Bob G offers different size replacement shafts, among them a 5/8", which works out to 15.875 mm... close enough for rock and roll as they say.
$19.
http://www.urbbob.com/gaineswheel.html
FWIW, i had a wheel ages ago and bought a new bass that had a huge 1" endpin. I just had a welder attach a new, thicker shaft on it. In retrospect that was not too wise, since I have never seen a 1" endpin before or since, and that bass is long gone.