I have an 02 WRX wagon and i have read that the ideal rear sway bar is 20mm but what happens if i run 21mm?

 

You can read my reply under 2006 wrx swaybar post a few above yours,but to elaborate I'm an old guy who's been playin w/perf cars and bikes for 40+ yrs.The first thing I try to do is to look around for the oldest guy who has been working on these kind of cars the longest in the performance arena. There's a good example at the website www.spdusa.com. GO THERE and READ IT ALL!! The first thing I picked up is don't do anything until you put on a first class set of tires (of course you do need to be able to afford 'em) but I did put on Michelin pilot sport ps2's-you wouldn't have much change from $900 but they are worth it. But as I said the car was still lacking. The web site didn't really recomend anything much bigger than 22mm for everyday driving,I agonized over it and over it until talking to some reliable and knowledgeable shop people they talked me into the hollow Cobb bars-Cobb doesn't even list a size for these-but the improvement is fantastic.If they don't make a wagon bar look around on the web,perrin,whiteline,hotchkiss etc. BUT get both and stick to something around 22mm-these are not harsh. I don't think you will believe how much they improve turn in and feel in the corner and the understeer of the car is reduced greatly. One last thing on the rear bar get one that's adjustable-It just means there are 3 holes in the bar to attach to.Start in the middle.