For all you modders...18R(RG) and 20R(21,22R) flywheels are interchangable. The 20R wheel uses bigger bolts, so the holes are bigger, and spaced slightly farther apart. The 20R wheel will bolt onto an 18R crank though.

Clutch side. 18R, 20R stock, 20R machined and lightened. Look how much bigger the 20R clutch surface is. If you are slipping a stock clutch on an 18R, this is a good upgrade.

Here is the crank face. You can see that a little material came off of the lightened wheel (18 lbs). The stock 20R (22 lbs) wheel is actually several pounds lighter than stock 18R (24 lbs).