I've had different drain down spouts with different threads so good
luck.  BUT, Try a boat supply shop for an expandable/adjustable
stopper that goes in the back of a boat - a drain plug.  But be sure
to put a metal band around the plastic water drain pipe so that you
do not break it with the expanding plug.

Alternatively, I found that a hotel shampoo bottle (thumb size) fits
nicely going down from inside the tank in the hole, maybe a cork
would do that too.   good luck.  malcolm s

frank wrote:

> Where can I get a cap to go on the water-tank outlet pipe underneath
> the van?  It is an 87 Westy,