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.

Air Cooled Engine Technology. Roth 9 th Ch 16 Pistons & Piston Rings Pages - PDF Free Download

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


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


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


16H (500cc) 79mm

Big 4 (650cc) 82mm

ATLAS 650 68mm

ATLAS 750 73mm

COMMANDO 750 73mm

COMMANDO 850 77mm


VENOM/CLUBMAN/MSS (500cc) 86mm

MAC (350CC) 68mm