Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 66% with no modifications to the crank. This means that your Commando will shake less vertically but will shake the same horizontally as explained below. Note that the Norton factory produced their Commando Production Racers with a 62% balance factor. Solid mounted motors run smoothest at approx 66% wet BF for street and racing. Detailed balance specs and recommendations are provided below (Note: 50 grams of crank oil is included in these “wet” specs).
WET BALANCE FACTOR FOR LIGHTWEIGHT 750 PISTONS/RODS IF YOU DO NOT RE-BALANCE
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 66% (stock 750 crank counter weight approx 1,197)
Example: 750cc with 66% balance factor
Lightweight piston 175 grams per cylinder
Rings 18
Pin & retainers 51
Ultralightweight Carrillo rod – small end 95
339 X 2 cylinders = 678 total reciprocating weight
Reciprocating weight X balance factor: 678 X 66% = 447
Big end of one rod including cap, nuts, bolts and shell bearings =370
Example: Ultralight Carrillo rod big end = 350 grams X 2 cylinders = 700 big end weight plus 50 grams (wet) for oil in crank = 750
Rotating weight X 100% = 750
Add the total rotating weight (750) to the 66% reciprocating balance factor (447) for a total crank counter weight of 1,197 grams for a 750cc.
WET BALANCE FACTOR FOR LIGHTWEIGHT 850 PISTONS/RODS IF YOU DO NOT REBALANCE
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 66% (stock 850 crank counter weight approx 1,273)
Example: 850cc with 66% balance factor
Lightweight piston 185 grams per cylinder
Rings 18
Pin & retainers 58
Lightweight Carrillo rod – small end 105
366 X 2 cylinders = 732 total reciprocating weight
Reciprocating weight X balance factor: 732 X 66% = 483
Big end of one rod including cap, nuts, bolts and shell bearings = 370
Example: Carrillo rod big end = 370 grams X 2 cylinders = 740 big end weight plus 50 grams (wet) for oil in crank = 790
Rotating weight X 100% = 790
Add the total rotating weight (790) to the 66% reciprocating balance factor (483) for a total crank counter weight of 1,273 grams for a 850cc.
WET BALANCE FACTOR FOR LIGHTWEIGHT 920cc PISTONS/RODS IF YOU DO NOT REBALANCE
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 65% (stock 850 crank counter weight approx 1,307)
Example: 920cc with 65% balance factor
Lightweight 920 210 grams per cylinder
Rings 20
Pin & retainers 58
Lightweight Carrillo rod – small end 105
423 X 2 cylinders = 796 total reciprocating weight
Reciprocating weight X balance factor: 796 X 65% = 517
Big end of one rod including cap, nuts, bolts and shell bearings = 370
Example: Carrillo rod big end = 370 grams X 2 cylinders = 740 big end weight plus 50 grams (wet) for oil in crank = 790
Rotating weight X 100% = 790
Add the total rotating weight (790) to the 65% reciprocating balance factor (517) for a total crank counter weight of 1,307 grams for a 920cc.
NOTES:
– All Atlas cranks should be checked for proper balance.
– It is not so important to achieve an exact balance factor. Increasing the balance factor 1% or 2% (e.g.: 67% instead of 66%) simply makes the motor shake front to back a little more than up and down.
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 a Commando crank will run smoother and with less stress with lighter pistons. As noted earlier, the Norton production racers used a 62% balance factor.
REBALANCING THE COMMANDO CRANK BACK TO FACTORY SPECS WITH LIGHTWEIGHT PISTONS
You can re-balance your Commando crank at 52% for the lighter pistons. To do so you will need to remove approx 90 grams from the counterweight – that’s roughly two 1/2″ holes drilled 1″ deep in the counterweight exactly opposite the journals. See specs below. Note that a wet balance factor of 52% equals a dry balance factor of 63%.
FACTORY SPECS
FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 750 PISTONS/RODS FOR ISOLASTICS AT 52%
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 750 crank counter weight approx 1,103)
Example: 750cc with 52% balance factor
Lightweight piston 175 grams per cylinder
Rings 18
Pin & retainers 51
Ultra Lightweight Carrillo rod – small end 95
339 X 2 cylinders = 678 total reciprocating weight
Reciprocating weight X balance factor: 678 X 52% = 353
Big end of one rod including cap, nuts, bolts and shell bearings =350
Example: Carrillo rod big end = 350 grams X 2 cylinders = 700 big end weight plus 50 grams (wet) for oil in crank = 750
Rotating weight X 100% = 750
Add the total rotating weight (750) to the 52% reciprocating balance factor (353) for a total crank counter weight of 1,103 grams for a 750cc.
FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 850 PISTONS/RODS FOR ISOLASTICS AT 52%
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 850 crank counter weight approx 1,170)
Example: 850cc with 52% balance factor
Lightweight piston 185 grams per cylinder
Rings 18
Pin & retainers 58
Lightweight Carrillo rod – small end 105
366 X 2 cylinders = 732 total reciprocating weight
Reciprocating weight X balance factor: 732 X 52% = 380
Big end of one rod including cap, nuts, bolts and shell bearings = 370
Example: Carrillo rod big end = 370 grams X 2 cylinders = 740 big end weight plus 50 grams (wet) for oil in crank = 790
Rotating weight X 100% = 790
Add the total rotating weight (790) to the 52% reciprocating balance factor (380) for a total crank counter weight of 1,170 grams for a 850cc.
FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 920cc PISTONS/RODS FOR ISOLASTICS AT 52%
Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 850 crank counter weight approx 1,310)
Example: 920cc with 52% balance factor
Lightweight 920 210 grams per cylinder
Rings 20
Pin & retainers 58
Lightweight Carrillo rod – small end 105
423 X 2 cylinders = 796 total reciprocating weight
Reciprocating weight X balance factor: 796 X 52% = 414
Big end of one rod including cap, nuts, bolts and shell bearings = 370
Example: Carrillo rod big end = 370 grams X 2 cylinders = 740 big end weight plus 50 grams (wet) for oil in crank = 790
Rotating weight X 100% = 790
Add the total rotating weight (790) to the 52% reciprocating balance factor (414) for a total crank counter weight of 1,307 grams for a 920cc.