I got a rear rack for my bike, I go to put on, and I realized the eyelets don't have threads. This is probably an early 80's steel bike, of unknown brand and origin. It has 27" tires (not that I think this matters for this question).

It looks like I probably have enough clearance for a nut on the back side of the eyelets, but I won't know until I can get to the hardware store. Is there something different I should be looking for to use for this? Some kind of threaded clip or something? I bought this for a backup commuter / medium distance rider. I have it all ready to go except for the rack (and fenders, but I'm not sure I have adequate clearance in the back). I'm gonna take it to the LBS and see if they have any ideas, unless someone here has any suggestions for a tight clearance on the rear wheel?

What size are the holes in the frame and what size are the holes in the rack you want to mount? You can use bolts with thin hex heads, mounted with the head on the inside, or you can thread the frame. 5mm is the size I most commonly encounter, but if the frame holes are bigger, you may be able to thread them for 6mm, which is another common size. If all else fails, and you don’t mind burning the paint, you can get a couple of threaded flanged mounts that are used for brazed in water bottle mounts and drill the eyelets for them and braze them in place. Silver has a lower melting temp than brass and can be used to avoid damage to the brazed joints of the dropouts.

No brazing is in my future. I'll see about thin hex head bolts. I don't know the size. I'll have to measure later. If they're small enough, threading sounds like the way to go.

Standard 5mm cap screws seem to have heads 3.5mm high, roughly .138”. I also looked up pan head machine screws. They seem to have head heights of 3.3mm to 3.8mm. You need to look at the clearance you have on the inside of your right dropout to see it you have clearance for the chain and outer sprocket.

It just occurred to me that you could put in bottle holder flanged receivers by drilling for them and putting them and holding them in with something like Super Glue. I would probably put them in from the back side. No burning the paint.

Right side , fit bolt head in, there are some pretty flat head machine screws..
for chain clearance in top cog .
left side , plenty of room..

Try a blind nut if you can find one small enough to fit in the hole.

Otherwise as you already figured out, a simple thru bolt and lock nut with nylon insert should do the trick.

I suggest a button head bolt threaded from the inside, with a nylock nut on the outside. I often do this anyway, it solves the problem of rack bolts unscrewing.

Unless the holes are way too big, I'd just an appropriate sized tap to thread them. If you are in doubt you could have a bike shop do it, but if you are mechanically inclined it is pretty easy.

Thanks for all the suggestions. Need to measure, and visit the hardware store / LBS to see what I can find.

5mm pan head machine screws, stainless of course, with a nyloc nut as mentioned above. I know the Parts Source stores around here sell 'em... stainless nyloc nuts were 0.11$ each. I've had to do this with a few French bikes, except the holes are bigger so you can 6mm if your racks will accommodate them.. I usually install my fender/rack screws pointing out, just in case I ever break one off I can just unscrew it and put a new one in.

I recently got a similar frame and when I asked my LBS if they could put a Helicoil insert they said no (too small). I could do it myself, but I'd rather have someone experienced do it and my LBS is obviously not that someone.

Other than clearance, is there any advantage to a threaded hole vs an unthreaded hole with a nut on the other side? For all the trouble with threading, possible stripping, etc I'm surprised that they're threaded on so many frames...unless there's some hidden advantage?

It's easier to manufacture and easier to people to install a screw rather than putting a nut and washer on a bolt, that'd be my guess.