son was riding it and all of a sudden a metal on metal grinding noise. i immediately noticed the chain was grinding the guard down and the front engine side sprocket had slide off some. pulled the cover and sure enough. what typically holds this thing in place? what should i replace so this doesn't happen again?

 

That sheet metal locwasher bolted onto the sprocket should have splines on it that are opposite to the ones on the sprocket, buy a new one. You shove the sprocket and the sheet metal lockwasher separately onto the shaft until the lockwasher gets to the groove in the shaft. You can then turn the lockwasher round in the groove until the bolt holes line up with those on the sprocket, then fit the two bolts. There are lots of different locking washers, so get the right one for your engine.

 

ok sounds good. i will probably also get a 12T sprocket to help with some low end torque and a new chain.

 

"should have splines on it" - yeah they were completely worn off. ordered 12T 428 sprocket, new chain (current chain is stretched), and new retainer. should be good to go after that. i might change the tooth count on the rear sprocket to complete the "regear" at a later date.