More photos. 2 bbl rotary barrel. I included my prop hub for size comparison. The hub is 1.5" long. The smaller section of the carb barrel is the primary and the larger diameter is the secondary. The needle going through it is not removable to modify this item and now I don't think it is necessary. I made the Prob Hub from Fortal, it is harder than 6061, which I wanted for the same reasons Rhysium stated. The rear of the hub has 8mm x1mm threads, 1/2" of them and the front of the hub, 1" long, has 5/16"x24TPI threads. These two thread are familiar enough looking that i didn't want to have prop bolts of both types and risk mixing them up. I use the 5/16x24 bolts on other engines also. This will keep it simpler at the field during a flying session. (I can screw up enough without adding possibilities, excuse the pun.)