Add the total rotating weight (792) to the 65% reciprocating balance factor (467) for a total crank counter weight of 1,259 grams for a 750cc.

RECOMMENDED BALANCE FACTORS FOR 850 PISTONS/RODS

Isolastic mounted Commando racers & street bikes at 65% (850 crank counter weight = 1,300)
Solid frame bikes at 68% (850 crank counter weight = 1,317)

Example: 850cc with 65% balance factor
Lightweight piston 210 grams per cylinder
Rings 18
Pin & retainers 58
Lightweight Carrillo rod - small end 105
391 X 2 cylinders = 782 total reciprocating weight
Reciprocating weight X balance factor: 782 X 65% = 508

Big end of one rod including cap, nuts, bolts and shell bearings = 371
Example: Carrillo rod big end = 371 grams X 2 cylinders = 742 big end weight plus 50 grams for oil in crank = 792
Rotating weight X 100% = 792

Add the total rotating weight (792) to the 65% reciprocating balance factor (508) for a total crank counter weight of 1,300 grams for a 850cc.

RECOMMENDED BALANCE FACTORS FOR 920cc PISTONS/RODS

Isolastic mounted Commando racers & street bikes at 65% ( crank counter weight = 1,342)
Solid frame bikes at 68% (920 crank counter weight = 1,367)

Example: 920cc with 65% balance factor
Lightweight piston 240 grams per cylinder
Rings 20
Pin & retainers 58
Lightweight Carrillo rod - small end 105
423 X 2 cylinders = 846 total reciprocating weight
Reciprocating weight X balance factor: 846 X 65% = 550

Big end of one rod including cap, nuts, bolts and shell bearings = 371
Example: Carrillo rod big end = 371 grams X 2 cylinders = 742 big end weight plus 50 grams for oil in crank = 792
Rotating weight X 100% = 792

Add the total rotating weight (792) to the 65% reciprocating balance factor (550) for a total crank counter weight of 1,342 grams for a 920cc.

Notes:

- All Atlas cranks should be checked for proper balance.
- It is important that the center mass of the crank counter weight is accurately located opposite the rod journals.
- It is not so important to achieve an exact balance factor. A margin of error of +/- 1% or 2% (eg: 70% instead of 72%) simply moves the smoothest range of the motor up or down about 100 RPM.

SHOULD I RE-BALANCE MY COMMANDO CRANK FOR THE LIGHTER PISTONS?

NOT NECESSARY and here is why:

If you just lighten the pistons and don't change the flywheel counter weight of a Commando crank, the balance factor goes up and the motor runs smoother because - the higher balance factor reduces the up & down shaking of the pistons (reciprocating weight), but the fore & aft shaking (rotating weight) stays the same because the lighter pistons do not affect the fore/aft counterweight shaking forces when the flywheel counterweight is at 90 and 270 degrees (half way through the stroke).  So the fore & aft shaking stays the same but the up & down shaking is significantly reduced and this is why you don't have to rebalance a Commando crank for lightweight pistons - it will always run smoother and with less stress with lighter pistons. As noted earlier, the Norton production racers used a 62% balance factor. You can re-balance your Commando crank at 54% for the lighter pistons but in my opinon, a balance factor of around 65% is an excellent choice for Commandos and that is what you get by just installing the lighter pistons.