Why is the overall length of a guidebar different from the
descriptive length of a guide bar?
A guide bar that is called a 12" bar on one saw may have a different overall length from a 12" guide bar on a different saw. This is because the drive sprocket on different powerheads are positioned at different distances from the front of the saws.
Descriptive (Called) Length
ICS Describes their guide bars by how deep the saw is able to cut into a concrete wall. A 12" guide bar cuts 12" into a concrete wall. The saw is able to cut 12" because the bar extends 12" out from the wall-walker.
That is why these two lengths are different.
It is best to use the overall length when your not sure of which guide bar to buy.
The overall length will always remain a constant.
As your chain stretches during normal use, you'll use the chain tensioner to extend the guide bar,
and it is therefore a less reliable measurement.