Oringinal piston specs.

Piston specs for Bros J and K.


Note: First number is for 400cc and the number in corner brackets is for 650cc.
The numbers in double corner brackets are service limits.

Piston specs for Bros L.


English piston specs for Bros NT400 J/K, accuracy not guarunteed.
Item. Standard NT400 J/K. Service limit NT400 J/K.
Cylinder bore 64.000 mm - 64.015 mm 64.1 mm
Cylinder warpage across top - 0.1 mm
Cylinder taper - 0.05 mm
Cylinder out-of-round - 0.05 mm
Piston Diameter 63.96 mm - 63.99 mm 63.9 mm
Cylinder to piston clearance. 0.01 mm - 0.55 mm 0.1 mm
Piston H (look at picture) 10 mm -
Clearance between piston and piston pin 0.002 mm - 0.014 mm 0.04 mm
Clearance between piston pin and connecting rod small end 0.016 mm - 0.040 mm 0.06 mm
Piston pin OD 14.994 mm - 15.000 14.98 mm
Piston pin bore in piston 15.002 mm - 15.008 mm 15.05 mm
Connecting rod small end bore 15.016 mm - 15.034 mm 15.07 mm
Piston ring to ring grove clearance, top. 0.025 mm - 0.060 mm 0.08 mm
Piston ring to ring grove clearance, second. 0.015 mm - 0.045 mm 0.06 mm
Ring end gap, top. 0.20 mm - 0.35 mm 0.5 mm
Ring end gap, second. 0.35 mm - 0.5 mm 0.7 mm
Ring end gap, oil. 0.2 mm - 0.7 mm 0.9 mm