Callaham's blocks make use of their Enhanced Vintage design, which utilizes a Delrin bushing in the arm hole for the arm to bear against (see illustration below).
With traditional threaded block designs, when you use the arm, you create forces that are concentrated right at the threaded portion of the arm - its weakest point. This is where arms typically break. Callaham's solution of leaving this bearing area of the arm unthreaded - coupled with the Delrin bushing that the arm bears against - virtually eliminates broken arms.
Callaham's pop-in arms have a reduced threaded length as compared to traditional arms (as seen in the image above right). It is therefore unthreaded where it bears against the Delrin bushing in Callaham blocks, and this allows you to push it in, then spin it once or twice to engage a thread or two. The threads are there simply to keep the user from inadvertently pulling the arm out during use. So the arm is virtually a pop-in design (though engaging a couple of threads is suggested), hence the name.
Made of solid stainless steel. Unplated, so you can bend the arm to your liking without fear that plating will begin to flake off.
Note: Callaham's pop-in arms are suitable for use only with Callaham Enhanced Vintage blocks. If you have a traditional threaded block, then please choose arms with a “Full Thread”.
