How much tolerance is acceptable on the chainline of a fixed gear conversion? When converting a geared bike to fg/ss, at some point there will be adjustments to be made regarding chainline. Without getting into details about what could be made to make it better (bb axle width, hub spacers, different cogs, whatever), how much disalignment can we have between chainring and rear cog?



Currently, I am building a fixed gear out of 90s MTB frame, track rear wheel and road crankset, and I am sure it will work eventually, because I've done this before and I don't care about the hassle. What I hope is someone could give this question a final answer, to enable (or not) people who are into that "DIY fixed gear spirit" to have some confidence (or to know the risks with more certainty) before starting a new conversion project instead of simply buying new stuff.
There is no hard and fast limit. The farther off the cogs are, the worse vibration and wear you will get. My personal experience is that a couple of millimeters off is still smooth and at 5 mm the chain will be noisy but still work. 

On a singlespeed it's virtually impossible to throw a properly tensioned chain without breaking cog teeth, but bad enough chainline will wear them out and make that easier.