Check your balls...

[panheadpete]

...'cos size DOES matter; just measured the size of the towball on my newly acquired, American made towbar and it's 47mm - standard American one and seven-eighths, I believe.
Not too clever if you're using a standard 50mm hitch, not at all clever if you've got an old British 2" trailer hitch (50.8mm).
Just a thought.
Cheers,
Pete

[Mr B]

1 7/8", that's the American standard. My towbar came with this size ball on it. I drilled the mounting plate out to take a 50mm chrome ball off Ebay....£13.50 or thereabouts.

Brian.