Katzenjammer

I discovered that the rack eyelets for a used touring bike I bought don't take the standard M5/0.8 pitch cap screw. Nor, as I discovered, do they take an M6, or at least not those with 1.0 pitch which is what my LHS had. My off-road bike uses M5 screws for the rack as well as for everything else, so I thought that was the standard (I'm influenced by computers, I think--I expect everything to be standardized)

My experience isn't extensive, so this might be a common variation that everyone knows about except me. Any idea what size/threading cap screw I actually need?

Retro Grouch 

I've never encountered one that wasn't a common size. Are you sure it doesn't have a gob of paint that's making it hard to make a screw thread in? That's a common problem.

Katzenjammer

I was sure you were right, but it actually turned out to be something else. Apparently the two I tried had slightly chewed threads that weren't obvious even under magnification. And since I was trying to be really gentle so as not to chew the threads(!) I only tried turning the screw in with my fingers. Just now, to test the paint thesis, I tried the M6 screw in one of the pristine-looking front eyelets and it went in like butter. So I used a wrench and it went into the ones that previously had resisted, too. Así es la vida, I guess.

But now, unless I can get some kind of M6->M5 reducing grummage (if such a thing even exists), it looks like I'm going to have to bore out the mounting holes on the racks to suit. Which doesn't feel like a good idea since there isn't that much spare metal.

Retro Grouch 

Can you run the bolts through from the opposite direction?

Katzenjammer

From the wheel side out, do you mean? If so, yes, easily.

GTcommuter

I had a similar problem on an older Peugeot. M6 size holes, but un-threaded. I put M5 bolts with flat hex heads sticking out like Retro Grouch suggests and sandwiched them with washers. Then mounted the fender mounts to these and put a nut on the outside. It worked fine, but the fenders aren't weight bearing.

Katzenjammer

If it were only going to be fenders, I'd do that too. But it's racks and fenders

GTcommuter

Yea, I figured. Sounds like it's time to machine some custom step-down bolts with a short M6 section and ending with an M5 stud.

Let me know what you work out.

Katzenjammer

The only thing that comes to mind is maybe make 1 inch long bridges out of, say, 0.25 aluminum. Run an M6 hole through one end for the eyelet mount, and then an M5 hole at the other end for an M5 screw and locknut, presuming they make M5 locknuts (I didn't see any at the LHS, but on the other hand I wasn't looking). As long as I have enough clearance at the top of the tire, it seems like it should work.

What I really need is an M6 sleeve that would accept an M4 screw...I could always cut a slice of tube to shim out the rack mounting point. *sigh*

HillRider

Can you use M5 bolts in the existing holes and retain the bolts with nuts instead of the threads in the dropout eyelets?

Katzenjammer

I think that's what Grouch was obliquely suggesting, too, but I didn't immediately pick up on it. Good question. I don't want to wreck the threads in the eyelet, but maybe I could protect them by shimming with some nice soft aluminum tubing. I'll think on that--thanks for the idea!

Additional ideas gratefully received!