What size are the bolt holes on my bicycle?
November 5, 2007 10:08 AM   Subscribe

What size are the bolt holes on my bicycle?

Everything I buy for my bike (rack, fenders) comes with the right size bolts, so the bolt size must be standard. But now I have 3 or 4 different types of bolts on my bike and I want them to all be the same. BikeUSA has lots of different bolts, but which ones are the right size for my bike? It looks like the two choices are 5 or 6, but which one is it?

I have an old Cannondale M800 if it matters, but like I said, everything comes with the right size bolts, so there must be a standard size for frame accessory bolts.
posted by GuyZero to Sports, Hobbies, & Recreation (9 answers total)
Somebody's probably going to come in with the real answer but for future (non-bike) reference most hardware stores have a gauge you can use to size bolt diameter and thread pitch.
posted by Opposite George at 10:11 AM on November 5, 2007

I don't really understand the problem here. If you installed the current set of bolts, you must have the right size allen wrenches to have installed them. If a 5mm wrench fits it, then it's a 5mm bolt. Most of the bolts probably are 5mm, but some of them might be 6mm, which is why you can buy both kinds. If you don't own 5- and 6-mm allen wrenches, then you should - and you can determine what size bolts you have.
posted by Wolfdog at 10:44 AM on November 5, 2007

Dunno what bolts you've been working with, wolfdog, but the hex-size=bolt-size mantra only ever got me in trouble.

That said, pull one of the bolts. measure across the threads, and that's what the threads are. 5 or 6 mm. If you're not sure of your measurement, go to autozone or whatever discount auto parts store is around. They'll have gauges. Beware, though- metric bolts can be different diameter sizes but still hace the same thread pitch, so make sure it's a snug fit and wobble - and thread it into the gauge pretty far.
posted by notsnot at 10:48 AM on November 5, 2007

Most bolts on most bikes are 5mm, but some are 6mm, 4mm, 3mm, and a few are even odder sized (2mm, 2.5mm).

If I understand correctly, you want all your bolts to look the same? In that case, I would recommend getting a bunch of each of those sizes (mostly 5mm) with the same general shape.
posted by JMOZ at 11:19 AM on November 5, 2007

Racks, fenders, and water bottles are all 5mm ( by .8mm pitch, but you can ignore that). You can go extreeeeeme! on all-titanium bolts, but I would just walk into a shop and ask them. They will charge you a few bucks for some bolts, and they will do all the work of making sure they're the right size.
posted by wzcx at 11:42 AM on November 5, 2007

you can't ignore pitch. if you go anywhere but a bike shop to get your bolts, you'll get the wrong pitch with the right diameter. you'll screw those suckers in, destroying the threads as you go and strip them out.
posted by klanawa at 12:13 PM on November 5, 2007

Titanium bolts on bikes tend to have a seizing problem.

Pull a bolt from your stem (or somewhere), test it in the holes you want bolts in, then take that bolt into your LBS (local bike store) and tell the guy what you want in relation to the bolt you're showing him. You'll get the appropriate head, thread, and bolt size AND you'll have the right length bolts.
posted by lothar at 12:37 PM on November 5, 2007

Every ACE or Orchard Supply hardware store around here (SF, Oakland) has a huge selection of bolts, nuts, washers, caps, and other items you'd need, stored in open trays so you can mix & match just the pieces you need. Take all your bolts off, tag them so you know where they belong on the bike, and budget an hour to dork out in your local hardware store.
posted by migurski at 2:11 PM on November 5, 2007

Most, if not all accessory bolts on bicycles are M5 x .8 socket head cap screws. This includes, but is not limited to: water bottle cage bolts, fender bolts, pannier rack bolts, some seat binder bolts, disc brake attachment bolts and a few others.

The bolts that come with accessories vary from high quality to made-from-cheese. It's a good idea to replace them all with stainless, if you can be bothered. You'll sometimes see a relative of the socket head cap screw, the button or wafer head cap screw. These have a lower profile head than a standard SHCS, and can be useful in some circumstances. Bear in mind that the hex socket in a button head is usually one size smaller than a standard socket head, so they aren't really suitable for applications where you really need to crank down on the bolt eg pannier racks.

Buy stainless and put a little dob of grease on the threads when you re-mount everything. Truly much easier than trying to travel back in time and do it after the shitty plated bolt that's currently in there has galvanically corroded to the frame.
posted by tim_in_oz at 2:26 PM on November 5, 2007 [2 favorites]

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