Doesn't the TRF501X (non worlds version) use different belts, hence why Jimmy bought some in his 'Big Belt Pulley conversion' report?
http://www.oople.com/rc/photos/cars/501x/bigbelts/
TRF501X belts are 363mm according to this -->
http://cgi.ebay.co.uk/Tamiya-51285-R...item4cf46d2279