What's the difference between a 420 and a 428 chain?

A 428 should have a higher tensile strength than a 420. I would assume that if it has a 428 on it and there are no problems, that you could replace with a 428 and be better off than with a 420.

420NZ Pin Length 16.65mm, Roller Dia. 7.77mm, Plate thickness 1.8mm

428NZ Pin legth 18.90mm, Roller Dia. 8.50mm, Plate thickness 2.0mm

Both chains have a pitch length of 1/2" but are different size chains.
There is only ~700lbs. difference in the tensile strength of the two, due to the bigger plates on the 428.

Buy the chain that your sprocket is made for.

Some KX-100's have 420's, others have 428's. You were told correctly.

I have owned a KX-100 with a 420 chain, and another with a 428, and trust me...you are better off with the 428. The 420 is simply too wimpy for the power output of this bike, particularly when hauling around an adult (me). I found that the chain (420) wore enough to need adjustment almost every ride, even with proper lubing. This is/was truly one of the few weak spots of this bike.

What else? The fork springs are wimpy, and the cooling system is marginal...the bike will overheat if pushed hard enough. A higher pressure radiator cap helps.

Otherwise, congrats on owning a great bike! :yeehaw:

8, but I'm no math major ;)