I have a similar problem with my chainring. I have an old 5 spd and replaced the crank and corresponding chainring. I did my best to have a straight line to gear 3, but still lose the chain from time to time. I'll will implement some of the ideas I read here to see what works. I can easily mount a fixed front derailluer cage to keep the chain, or look for some sort of chain keeper for my BCD. Where would one find a chain guard disc to attach to the crank spider? Robert