I've had to borrow my buddies Fin straightener before, and my saying is borrow twice, then I need my own lol, so I ordered my own, and have used it on every one of my bikes, it does take some patience to work with it, but your time pays off!
I saw this tool advertised online and thought- what a great idea. Found it on this site and was bummed about the poor reviews. But for the price I thought..I'll try it. Glad I did.
Step one: Carefully insert tool in straight sections of fins to find the correct size for your radiator. On our 2015 YZ250F size 14 fit perfect.
Step two: Again carefully insert tool between fins a small amount, and when you are certain the alignment is good, push all the way in. Its best to start out on a section that does not need to be straightened then work one fin at a time to bend back folded over sections. If you are a perfectionist or just care about taking care of your equipment this is a tool you should own.