JS Motorsport
 
 
motorcycle
HIGH-PERFORMANCE
PARTS
FOR NORTON TWINS
JS Motorsport
4480 North Academy Avenue
Sanger, CA 93657
Phone/Fax: 559 875 0659
Email: jim@jsmotorsport.com
JS Motorsport Technical
Balance Specs
Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 65% with no modifications to the crank. This is an excellent factor for street bikes and racers with Isolastic mounted frames. Consider that the Norton factory produced their Commando Production Racers with a 62% balance factor. Solid mounted motors also run smoothest at approx 65% 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).

RECOMMENDED WET BALANCE FACTORS FOR 750 PISTONS/RODS

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 65% (stock 750 crank counter weight approx 1,260)

Example: 750cc with 65% balance factor
Lightweight piston 185 grams per cylinder
Rings 18
Pin & retainers 51
Lightweight Carrillo rod - small end 105
359 X 2 cylinders = 718 total reciprocating weight
Reciprocating weight X balance factor: 718 X 65% = 467

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 (467) for a total crank counter weight of 1,257 grams for a 750cc.

 

RECOMMENDED WET BALANCE FACTORS FOR 850 PISTONS/RODS

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 65% (stock 850 crank counter weight approx 1,310)

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 = 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 (508) for a total crank counter weight of 1,298 grams for a 850cc.

 

RECOMMENDED WET BALANCE FACTORS FOR 920 and 1007cc PISTONS/RODS

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 65% (crank counter weight approx 1,340)

Example: 920cc with 65% balance factor
Lightweight 920 or 1007cc piston 235 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% = 543

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 (543) for a total crank counter weight of 1,333 grams for a 920 or 1007cc.

 

NOTES:

- All Atlas cranks should be checked for proper balance.
- It is not so important to achieve an exact balance factor. A margin of error of +/- 1% or 2% (e.g.: 67% instead of 65%) 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?

No - 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 wet balance factor of around 65% is an excellent choice for Commandos and that is what you get by just installing the lighter pistons. Note that a wet balance factor of 65% equals a dry balance factor of 72%.