Take me through bolt sizes or at least how to work it out

ishaw

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I've never really had to buy bolts without stating their intended purpose before. I'd usually just go to the LBS and ask for the right on and they would provide. Now however, wanting to find missing bolts or uprate what I have to stainless, anno or even ti, I am never sure what I need, so how do I know? I am aware of all of the M numbered sizes, but that's about all I am aware of. What does M5 mean, what or how do I discover what M number the bolts I need are?

Seems an odd question, as I can build and service almost all of a bike, nearly build a wheel, but I've never needed to consider bolts and their special qualities and sizes until now.

Can anyone show me the light?
 
Re: Take me through bolt sizes or at least how to work it ou

M = metric
# = diameter of threaded part (around the thread)

So if you have as bolt/screw hole you need to include the thread, often ~1mm more than the internal hole only.
Best really is to just keep a collection of each size and screw one in. That checks if it is metric and if a different thread might have been used.

For anything else use the link above or look at screws in wikipedia
 
Re: Take me through bolt sizes or at least how to work it ou

If you want to be anal about it you can get a set of digital verniers to measure diameters for less than a tenner - otherwise a ruler is good enough for most uses.
 
Re: Take me through bolt sizes or at least how to work it ou

If you're going down the upgrade route then get a bolt gauge from Pro-Bolt...

http://www.pro-bolt.com/workshop-other/ ... XKOmkpBpWl

They do a nice line in titanium, aluminium and stainless steel fasteners and are better quality than tibolt/superstar.

They also have a useful pdf that tells you everything you need to know about bolt and washer sizes.
 
Re: Take me through bolt sizes or at least how to work it ou

M5 is 5mm major (largest) diameter of thread, and would need a 5.1mm hole for it to fit through. M6 6mm etc.

Standards on a bike:
Bottle cages / shimano jockey wheels M5.
Ahead top cap M6.
Canti/V Brake bosses (to hold the brake on) M6
Rear deraileuer bolt is M10 fine pitch

All varieties are available with normal / fine pitches, custom threads are often used to ensure that the manufacturers part has to be used.

Confusingly, thread size pitch and length are described in similar formats & common sense needs to be applies.

i.e.

M10 x 1.5 is M10 normal pitch 1.5mm.
M10 x 25, would be M10 & 25 length.

It's very rare that you'd get a bolt so short the length / pitch would be confusing .
 
Re: Take me through bolt sizes or at least how to work it ou

^^^^ what he said.
also disc brake calliper bolts are M6.

seatpost clamp tend to be M5 or M6.
 
Re: Take me through bolt sizes or at least how to work it ou

Thanks for the replies. My need has arisen as I would like to upgrade and/or change some rusty bolt heads. Ordering online from the likes of Toronto cycles or pro-bolt means comparing directly before buying is tricky. I've got some digital verniers so can measure the 'M' value, so the only real issue is thread pitch.

How likely am I to buy a bolt for a bike frame/part from the 90's onwards, and find it is the wrong pitch?
 
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