The internal motor retainers are designed to retain Aerotech and Loki motors that have threaded closures for rockets that are using electronics for deployment (they are not compatible with motor ejection). The retainers can be used in minimum diameter rockets as well as larger rocket air frames.
The 38mm internal 1/4" motor retainer is used for both minimum diameter rockets as well as general motor retention for any rocket motor that has a Loki forward closure with a 1/4 threaded end.
The 38mm internal 5/16" motor retainer is used for both minimum diameter rockets as well as general motor retention for any rocket motor that has an Aerotech forward closure with a 5/16 threaded end.
The BT60 internal 1/4" motor retainer is used for both minimum diameter rockets as well as general motor retention for any rocket motor that has a Loki forward closure with a 1/4-20 threaded end.
The BT60 internal 5/16" motor retainer is used for both minimum diameter rockets as well as general motor retention for any rocket motor that has an Aerotech forward closure with a 5/16 threaded end.