Gdub

You can machine weight out of a standard flywheel and in a straight road car will probably get away with it. The one in my yellow RS weighs only 5.5kg and has had pretty much all of the counter weight machined off the back but it doesnt get revved hard so the risk is low.

With the counterweight machined off, the standard flywheel must be much stronger & safer than an original non lightened one. With the counterweright removed, there is less mass & thus less centrifugal force acting on the 'web' of the flywheel. Reducing the thickness of the web would be a worry though.
 
Cheers Gregg

Noono

Gdub

You can machine weight out of a standard flywheel and in a straight road car will probably get away with it. The one in my yellow RS weighs only 5.5kg and has had pretty much all of the counter weight machined off the back but it doesnt get revved hard so the risk is low.

With the counterweight machined off, the standard flywheel must be much stronger & safer than an original non lightened one. With the counterweright removed, there is less mass & thus less centrifugal force acting on the 'web' of the flywheel. Reducing the thickness of the web would be a worry though.

Cheers Gregg

 
You also alter the structure of the cast when you machine it which can weaken it.  Lightened flywheels are OK when done properly, they're dangerous when they're not.  If you have an all out racer a non standard one is the way to go regardless.
 
Woz