piston grade

[Andy Cost <andycost@earthlink.net>]

I have some standard bore pistons that measure 87.20 at the skirt.  They are 
stamped on top with a 3 inside a diamond shaped box.  The manual says it it a 
grade 3 stamp which is between 20 and 30.  It says something about 1/1000 mm 
units.  The only thing I can come up with is that the grade is some kind of 
weight.  Please let me know what all this means.

Thanks,

Andy Cost