I had a 415 front made for a DRZ125 minimoto project, and ran it with a couple of different 420 rear sprockets and a 420 chain. I did this just to figure out the gearing before having the proper 415 rears made. The pitch and pin diameters are the same between 415 and 420, the 415 stuff is just narrower. I'm not sure I'd try it on a triple digit speed 300/400 class bike, but I didn't give it a second thought on a 60mph mini.
No kidding! The penalty for failure isn't nearly as severe, so caution is sometimes thrown to the wind.
When we ran RS concerted chassis with 65, 85, and 150R motors we did exactly this and so did most everyone else as far as I know. A friend found a place that made custom front sprockets and then we switched everything to 415.
This is what I'm being told but just doesn't seem right. All front sprockets i can find are 420. The chain that is on there now is 415. I gotta have the rears made and I need it by this weekend. I was told I could run a 420 front and and 415 rear with a 420 chain, but that just seems like disaster to me.
We did it for a couple years. Everything is the same except the chain is a little wider, so a little more side gap on the back sprocket.
Google PBI sprockets and then order a couple of 415 CR85 sprockets. Don't send your kid out on anything not as perfect as you can make it.