chain size?

[ecy063]

I would like to convert the chain and gears on my sons cr85b. I was told that there are conversion gears made that step chain size up to 428 from the smaller 420. Question is what is the difference? Wider or longer? Both? Do I just change the front sprocket? Please help as I am getting conflicting imformation from the local shops.

 

[Cody305]

I tried to do this on our '05 CR and it hit the ignition cover. I guess if I went down a tooth it would have fit, but that would have changed the gearing to much.
The 420 chains work ok, just buy the heavy duty type.

 

[ecy063]

Thanks Cody305. The reason I wanted the wider gear was the output shaft spines seem to be wearing out. I have only four races left in the season and the bike is being retired. Hoping to not have to split the cases to reploace the shaft.