Aero Pack 54mm tool less mount not compatible with CTI engine case?

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Update of this thread so everyone knows the conclusion. The CTI cases are not uniform, I worked with MadCow and sent them my two new cases and the retainers so they could see for themselves what was going on. I also went to AMW where I got my 54MM cases and sure enough the CTI cases have a .003 difference from top to bottom. AMW was kind enough to check a few new cases and they all had the same problem. Robert even checked one of his older rockets with a 54MM retainer in it and same problem. He then went and pulled an older CTI case and everything fit just fine. So warning to new CTI 54MM buyers, the Aero Pac retainers will either have to be modified or you will not be able to get the motor into the rocket.
Hold the phone here folks. Normally I would be happy to rail on CTI 54mm "quality" but lets revisit the numbers here. ID of OP's retainer was calipered at 2.118, and when I calipered my (2) they were both exactly 2.144, a difference of 0.026. In precision machining world 26 thousandths is a proverbial foot. If you know a machinist/tool maker, they would bark at that tolerance immediately!

Just because your CTI case differs by 3 thousandths top to bottom does not explain your retainer ID being off 26 thousandths.

Root cause of this issue is the retainer being out of spec, gonna have to give CTI a pass on this one... I feel dirty saying that...