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Norton could not bore as accurately as the piston makers could turn on the piston OD (including taper etc), so the piston makers made 2 slightly different sizes of piston ie grades. Norton then measured the machined bores and selected the piston that gave the best fit. A workshop borer can hold better tolerances than a factory borer so no need for Graded aftermarket pistons.When I worked for a company making shell bearings one customer wanted 7 grades of bearing thickness with a difference of 3 micron between each grade. They measured their cranks off the line and selected the grade of bearing to fit. Nothing to do with keeping oil pressure, they stated it was to reduce vibration by controlling the clearance between crank and bearing.1 micron = .00003937 inchesIgnore the bottom corrosion, rings will never go near it, the top will cause a very small bit of extra oil to be burnt if the ring goes over it and leaves some oil in there. But if you are only going to do a few thousand miles a year then a few extra drops of oil is nothing.
Norton could not bore as accurately as the piston makers could turn on the piston OD (including taper etc), so the piston makers made 2 slightly different sizes of piston ie grades. Norton then measured the machined bores and selected the piston that gave the best fit. A workshop borer can hold better tolerances than a factory borer so no need for Graded aftermarket pistons.
When I worked for a company making shell bearings one customer wanted 7 grades of bearing thickness with a difference of 3 micron between each grade. They measured their cranks off the line and selected the grade of bearing to fit. Nothing to do with keeping oil pressure, they stated it was to reduce vibration by controlling the clearance between crank and bearing.
1 micron = .00003937 inches
Ignore the bottom corrosion, rings will never go near it, the top will cause a very small bit of extra oil to be burnt if the ring goes over it and leaves some oil in there. But if you are only going to do a few thousand miles a year then a few extra drops of oil is nothing.
