Its a RKsport TB but how can I tell if its 52mm or 58mm?

 

You could measure the diameter of the openings 25.4mm/inch if you don't have a mm ruler.

Also, if you have it off look at your intake. Most intake port jobs it is "obvious" they have been ported because normally a Dremel or similar hand held tool is used. The stock openings are 52mm. If yours have been bored larger for the throttle body, chances are it's a 58mm tb. If the intake holes are nice and perfectly round and the tb mates up good then it's likely a 52mm.

However, easiest way by far is to just measure the openings (across the center).

 

Thanks, so measure each opening separately correct? (aka each circular opening)

 

Yep!

 

t-gage and calipers would be the most accurate, but a tailor's tape should be enough to find out. 48mm~1.89" 52mm~2.05" 58mm~2.28"