so i have been reading up in this forum for a while and lots of people ask about flywheel lightening and weights so here is what i found before i started machining the flywheel weighed 7.9 kg , i believe this is a standard 4k flywheel after machining the back only the flywheel weighs 6.2kg , i think this should make some difference to the way the car revs , also after maching i shot peined the back to combat the cracking issues that come with lightening cast flywheels , although i am sure it would not break anyway. after maching i balanced the flywheel on the end of the carnk shaft, and it was only out 1.8 grams , so it seems fi you set the flywheel up well in the lathe there is very little balancing to do . i hope this helps !