How close is too close?
 

Having several low end Raleighs, I'm familiar with the mystery threading in the eyelets in the rear dropouts. I know a 5mm screw will go in there, but not exactly tighten up very well. Most of my bikes have rear racks, and I usually insert a 5mm Allen head, button head screw from inside the dropout, and then use a nut outside the rack leg. I'm trying this on the Super Course I'm currently working on, and using a 6 speed Shimano freewheel. There's a point when -- passing from the smallest rear cog to the second smallest -- the chain only has about a 1/16" clearance from the button head of the screw. Is that enough? Is a miss as good as a mile? How much minimum clearance do I need?

If it works, it works. If it doesn't , it doesn't. I would try it out. If it becomes a problem, use a short screw from the outside and lock tight it.

In this case, a miss is a miss in my book. I have a couple bikes where the chain in the high gear barely clears the inside of the dropout. I know it's not quite the same situation, but there's negligible flex between the cogs, chain, and dropout--unlike clearance at the rim or bottom bracket, where frame/wheel flex can turn a miss into contact. The only slight concern I might have is if you get a stiff link that impacts the nut. Perhaps file a bit off the nut. Alternatively, you could drill out the eyelet and tap it to 6mm.

I'm probably going to try riding it like this, and I guess time will tell. Just wondered if anyone had any specific reason not to trust the close tolerance.

I've had the same type of issue on some bikes. On a tour I noticed that every once in awhile the chain would hit the head of a screw that attached the fender to the dropout. When the outer link just lined up wrong, it would hit. Solution was to swap out the screw for a button head. Maybe 100 microns of clearance in the worst case, but it never hits now.

How much clearance do you need? Just enough.

Good life advice. :)

You and I are at least at the right distance:).