flywheel or the gearbox input shaft seal in front of the bellhousing - and both must be fixed before a new disc goes in.

Either seal puts oil directly into the bellhousing, where it is thrown onto the friction faces.

Oil on a facing causes slipping under load and judder on take-up, and the facing cannot be cleaned because the oil has soaked in.

The evidence is visible when the gearbox comes off: oil on the bellhousing interior, a wet flywheel face, or a dark ring on the disc.

Both seals are cheap and both are accessible only at this moment, so they are renewed as a matter of course on a high-mileage engine.

Fitting a new clutch without curing the leak guarantees the job returns, usually within months.

Almost always with the raised hub boss towards the gearbox - and most discs are marked, but check before assembly rather than after.

The hub protrudes further on one side, and that side must face away from the flywheel or the disc will not sit flat.

Fitted backwards, the hub fouls the flywheel bolts and the clutch will not release - and the fault is only discoverable by removing the gearbox again.

Markings vary: 'gearbox side', 'flywheel side', an arrow, or nothing at all on some discs.

Where a disc is genuinely unmarked, the deeper hub projection faces the gearbox on nearly all designs, and offering it up dry will show which way clears the bolts.

Take a photograph of the old disc before removing it. It answers this and several other questions later in the job.

Contamination, a distorted flywheel or disc, or worn mounts - and the disc is the least likely of the three if it is new.

Oil on the facing is the most common cause and points at a seal.

A flywheel face that is not flat, or that has hard heat-spotted areas, gives uneven grip as it rotates.

A dual-mass flywheel that has lost its damping produces judder that a new clutch will not cure.

Worn engine and gearbox mounts let the whole driveline rock as torque comes on, which presents as judder and has nothing to do with the clutch.

Diagnose in that order. Replacing a clutch to cure judder caused by a mount is a common and expensive mistake.

For heavy commercial and specialist applications yes; for ordinary vehicles exchange discs are cheap enough that relining is not worth the handling.

Relining rivets new facings onto a sound disc carrier, and the carrier and hub springs are frequently fine.

It is well established in heavy vehicle and agricultural work, where discs are large and new units expensive.

The carrier must be checked for flatness and the hub springs for free play, because neither is renewed by relining.

Turnaround is the practical constraint: a relined disc means the vehicle waits, which for a fleet is usually more expensive than the saving.

Keeping an exchange disc on the shelf for the highest-volume platform generally beats relining on economics once downtime is counted.

By the facing thickness above the rivet heads - and in practice it is only measurable with the gearbox out, which is why symptoms drive replacement.

Manufacturers publish a minimum thickness, usually expressed as the material remaining above the rivets.

Worn to the rivets, the disc scores the flywheel and pressure plate and turns a clutch job into a clutch and flywheel job.

There is no inspection port and no external measurement, so nobody measures it until the assembly is already apart.

That is why slipping under load is the practical trigger for replacement, and why the test against the handbrake is the standard check.

When the disc is out, measure it and record the reading against the vehicle's duty - over time that builds a genuine expectation of life for the fleet.