Originally Posted by DESERT SCORPION
Yes, the gas cap wants to cross thread very easily. This was not always the case on my bike. It swells up after continued exposure to gasoline. I partly remedied this situation by baking the cap in the oven at 175 degrees F. for about three hours. Much smoother fit after this, but after a few weeks its as sticky as ever. Next I tried sanding the entire threaded portion of the cap down with 220 wet or dry sandpaper. This barely had any effect because the plastic is extremely abrasion-resistant. I could not remove enough material. The trick I use presently is to wind the cap counterclockwise until I feel the tank and cap threads jump by one pitch. Now firmly push the cap againt the tank opening while slowly turning clockwise. One must be very persistent to keep the cap level with the face of the filler neck. After about half a turn it is engaged enough to finish tightening it. Naturally BMW chose not to use a standard pitch, or I would just replace the damned thing with a $10 O'Reilly non-vented cap. There is a nice alloy cap that Husky lists in their Special Parts Catalog for the TE449; Cod. 8000H4069. I wonder if it will work on the BMW?
I use a dab of Vaseline (petroleum Jelly) on the threads. I no longer have a problem..