14mm. Seems about right. We're going to try a 12.7mm this weekend, probably work OK, we'll see. The big downside is the stopper slams into the ferrul and adds friction to the setup. It still works fine, but maybe not for teams that adjust a lot (the pros adjust every time they tack). That is why we generally see double blocks and why I wanted to try 2 shock blocks tied together. I may still give that a go but it's a fair amount of splicing vs. 1 single ferrul.