The back side of the flywheel is machined to reduce weight. If you want a lighter flywheel ACT has a Prolite flywheel which weighs in at 9.4 lbs for a 10 lb weight savings. The Pro Lite has more material machined from the forging to save the weight.
The flywheel is also balanced by ACT before it is shipped out. The large partially drilled hole on the left side of the flywheel was done for balancing.
A strong clutch disc starts with the hub. The hub is a failure prone part on many clutches and ACT takes pains to make sure theirs will not. Most hubs use a thin steel stamping with the input shaft splines riveted or welded in place.
The ACT hub uses a super strong one piece forging with the splines directly broached into the hub body for a precise fit to the input shaft. A tight fit reduces step wear on the shaft.
The retaining windows for hubs springs often wear heavily and develop play that allows the springs to rattle, make noise and even fall out jamming the clutch. To reduce wear, the ACT hub's windows are induction hardened where the springs rub to prevent this.
Many aftermarket sprung hub clutch discs leave out the anti rattle bushing and tensioner spring. ACT uses OEM quality parts here to assure that the clutch will not contribute to a car's NVH.
To make sure that the clutch hub springs will not bottom out violently and stress the clutch disc internally, ACT uses dual springs to give additional stiffness.
The hub uses these heat treated, stamped spring retainers to hold the springs and anti rattle bearings in place.