WHAT IS IT?
This is a measurement used by many aftermarket pulley manufacturers. What we are trying to get to is the offset distance between to different planes. The plane that the crank pulley bolts onto and the plane that the water pump pulley bolts to.
HOW IS IT MEASURED?
One way to measure this is depicted in the photo below.
1) Zero a set of calipers on the thickness of a metal straight edge.
2) Hold the straight edge flat on the flange that the water pump pulley bolts to so that hangs down in front of the balancer.
3) Extend the caliper rod from the straight edge to the face of the balancer where the crank pulley will sit.
Note: This does not have to be measured this exact way. As long as you have a good firm straight edge and something to measure with you should be good. A lot of people will use a framing square and a tape measure or a tape measure and a level.
WHY IS IT IMPORTANT?
Many of the blocks we work with spanned decades and through that time water pumps changed. Or balancers. Or both. This could be akin to something like Chevy Small Block where there were three different water pumps or Ford Small Block where there were three different water pumps and two different balancers but also a whole slew of aftermarket components and lots of interchangeability and crossover. Where this can also come in handy is something like Ford Big Blocks where there was both an internal and external balance difference.