I converted all my Dillon 550 and 650 toolheads to locking. Tapped the hole where the retaining pin goes through the toolhead and inserted #4-40 Helicoil inserts. Replaced the pin with #4-40 x 1/2" SHCS. Now the toolhead stays tight up against the top of the press. I was getting seating depth variations on my 550 with precision ammo. I think this will fix it.