Bicycle Mechanics - Standard Thread for Rack/Fender Eyelets

Is there a standard thread for rack and fender eyelets. I don't want to buy a bunch of bolts I don't need/go to the hardware store 10 times.

It's an old schwinn mtn bike if that helps

All of the bikes I've worked on use M5x.8 bolts to fasten racks and fenders to the dropout and fork eyelets.

All of the bikes I've worked on use M5x.8 bolts to fasten racks and fenders to the dropout and fork eyelets.

Another no-nonsense reply by HillRider. Seriously, you should be awarded a "Bike Mechanics subforum poster of the decade" - your posts are consistently great.

I have an oddball Fuji that uses 6mm bolts, but it's the only one I've seen that does.

My Axis uses 6mm on the rear and 5mm on the front. But I think all my other bikes have used 5mm.

All of the bikes I've worked on use M5x.8 bolts to fasten racks and fenders to the dropout and fork eyelets.

A 10-32 will work also and a 4mm allen wrench will fit in those.

Careful with those 10-32 bolts! A #10 screw is just enough smaller in diameter than a 5mm screw that it'll strip the threads in the hole pretty easily. Instead of torquing the 10-32 screw in very tightly, I use a drop of blue loktite and just snug the screw in place.

Regards,
Bob P.

Why not just use a 5mm bolt? It's not like they are very hard to find. Any decent hardware store will have them, or get a couple of friends together and buy a box from mcmaster.com. Using a #10 in a 5mm hole is like using vise grips instead of a wrench. Sure it will work, but you're going to damage stuff.

Careful with those 10-32 bolts! A #10 screw is just enough smaller in diameter than a 5mm screw that it'll strip the threads in the hole pretty easily. Instead of torquing the 10-32 screw in very tightly, I use a drop of blue loktite and just snug the screw in place.

Regards,
Bob P.

They've never been a problem in the hundreds of bolts I've...um...obtained from work. I have yet to strip out a rack mount on any bike I've used them on.

Why not just use a 5mm bolt? It's not like they are very hard to find. Any decent hardware store will have them, or get a couple of friends together and buy a box from mcmaster.com. Using a #10 in a 5mm hole is like using vise grips instead of a wrench. Sure it will work, but you're going to damage stuff.

I use the 10-32 because finding a 5mm bolt isn't...or at least wasn't... as easy as you think. My local hardware store which is a really good one, didn't start carrying metric bolts until just a few years ago. I'm not sure you could find one at the big box stores now. Back when I started riding, metric bolts were a very special order item. In other words, don't ask because you'll get one of 2 reactions, either outright laughter or such a blank stare that you'd think the clerk had just gone comatose! 10-32 bolts worked and...as I've said earlier...have never damaged any of my bikes.

Any japanese motorcycle shop will have stocked them for decades. All the shops I worked it had a Wurth jobber come around and keep our bins full.

One shop even had Stainless Steel . :)

My local hardware store which is a really good one, didn't start carrying metric bolts until just a few years ago. I'm not sure you could find one at the big box stores now.
ACE, Lowes, Home Depot and OSH all carry metric fasteners. I can understand why they may have been hard to find 20 years ago, but even GM has been using a handful of metric fasteners for 10 years (which I discovered changing the brake pads on a 97 pontiac).

.....even GM has been using a handful of metric fasteners for 10 years (which I discovered changing the brake pads on a 97 pontiac).

Longer than that. I had a 1978 Pontiac Grand Am that had a mix of metric and SAE fasteners. Apparenly the more recently redesigned components were metric and the older ones SAE. The general rule was if the bolt had a blue painted head, it was metric.

Longer than that. I had a 1978 Pontiac Grand Am that had a mix of metric and SAE fasteners. Apparenly the more recently redesigned components were metric and the older ones SAE. The general rule was if the bolt had a blue painted head, it was metric.
Interesting. I assumed the difference between metric and SAE parts was where they had been sourced. I guess the lesson here is that auto parts stores have always been a good place to find metric fasteners.

ACE, Lowes, Home Depot and OSH all carry metric fasteners. I can understand why they may have been hard to find 20 years ago, but even GM has been using a handful of metric fasteners for 10 years (which I discovered changing the brake pads on a 97 pontiac).

It wasn't 20 years ago that they were hard to find...try less then 10...in a general hardware store. Lowes carried more stuff earlier (back when they were Eagle and had a far better selection of just about everything;)) and I've seen them popping up more in other hardware stores.

However, after talking to the machinist (and fellow bike rider) here at work, he was very surprised at how close the thread pitch and the diameter of the bolt are. He said that for a short bolt, like a rack bolt, there wouldn't be any issue. The diameter is 5000th's smaller but, for certain applications, they often drill their holes that much larger prior to tapping them so as not to break taps. He wouldn't suggest it for a bolt that has to go into something an inch or more but for a short thread like a braze-on fitting, it won't hurt anything and will hold just fine.

Something else to consider, if you don't want to chase the threads or don't have the taps to do it, bikes are painted after the braze-ons go on. I have yet to see one that has had the threads chased after painting so the hole size is even smaller than the M5 should use. A smaller diameter bolt is easier to put in then the larger M5 would be.

Essentially, a 10-32 isn't going to hurt anything. It certainly isn't a 'visegrip vs a proper wrench'.

The pitch is shockingly similar. 10-32 = 32 threads per inch. M5x0.8 = 0.8 mm / thread. 1/0.8 = 1.25 threads per mm. 1.25mm * 25.4mm/in = 31.75 threads per inch.

They both use a 60deg angle on the threads.

Pitch diameter of 10-32 is 0.1697". M5x0.8 is 4.480mm = 0.1763".
10-32 UNF-2A (external thread) Major diameter= 0.1891" max - 0.1831" min
M5x0.8 Major diameter female = 5mm = 0.1969"

That gives you about 0.010" overlap, where normaly on a M5 thread you'd have 0.017-0.018" overlap. So, approx. half.

Mind you, except maybe water bottles, the majority of fender and rack mountings are largely subject to radial as opposed to axial forces. So, in general, you should be alright, but in my opinion you'd might as well us a proper M5 bolt if you can.

On a lot of parts we manufacture where I work, we'll cast one hole size and use it for both M5 and #10 tapped finished products. Note that we do use the appropriate taps though. I have installed #10 screws in M5 holes before but I'm pretty sure it was by accident. You can definitely feel that something is wrong although on a bike frame with paint in the threads, you'd be hard pressed to notice.

FWIW, I've never had a problem finding metric hardware in the past 15 or so years that I've had a use for it. Napa stores are a terrific source for hardware when you don't have time for mailorder.

If you ever want to tell if a bolt is metric or standard, an easy way to tell is to check for marking on the bolt head. Steel metric bolts using a numerical system for their strength rating (common ratings are 5.8, 8.8, 10.9, and 12.9). Stainless steel metric bolts are generally marked by their material (genereally A2 or A4 for 18-8 or 316 stainless respectively). Steel standard bolts using a dashed marking system that I'm very unfamiliar with only because the relatively moderns cars (mid 80's and up) I've worked on (where most of my wrenching is done) all use metric hardware, save for my parents' Canadian-built 1988 Suburban which uses a mix. Stainless steel standard bolts (and sometimes washers) can be found marked with the actual stainless steel type (such as "316"). If it's unmarked, it could be either.

Important question: I need, for a vintage computer (Horizon), four imperial-threaded fasteners. No metric fastener will fit - and I don't want to destroy/strip the threads in the case. Where from can I order imperial fasteners (I live in Europe)? Remember, I need only 4 - that means, some shops won't even bother to answer me.

Look for a moto shop that does older (pre '70) British bike restorations. IIRC BSA, Triumph, Royal Enfield, etc. all used Imperial thread back in the day.