I have put in a number of Belt Mtn. cylinder base pins in Ruger single action revolvers. The pins are slightly oversize compared to the Ruger pins. They usually slip in, sometimes with a little effort. Once in a while, the frame hole has some imperfections which permit the factory pin, but don't permit the Belt Mtn. pin. I have used the original pin to lap the frame holes on a couple of guns to open the frame hole just enough for the Belt Mtn. pin.