CLUTCH-03, Checking Clutch Disc Wear
The clutch disc wear should be checked every 15,000 miles as a minimum. If the car is driven hard or is used on the track (Driver's Ed events, Autocross, Time Trials, Full Race) the clutch disc wear should be checked more frequently. I generally recommend every 7500 miles for these cars OR at the end of each race season, whichever comes first.
In this particular picture the end of the slave cylinder and side of the inspection port are difficult to see. That's because this picture was taken on a turbo with the heat shield for the cross-over pipe in place. If the heat shield is removed it's much easier to see the inspection port from above the cross-over pipe. Also, on normally aspirated cars it's much easier to see since there is no cross-over pipe to obstruct your view.
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