I replaced my old castle nut with a self-locking nut and I hated it. The darn thing tried to cut the threads off the end of my axle. I used it twice and went back to the castle nut. I've been using this and it works pretty well (sprung snug, doesn't vibrate). But I like Emmbeedee's idea even better. Although I've had the DR for 'awhile', I don't like to check my chain tension as described in the manual. I frequently stand on the right side of the bike, lie over the seat (like a deer on the fender), grab the axle/swingarm with my left hand, pull my weight down on the seat, and check the chain tension with my right hand. Do NOT pull the bike over and pin yourself to the garage floor: your wife will NOT be happy. There should be a noticeable free play, NOT snug (but not the 1.2-1.8" in the manual, because that's for an unloaded check). About the upper chain roller, yes remove it; but I don't recommend using a bolt to plug the 'now empty' hole. Here's a pic of where my chain was hitting the bolt that I used to plug the hole. Sooooo, use a set-screw, available at the nearest Home Depot (8x1.25mm). Look closely and you'll see the blue Loctite that I used to keep it there.