Dmftoy1 said:
I was measuring across the crown of the piston and seemed to get 73.75mm. Should I be measuring across base Of skirt? My assumption about it being a .040 piston is on andover's site the show 74mm piston for .040 and these are stamped 74. (Sorry if this is a stupid question)
You definitely have a set of 74 mm(73 mm +.040") pistons.
As Rohan pointed out, pistons are always measured at the bottom of the skirt, usually 1/2" or so above the lowest point, perpendicular to the pin, when measuring for clearance purposes.
The crown of the piston is always smaller than the measurement at the bottom of the skirt, to allow for the extra expansion of the crown, which runs much hotter than the skirt. The diameter at the top land (the crown) is typically something like .005" smaller per inch of bore, depending on the piston material. For a 74 mm bore, that means the top would measure something like .015" smaller than the skirt. The actual amount varies between manufacturers, and depends on detail piston design.
I've just gone out in the shop and measured the diameters of a stock, slightly used, Hepolite Powermax 73 mm +.040" piston, and it is 73.54 mm at the top land and 73.88 mm at the skirt. If new, the skirt diameter would be slightly larger, as the skirt always collapses slightly in use.
Ken