Alright, my NSR in the final form will likely be a 90+cc bored up 80 over 20hp. I have a ridiculous extended swingarm which is going to impart yet more stress on the chain as it will likely be half again as long as original. I ride on the street and will be hanging the throttle wide open pretty much all the time. I want to be 100mph capable if the motor can pull my fat ass.
I am concerned that the 420-size chain might not be up to the task and the idea of a chain breaking at that speed is not encouraging. I think though if the NSF guys running bored up 100s on the track are ok I will be too. Are any of you guys busting chains?
I don't think you will have a problem. We run 420 chains on RS conversion bikes that have CR85's and 150R 4 stroke motors in them. Also RS125's with over 40HP run a 415 which is basically the same chain, just narrower for less weight.
I've run 420 Gold RK Chains and end up replacing them every 1500-2000 miles on my big motor builds. They stretch out a little bit here and there and once they do that they tend to eat up your nice expensive aluminum custom sprockets.
It will hold fine, but do notice that you don't move your wheel and the chain gets looser, you may want to think of replacing the chain. Your mileage will vary depending on what kind of beating you're giving it.
I only bought it cause it was the last 420 chain on the shelf at my local shop and I wanted to ride that day. It was a cheap 18 dollar d.i.d. black chain ( I almost always ran gold chains before) and after at least 10hrs on my heavily modded 22+hp cr85 It is stretched but not like I thought it would have been (3/16 of an inch).