Dr. Rocket rear closure measurements.

I'm getting ready to put in the final ring to support my Aero-Pac motor retention system. I'm just rechecking the measurements so I know whether to set the retainer at 3/8" or 1/4". I've got it marked for 3/8" but want to make sure.

Unfortunately I do not have a 29/180 or 240 to check. All I have is the Aerotech 29/40-120 and the rear closure measures 1/4". Can someone tell me how long the rear closure of the 29/180 or 240 Dr. Rocket case is?

Thanks

The Dr. Rocket 29mm rear closure measures 1cm or 3/8".

Thank you kind sir.