Bicycle Mechanics - Is the standard chainring bolt size M8?

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bikefreakvinnie
01-02-10, 06:41 AM
I want to buy 5 chainring bolts for an old chainset which has been disassembled, I do have one bolt from the old chainset and it is an M8 thread.
The bicycle shop website I've been checking out does not specify the thread size of the bolts which makes me think that M8 is the standard and they do not need to specify, if its taken for granted. However I have another chainset which has M5's.
I haven't been able to find any other information on this, would anyone be able to shed some light on this!?

HillRider
01-02-10, 07:29 AM
Chainring nut/bolt pairs for double rings are all M8 that I've ever seen as are the bolts for the granny ring of triple cranks. I have seen a couple of older cranks where the bolts for the granny ring are M6 but that's not common.
bikefreakvinnie
01-02-10, 07:39 AM
Chainring nut/bolt pairs for double rings are all M8 that I've ever seen as are the bolts for the granny ring of triple cranks. I have seen a couple of older cranks where the bolts for the granny ring are M6 but that's not common.

Great! Thanks for that, it is a double that I am messing around with! The other chainset I have that was the M5 is a triple and an oldie, it a SUGINO PX... Thanks for clearing that up I can go ahead and order now!...

MnHPVA Guy
01-03-10, 07:17 PM
M8 generally refers to an 8mm fastener with ISO std. thread pitch.
ISO is 8mm x 1.25mm
Square taper crank fixing bolts have a finer pitch, 8mm x 1.0mm
Chainring bolts are extra fine pitch at 8mm x 0.75mm threads
DannoXYZ
01-03-10, 07:53 PM
There are multiple ISO standards for metric thread. Availability is typically more an inventory decision more than anything else.
HillRider
01-03-10, 08:26 PM
M8 generally refers to an 8mm fastener with ISO std. thread pitch.
ISO is 8mm x 1.25mm
Square taper crank fixing bolts have a finer pitch, 8mm x 1.0mm
Chainring bolts are extra fine pitch at 8mm x 0.75mm threads
Ahaa! I know the most common M8 pitch is 1.25 mm and crank bolts are 1.0 mm but I didn't know what chainring bolts were.

I tried both an M8x1.25 and an M8x1.0 bolts in a couple of chainring nuts and neither threaded in more than a couple of turns so it was obvious the thread pitch was different. Now I know what it is.

Incidentally, as DannoXYZ mentioned, there are several ISO and SAE thread standards. SAE bolts are available in both NC (National Coarse) and NF (National Fine) series. For example 14/-20 is NC and 1/4-28 is NF but there are other thread pitches also defined.