I have SKS fenders on a couple of bikes -- a 1972 Puch with non-threaded eyelets and a modern (2004) Felt with standard eyelets. The Felt takes the SKS M5 bolts without issue, and the Puch works just fine because I have the fastening nuts.

However, when I try the SKS bolts in the threaded eyelets of my 72 Motobecane Grand Record and my 86 Univega Gran Premio, the bolts don't thread in. Do these older bikes have a different thread pitch and/or diameter for their eyelets?

I found this thread that alludes to arcane threading in older bikes, but could find no further discussion:

https://www.bikeforums.net/bicycle-me...ifference.html

Thanks so much...

A mm or two?

I am not sure about the threading but I would never run a screw into a eyelet without running a tap first, especially on a older frame's dropout eyelets.

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...AFAIK, in ancient times there was no standard for fender eyelet threading. As you already know, an M5 works in the majority of cases. Sometimes they came without any threading at all, but they are relatively easy to tap with the appropriate size tap.

I've had one or two that needed to be tapped M6.

I see -- so the old threading really could be anything, and the thing that folks typically do is simply to update the old threading to the current accepted standard. Yes?

Yes and No LOL I should have stated that I use the tap just to clean and 'chase' the threading. The Motobecane could well be French threading. The Univega may be an issue of the threads just being dirty from lack of use.

Yep, even straight from the production line back-when eyelets were fouled w/ paint and/or excess chrome.
It was SOP to use a tap prior to installing racks, mudguards and such to chase the threads. If no accessories were installed the OEM finish is still there to be cleaned up decades on with whatever else has accumulated since.

Except for Chicago Schwinns M5 was the most common size we saw on eyelets.

-Bandera

The vast majority of forged dropouts with eyelets, whether Italian, French, Japanese or whatever, were threaded 5.0mm x 0.8mm. A few oddballs came with unthreaded eyelets, or 6.0mm x 1.0mm threading.

...if your hole looks like it's never been threaded, try an M-5 tap in there and you should be good to go. If the M-5 is too small to cut decent threads, you might need to go larger (M-6). If it's obviously threaded, try an M-5 tap and see how it fits in there.

That's pretty much worked for everything I've racked or fendered. There are probably some exceptions that some one knows about, like Whitworth British bikes or something. That's what always happens here when I make a general statement.

Some old French bikes have large unthreaded eyelets for fenders and racks. Those are the worst.

This is super helpful info, everyone. Thanks!

I should mention 1980's Cannondales with 10-32 threading, at least at the bottle cage riv-nuts, and late-1980's Raleigh Technium (the "backwards" one with 753 steel tubes and alloy lugs) which used 10-32 threading at the seatpost clamp.
These were simple to chase out to M5x.8 since the two sizes/pitches are so near to identical, but tapping the Riv-Nuts in the Cannondale requires some care to prevent the nut's spinning loose in the tube wall.
Well, tapping anything requires care to prevent breaking the hard/brittle tap!!!

My '72 Motobecane Grand Record has standard M5 threaded eyelets, but I guess it's possible that a previous owner did that. I don't remember if I even tried them before sending it off to be powder coated, but the threads were nice and clean when it came back from the powder coater.

I used to have a '77 Gitane with M6 eyelets.

They do sound like it... Until you realize that fender draw-nuts (metal fenders, cantilever brake pad mounts) fit through the hole. Elegant!

Well damn. You are right-that is ingenious. I will have to see how it works.

Just make sure to throw a leather/rubber washer between the frame and the stay, to protect the paint.

Metric bolts are expressed in diameter and size of one thread. M5 is 5x0.8mm, so it's 5mm diameter and 0.8mm thread.