I never noticed the casehead stop in the pic being larger than the thread diameter, no wonder I kept getting cosine error. Thanks for pointing that out.

As far as using the step measurement technique with the head of the calipers, open them up more than enough to allow the body and head to rest in contact with the threaded rod sticking out of the support and clamp piece, holding it such that the head is abutted square to the back of this support so that the flat (face opposite that containing the rack) of the caliper body is in full contact with the OD of the threads. Then slowly slide the moving jaw toward the end of the threaded rod until contact is made. This is the std gap measurement method using calipers. It does depend on a repeatable banking surface for the sliding jaw which is why I place the caliper on the threaded rod where the sliding jaw will contact the end of the threaded rod pretty much parallele to the screw slot.

Hope that makes more sense.

This is the same technique you’d use to measure the distance from the casehead to the reference surface on a Wilson case gage for example. That is, the head of the caliper is held square to the casehead and next to the edge of the machined slot with the sliding jaw carefully brought into contace with the reference surface. Touchy but with a bit of practice, repeatable enough. A good test is to use this tecnique to mesure the step created by the machined slot that provides the go/no-go datums.

Pete