Riding to work today I felt something big hit me in the knee. I didn't see any birds or flying debris. I looked down a minute later to see that my HD flush mount gas cap had jack in the boxed itself out of the tank filler. Awesome, as if $200 a set wasn't enough, had to shell out for a replacement. If you use them, make sure you check it is locked real good after filling up.

 

Thank's for pointing that out , a jack in the box does share design features with flush mount gas caps.

 

Regardless of the fact that the flush mount uses a spring loaded twist lock to raise and lower the grip area, it still screws into the fill port the same as any other cap. A standard cap will just as likely fly off if not properly tightened, which is what had to have been the case here. The spring has nothing to do with retaining the cap on the tank.

Unless the OP is saying the cap broke and the pop up part of the cap separated leaving the cap itself still in the tank...

I've had them on two bikes and never have them been even slightly loose.

 

I'm usually pretty good about making sure it is screwed in tight. Not really sure what happened, but it went from being locked in at home to gone within 4 miles. Didn't even heat in rattle around, first at sign was hitting my knee. Not saying it wasn't operator error of some sort, just something to watch.

 

Definitely an unusual situation. No doubt. Will make me sure it clicks when I tighten mine.

 

I know they are prone to come apart but I have never heard of or seen one ejected.
I have had mine come apart at the pump, a couple of times, and had to fiddle with it to get it back together and make it home. Last time that happened, I ordered an "improved" replacement...It wasn't cheap either.

 

After 80,000km mine went the other way, it seized closed.

The parts guy said he'd never seen one wear out before. (Only lost/dropped/broken ones).