Here's my 2 cents on this. My back sprocket is as true as I can get it with my limited knowledge and tools, my chain kept popping off, I couldn't figure out why for the life of me until I saw it, the rear sprocket is just a bit too far out from the hub of the wheel and it does not line up w/ the sprocket in the engine, so the chain kept popping off on the inside of the sprocket next to the wheel. My first idler, with set screw got pulled into the spokes, the engine ripped the set screw right out and idler into the wheel, so $50 later and a new wheel, I did away w/ the idler, went to TSC and got some #41 chain, it's a bit wider than the 415 that my kit came with and it allowed for the small indifference in spacing of the front and rear sprocket. Also TSC sells a nice spring loaded idler that will mount to your seat down tube with a U bolt, I saw it installed on one of the threads on this forum, and it looked like a good setup, Much better than the crap idler these come with that when it fails will cause a catastrophic failure if you are going at a high enough speed. I believe you need to treat any moving part like the safety of a gun, it can and someday will fail, and if it fails you don't want to be on the business end.