It is possible to read a clutch like good engine builders can read a spark plug.
By looking at differing clutch trauma we can tell you about the state of tune of your engine, (too lean, ignition advanced too far, pre-ignition, detonation, etc.)
Keyhole fracturing is a function of detonation and sometimes to a lesser extent pre-ignition.
Picture the friction paddle as being one mass that is separated from the other five (or seven) friction masses by an expansion slot, the slot is a thermal necessity as without the slot the disc will warp up immediately from thermal exposure.
When the engine pre-ignites the combustion charge, instead of nice controlled combustion chamber pressure increase, you get a bang which is a pressure spike that can sometimes happen before TDC.
Picture in your mind this rapid increase in combustion pressure as being equal to hitting the top of the piston with a sledge hammer.
This rapid increase in pressure is harmful to all components in the engine and driveline.
When the pre-ignition happens slightly before TDC there is a harmful reversal in rotation that is imparted to the crank.
The way that the clutch disc reacts to this nano-reversal is the mass of the paddles try to go backwards instead of forwards momentarily, the mass of each of the six (or eight) paddles stretches the disc core material at the base of the keyhole causing it to fracture.
We can advise with a high level of confidence, that a clutch disc which has suffered keyhole fractures has been caused by an engine running with a pre-ignition or detonation problem, and if this problem is resolved not only will the clutch discs last longer but the life of many other components within the engine and driveline will be extended.
Competition engines are often far closer to the edge of safe running/tuning than many owners/drivers appreciate, and a change of fuel or even weather can often be enough for these high output engines to run into issues as described here.
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