Broken drive sprocket bolt

My son's drive sprocket bolt broke while he was practicing today on his '05 crf250, and the front sprocket came off. The splines on the countershaft are flattened out, and the inside of the front sprocket is almost totally smooth. So what happened? Did the bolt break first? Did the bolt come loose first? Did this problem start somewhare else in the drive train? The most recent work done on the bike was clutch basket, springs, etc. (He was still complaining about the clutch, however.) Could these problems be related?
I know that we have to split the case to replace the countershaft. He also needs new front and rear sprockets, and a new chain. Do we need to check anything else? This morning, two different people asked Cameron if he had a job yet. :laugh:

Bswift, Stillwater is slightly less dusty for a day every once is awhile. But, I drive past a crude oil tank construction site daily where those giant backhoes are digging up buckets full of dust from thirteen feet down. I wonder how deep the water is?

Dirtbikr99, we got our parts from Honda. I'm glad that you didn't tear anything up. We had to buy both sprockets, a chain, a countershaft, a bolt, and split the case to fix the bike.

We are going to keep an eye on that bolt, and periodically replace it.

Looks like we might have had the chain too tight then.

Bswift, I don't think that I have met you. My son had a broken leg at dirt week 04, and he rode one of the #515 crf250s at dirt week 05. He is riding less at the 500 these days, and is addicted to motocross. He's #27 on an 05 crf250, and he is usually with one or more of his friends at an area motocross track. I drive the truck, pit crew when his dad is at work, and bring the food. I haven't been on a bike since I scared myself on his 85. It was a little faster than the trail bikes or the 75 street bike that I had in the 70s. :laugh: