[IdahoDoug]

I think I had a helper pry on the pipe to get it aligned enough to properly start the threads. Of course, start the threads by hand, which means it needs to be pried hard enough there is no interference at all from the part you're screwing down. Those are aluminum threads, after all.... Once both bolts are in several turns, you can finish screwing it down without the prying.

 

[IdahoDoug]

Ooh. You may need to take Old Bonus' advice and fully remove the pipe to completely clean both surfaces. Obviously you are wishing you had noticed this before assembly. A razorblade will make short work of it..