Our door knocker installation instructions references a dimension called the bolt center or the BC dimension.
The phrase Bolt Center references the geometric center of the diameter of the hole that is drilled for a bolt to pass through. Theoretically the center of the bolt will pass through the center of the hole as it goes through the door. In the case of our door knockers this would be the mounting bolts for the through the door mounting.
We refer to this point as the bolt center because this is the exact point that the workman should drill the hole for the bolt so it matches the door knocker. Right in the center.
In the case of our door knockers the bolt center dimension is the same as the distance between the bolts. If you measured from the bottom of one bolt to the bottom of the other the distance would be equal the bolt center dimension. This is true because both holes being drilled are the same size.
Lets imagine for an example that the top hole does not use a bolt. Instead the top hole is an eye viewer that requires a hole 3/4 inch in diameter and the bottom hole is a standard mounting bolt that needs a 3/8 inch diameter hole. In this case you could not drill holes based on the bottom of each. You would need to know where the center of each hole is and drill there.
This is why we use the term bolt center for our through the door mounting instructions.
