Cracks in flywheel

I had an oil leak problem in the clutch area together with slipping clutch.
The leak problem is solved but I found 2 cracks on the flywheel. The cracks are running in radial direction, approx 3/4 inch apart and approx 1/2 inch long and are on the disk surface. I can not tell the depth of them but they are clearly visible.
Do any of you have any experience of cracks like this? Can I put this flywheel back and expect it to operate normally in the future or should I cancel the trackday on Saturday and get a new flywheel?
All inputs appreciated

Bjorn, 1977 911 with 3.0 engine

I understand these to be "HEAT" cracks. You should replace the flywheel. Of course, I didn't replace my flywheel on my 73.5 T (CIS). I did have it machined and the cracks were still there. I didn't have any problems for three years before I sold the car...

Man that car was fun. I want it back now... :-(

Nick
'85 Carrera

I feel very strongly that the flywheel should be relaced, because flywheels can explode ... literally, and the kinetic energy suddenly released when that happens can send shrapnel great distances and cause death or serious injuries, not to mention quite a bit of damage to the vehicle!

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Warren Hall
1973 911S Targa

I agree, replace the flywheel.

Nick

Magical Powers hold engines together while they're running. Let me illustrate. At 6000 RPM, the flywheel is spinning 100 times each second. Pistons are going out and getting yanked back in at that same rate. Magical Powers, as far as I can see. and these Powers go away if everything is not just right.

I have never seen the results a flywheel that has come part while the engine was running, but clearly you can say goodbye to your transmission case, and I wonder what happens to the main bearings with the out-of-balance flywheel remains spinning.

I just replaced a flywheel because it had heat blemishes but no cracks. Years ago I probably would have put it back in.

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'83 SC