[DraEagWol]

For those of use - metrically challenged into finding out what the heck are they talking about when they say M5 or 10mm... & all that - here's a couple of good reference sites:
http://www.boltdepot.com/fastener-information/Printable-Tools/Default.aspx

http://www.boltdepot.com/fastener-information/Default.aspx

Since we mainly have the Socket Cap bolts I'm trying to correlate that to the correct M# = what hex size = bolt head measurement that's used - I can't figure out why sometimes they list the M# & othertimes the size in mm... is this correct?

M#~~ Hex(Allen size) ~~~bolt head(mm) / alternate finding
3~~~~~~ 2.5~~~~~~~~~~~~ 5.5
4 ~~~~~~3 ~~~~~~~~~~~~~7.0
5 ~~~~~~4 ~~~~~~~~~~~~~8.5 /(8mm)
6 ~~~~~~5 ~~~~~~~~~~~~~10.0
8 ~~~~~~6 ~~~~~~~~~~~~~13.0 /(12mm)
10 ~~~~~8 ~~~~~~~~~~~~~16.0 /(14mm)
12 ~~~~~10 ~~~~~~~~~~~~18.0

i.e. i see listings for chrome caps and they just list the mm size..

others for the bolts (cept they sometimes call them screws... which always confuses me - because to my mind - bolts need nuts.. screws don't). Like this: i.e. M4 x .7 x 50 which is even goofier... The M is for the head size but so is the second # which is listed in cm (.7) and the last is for the shaft length - which is in mm - why use both cm and mm for crying out loud.... to describe what bolt/screw you need / want to use?

anywho.. sry for the ramble... & getting my terminology / descriptions messed up ....

 

[BillyD66]

This is an oversimplification but here goes:
-M"whatever" refers to the screw diameter
-The next number(if given) refers to the thread pitch(like we have coarse and fine threads for the same diameter screw[10-24 and 10-32]) If omitted then the default of 1 is assumed which is 1mm for every 360 degrees of rotation
- The 3rd number is the screw length.

And to make matters more interesting there are the different shaft measurement starting points just like SAE depending on the head type.
And not necessarily mentioned but very important is the hardness of the bolt. Ask the guys who have had saddle bag bolts shear off why...
As for bolt verses screw I think like SAE up to a certain size like 14 or so is a screw and when they change to fractions they are a bolt. Not sure where metric draws the line

The whole thing is a giant cluster-**** since we really should have gone to the metric system and thus avoided all the crossover we have now. Then again kitchen and bathroom plumbing is NOT the same even though you would think so. Many things like penny nails have interesting historical meanings/stories but again just switching to the metric system would actually be easier since it is all based on one standard(as far as I know) Once you are past the learning curve it would actually make more sense than what we have now but of course what would you do w/ all the existing stuff???

 

[wayne]

This is an oversimplification but here goes:
-M"whatever" refers to the screw diameter
-The next number(if given) refers to the thread pitch(like we have coarse and fine threads for the same diameter screw[10-24 and 10-32]) If omitted then the default of 1 is assumed which is 1mm for every 360 degrees of rotation
- The 3rd number is the screw length.

That about covers it... basically the m5, m6 number is diameter followed by pitch then length. Thus M5-20x40 would be a 5mm diameter with 20 threads/cm, 40mm long. The head size differs depending on the usage or design, there aren't really set standards. I seen M6 bolts with 8mm heads 10mm heads 11mm heads etc depending on what the bolt (cap-screw) was designed to do. It gets a little tricker as some people refer to a bolt by the head size and not the dia. which although not a proper reference, is very common.Back when metric bikes were young you didn't see many of the head sizes that roughly relate to SAE sizes, e.g. 11mm which is roughly 7/16" 13mm- roughly 1/2", now they are more common. Older metric wrench sets often didn't include these sizes. Typically went from 8mm-10mm-12mm-14mm etc. Now you will see those oddball sizes in use more. My guess from personal experience is that they became more common due to their use in Chinese products.

 

[DraEagWol]

Ya... see that whole lets randomly change the head size is what is throwing me... I get the diameter / thread / lenght - that makes sense... but throw in all these different size heads... crazy. oh & that with the sometimes using the M# x thread x length or they just use mm x thread x length -

So - Does a M4 = 4mm diameter? cuz i've seen different crap on that too.

 

[BillyD66]

So - Does a M4 = 4mm diameter? cuz i've seen different crap on that too.

Should. Ordering through the internet could be dicey. Ace has a fair selection but I don't think they carry the head type used on our bikes(at least I didn't see it) Come to think of it I'm not sure what it is called

 

[DraEagWol]

Should. Ordering through the internet could be dicey. Ace has a fair selection but I don't think they carry the head type used on our bikes(at least I didn't see it) Come to think of it I'm not sure what it is called

Socket button head (rounded) & socket cap (flat top) types of bolts are on our bikes... button heads all around the engine covers... etc... on the sissy bar they are the socket cap - I'd like to switch all the button heads out for socket caps... lot easier to find the chrome or black plugs that go with them...

 

[wayne]

Ya... see that whole lets randomly change the head size is what is throwing me... I get the diameter / thread / lenght - that makes sense... but throw in all these different size heads... crazy. oh & that with the sometimes using the M# x thread x length or they just use mm x thread x length -

So - Does a M4 = 4mm diameter? cuz i've seen different crap on that too.

Yup. M4 = 4mm diameter.
Unfortunately there are a few "standards" for head sizes so that's a bit up in the air..