I used a motorcycle 8mm nipple from this guy and it seems to be holding fine (I did put a blob of weld on the end though to make sure it didnt come off)
http://www.ebay.co.uk/itm/Motorcycle...em5890b 2b37d
Are you sure it's a temp sender on the polo rad? I thought it was the fan switch that went in that middle hole? If so I'm pretty sure its the same size/thread as the sierra 1.8 fan switch if that helps.