What causes C Series pins to strip when tightened?
This is caused by not having enough threads engaging the casting. Either your spotface is too deep or the material is just too thin. When faced with thin material, do not use the Pin Spacing Drill Jig. Install the pins one at a time so that each new pin is touching a previously installed pin. This helps them tighten without stripping. You can also snug them up and cut the head off to avoid over torquing in thin material.