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PostPosted: Fri Apr 19, 2013 11:28 pm 
Gold Trader
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Location: Surrey
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?


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PostPosted: Sat Apr 20, 2013 4:43 am 
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Joined: Sun Jan 15, 2012 8:24 pm
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Location: Toronto, Canada
http://lmgtfy.com/?q=bolt+sizes+and+grades


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PostPosted: Sat Apr 20, 2013 7:06 am 
retrobike rider
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Joined: Fri Aug 08, 2008 2:36 pm
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Location: Yorkshire, England
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


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PostPosted: Sat Apr 20, 2013 7:55 am 
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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.


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PostPosted: Sat Apr 20, 2013 1:24 pm 
Gold Trader / MacRetro rider
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Joined: Thu May 06, 2010 10:05 pm
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Funnily enough I've been on the Toronto Cycles website looking at bolts and shiny things today, here's a link to their (quite handy) page on determining bolt sizes...

http://www.torontocycles.com/measuring.html


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PostPosted: Sat Apr 20, 2013 2:27 pm 
retrobike rider
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Joined: Wed Dec 09, 2009 5:36 pm
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Location: North Yorkshire
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.


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PostPosted: Sat Apr 20, 2013 4:43 pm 
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Joined: Mon Mar 28, 2011 9:29 pm
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Location: Somerset
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 .


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PostPosted: Sat Apr 20, 2013 9:23 pm 
Old School Grand Master
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Joined: Mon Jun 14, 2010 9:06 pm
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Location: Herts UK
^^^^ what he said.
also disc brake calliper bolts are M6.

seatpost clamp tend to be M5 or M6.


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PostPosted: Sun Apr 21, 2013 3:11 am 
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Joined: Fri Jan 13, 2012 2:28 am
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Location: Near Gatwick
Get some calipers and a thread gauge?


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PostPosted: Sun Apr 21, 2013 9:13 am 
Gold Trader
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Joined: Fri Jan 23, 2009 2:30 pm
Posts: 12429
Location: Surrey
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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