Are you 100% sure? My machinery's handbook says different. As does the Complete Chain Guide, a valuable internet resource I've gotten good use out of in the past: The Complete Chain Guide, section 1.5.2: Motorcycle Chains. Tsubaki's website also has some useful chain dimension charts: Tsubaki ANSI standard roller chain.Yes, it WILL fit as the rollers are smaller although the pitch and roller width are the same.
The pitch will be the same but a 420 motorcycle chain is narrower than an ANSI 40 or 41 chain, so no it won't fit. A 425 or 428 chain, however, would fit just fine.
428 is old school! Lots of older dirtbikes from the late 70's thru the early 90's used 428 chain. The steady increase in power output from newer bikes has forced a move to 520 or 525 chain (in dirtbikes, at least). However, 428 chain is still used on kids bikes (eg; Kawi KLX110's, Honda XR70's/50's, etc). I'm confused though, you say it's too narrow? I'm pretty sure it's wider than 420 (width inside the links). The rollers are significantly larger though, maybe that's why it never fits for you?have you held up a piece of 428 chain next to a piece of 420??? Help me out here man, I've compared them, very seldom can I ever use 428 for go-kart purposes even when I want to given the awesome strength. Its just too narrow.