Check the top end of the engine. Many have roll over "burrs" that can start fraying a BT-20 size tube. This can happen with the thin blue (Estes) or yellow (Quest) engine mount tubing.
Better yet -
Sand off the burr using some 220 grit on a block. Just roll off the end.
Here's the rounded, sanded off burr.
Don't take off too much, just enough so it doesn't rub or lift the inside layer of the spiral wound tube.
Sanding the ridge off also makes it easier to insert the motor into the motor tube. I will sometimes use a Delta belt sander and grind down a batch of motors in one sitting, but hand sanding may be safer than a belt sander with its potentially greater risk of a static induced motor ignition.
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