24-06-2021, 10:06 PM
Just to expand a little more on Andrew's graded number in the diamond.
The Austin Service Journal, dated April 1934, states:
Each grade covers a limit in diameter of 0.0005" only. When built the same grade number is stamped both on the block face and the respective piston so ensuring correct clearance.
Where oversize, fractions such as +1/64, +1/32, + 1/16 etc will be stamped, subdivided into grades as per the standard sizes above. Where the + figure is a plain number. eg +5, this indicates the oversize in thousandth of an inch. +5 thus being + 0.005". All oversize stamping appear on both piston and cylinder block face.
Prior to '34 the nominal oversize figure of a bored out block was stamped on the valve cover facing.
The Austin Service Journal, dated April 1934, states:
Each grade covers a limit in diameter of 0.0005" only. When built the same grade number is stamped both on the block face and the respective piston so ensuring correct clearance.
Where oversize, fractions such as +1/64, +1/32, + 1/16 etc will be stamped, subdivided into grades as per the standard sizes above. Where the + figure is a plain number. eg +5, this indicates the oversize in thousandth of an inch. +5 thus being + 0.005". All oversize stamping appear on both piston and cylinder block face.
Prior to '34 the nominal oversize figure of a bored out block was stamped on the valve cover facing.