I am planning to build a minimum diameter 38mm rocket, using the Aero Pack minimum diameter motor retainers. I am planning to fly Loki Research motors in it. However, I have ran into a problem: the Aero Pack retainers have a 5/16-18 threaded bolt, but Loki plugged bulkheads have a 1/4-20 threaded hole. I could tap the Loki bulkhead out to 5/16-18, but would that count as "modifying the motor" which is not allowed by NAR/TRA rules? Could I modify the retainer in some way? Or should I use an entirely different retention method altogether? Any and all ideas appreciated.