1.882" Depending upon how far out of the pen you prefer the refill tip to extend, you can use a slightly longer tube and get lesser tip extension. 1.92" is probably about as long as you want to go.
I had several customers complain about the noise so I stopped making these. I've never found a click I liked. A $2 Parker Jotter has the smoothest clicking action but yet not one of the stupid click kits Berea or Dayacom makes is worth a crud. End of rant.