The rods are 1/2 inch. I bored the upper and lower blocks together and have only a couple of thousands clearance to the rods. I used stud locking compound to hold the rods in the lower block. They could just as easily be secured to the upper block but I preferred having them rise above the top rather than protruding through the bottom. In use, the anvils come together as close to exactly as I can measure and there is really no noticeable lateral motion, even when loaded. You do need to take care in drilling or boring all holes together, even the ones for the studs holding the anvils. Also, the anvils need to be made so that the mounting holes and working surface are on the same axis. Easy on a lathe although I managed to do it without one. The anvils themselves are heat treated A2: tempered, not full hard.