If you have a set of calipers or micrometers, I posted the actual dimensions I measured from my bulkhead. (in the post with the picture) I just measured the o-ring thickness, and subtracted .015 from that number, and thats how much I shaved down the bulkhead. That way the o-ring is under a little bit of compression and minimizes how much the arm "moves" during racing conditions. If you need a more detailed answer/pics, let me know...I have a spare bulkhead I could mod and post a step by step.