Do you have the original cracked cap? When that happened to mine I glued the halves together and then set that repaired cap into a PVC end cap from the HoDepot plumbing department and glued it in place for reinforcement. Leak free for years now.
Stephen
    On Friday, June 28, 2019, 11:00:26 AM EDT, Jack Elliott <pdaxe2gto@GMAIL.COM> wrote:  
 
 The drain cap for my 84 Westy's freshwater tank developed a crack. The
online vendors give us two size options: 34mm and 35mm. The smaller 34mm
(P/N  253-067-285A) seems to be the correct size for this model year, so I
ordered one from GoWesty.

It's too large, turns without engaging the threads. I double-checked the
dimensions with their customer service dept. and it's clear that they
didn't send the 35mm one by accident.

So . . . here's what I'm looking at: a Westfalia OEM water tank (GoWesty
also looked at pictures of the tank I sent and agreed this is a stock
tank); with a drain pipe that has an o.d. of 34mm for the unthreaded
portion; and the threads are in quite good condition with no sign of
stripping, cross-threading, or wear. Nice sharp edges.

Yet the 34mm cap turns and turns without tightening.

I have a clunky fix installed, using a PVC ball valve from Home Depot and
it works ... but still, is there some odd smaller-than-34mm size rarely
seen?