If I interpret your query correctly, you want to retain a Blackpowder F motor in the motor tube?
Some people use a small ring inside the motor tube called an Engine Block. The Motor pushes against it to transfer the thrust.
Others wrap 1/4 inch of tape around the back of the motor and build up a kind of thrust ring that pushes against the back of the tube (the motor will hang out by about 1/4 inch)
For retention (keeping the motor from being ejected out the back, you can use a thin metal strip or thick gauge wire to create a "motor hook" on the backside, or you can wrap masking tape around the motor case so it makes a very tight friction fit. If you do the 1/4 inch tape thrust ring, you can then put more tape over both the ring and the back of the motor mount which could retain the motor.
Is this what you're asking about? May help if you can explain more about what you're trying to figure out.