difference between hydro and mechanical rocker shaft diameters

The .874 and .872 were read from a dial mic that could actuall measure .872 on the .874 if i squeezed the caliper jaws against the shaft in an effort to get a 90 perpendicular reading , without a chance of rocking the jaws and introducing errors. So its not the best caliper in anyones opinion, perhaps somwone with a GOOD $$ caliper or 1.00 dial micrometer could measure their mech and hydros on the bench. The hydros i modded were drilled for mechanical oil ports and the ductile iron rockers slid right on with excellent fitment for a .002 'oversize'. Iron rocker bores were smooth, no wear patterns detected.