I had a cracked coolent expansion tank,  1" long split old tank ,  picked
up one from my flaps $18, cap leaked tried 3 other caps, still leaked.
Ordered a tank from the dealer $58. Guess what tanks had most of the same
markings and looked that they were made in the same place, well 98 % the
same. The aftermarket tank had 3 1/4 turns of threads and the VW tank had 4
turns of threads allowing the cap to tighten all the way.  Took me a while
to figure it out. If you buy a new coolent expansion tank check the threads
before you put it in, I put the cap on before but didn't tighen it down.
Lloyd in Eureka, Calif