This size list is for reference only.
Cylinders are bored based off the actual piston measurement using a micrometer and bore gauge. This measurement should be taken by the machine shop that is doing the work.
Pistons are measured at the bottom of the skirt perpendicular to the wrist pin (this is its largest diametrical dimension). The measurement should be taken 1/2 inch from the bottom on the thrust face of the skirt. You might notice pistons are slightly out of round and tapered near the crown. This is to account for expansion of material in the crown and that supports the wrist pin.
Standard Bore Sizes in Millimetres
For Inches convert by dividing MM by 25.4
IE: 76mm/25.4 =2.992 inches
Oversize IE .020 inches= .5mm
TRIUMPH:
3T (350cc) 55mm
3TA (350cc) 58.25mm
T15 (150cc) 57mm
T20 (200cc) 63mm
5T (PU 500cc) 63mm
T150/T160 (750cc) 67mm
T100/5TA/T100C (500cc) 69mm
6T/T110/TR6/T120 (650cc) 71mm
T140/TR7 (750cc) 76mm
BSA:
M20 (500cc) 82mm
A7 (500cc) 62 and 66mm
A10 (650cc) 70mm
A50 (500cc) 65.5mm
A65 (650cc) 75mm
B25 (250cc) 67mm
B40 (350cc) 79mm
B44 (440cc) 79mm
C15 (250cc) 67mm
B50 (500cc) 84mm
ROCKET 3 (750cc) 67mm
NORTON:
16H (500cc) 79mm
Big 4 (650cc) 82mm
ATLAS 650 68mm
ATLAS 750 73mm
COMMANDO 750 73mm
COMMANDO 850 77mm
VELOCETTE:
VENOM/CLUBMAN/MSS (500cc) 86mm
MAC (350CC) 68mm
VIPER/ VIPER CLUBMAN (350cc) 72mm