[Rich Biebel]

Mains were bored after caps were installed and caps don't all interchange well. The Register locates the cap so when replacing caps, it usually meant trying a number of them to find a close fit first. Once your close it can be align bored and or honed. You could find a cap that fit the register but the bore was off too far to align bore it. Aftermarket caps are small to allow for this issue. Most shops saved old caps just for this purpose.