gandini wrote
Unfortunately it is a special bolt. Cinelli don't make too many threadless stems, so I can't go by my older quill stems from them. And the bolt is very different from any I've seen on TTT or other threadless stems.
Also a correction: it takes a 6mm Allen wrench, but is a different thread to other 6mm Allen bolts on a bicycle (like the derailler clamp bolts.) It's about 20mm long.
Metric bolt of a certian size are always the same, there are differences in threading. It is most likely an oversized head that is causing the confusion, meaning the bolt is smaller then you think, sounds like an M6 bolt with an 6mm key, instead of the usual 5mm key.
Also, you sure the bolt isn't just worn or the threading on the bar isn't stripped? Only thing I can think of is a english bolt is used, but that wouldn't make sense.
Madcow wrote
2004 Ram, used 2 M6x16 bolts for mounting the stem and 2 M4x12bolts for mounting the computer arm. They did however use a 5mm allen key not a 6mm. I seem to remember the 05 version is the same, but I don't have any to check against right now.
Standard for M6 socket bolts is 5mm key. The number folloing the M, in the case M6 represents two things, the thread diameter and the head height, not the key size. Oversized keys can be found usually on taper head screws, like on Shimano deraileur bolts in which an M5 bolt has a 5mm key, instead of the standard 4mm key on a M5 socket bolt.
Hope that clearifies things. Just dont want to confuse people.
So, here is how it works,
M-stands for Metric
Number after M-stands for thread diameter and head height
To calculate head diameter you take the number following M, divide it by 2, and add the original number plus one half M plus 1 for head diameter.
Head diameter = M + (M/2) + 1, where M represents the number following M.
So here is an example, well use M6 since that is what we are talking about.
M6
Thread diameter = 6mm
Head height = 6mm
Socket = 5mm
Head width 6+(6/2)+1=13mm
And lenght of bolt is always from the bottom of the head to the bottom of the screw, head height is not included, unless otherwise noted.
Hope this helps for future purchasing of bolts! Or did I just confuse everyone?
ONE LAST THING!!!
If you are buying Ti bolts for weight savings purposes taper heads are about 5-20% lighter, depending on length, due to the head structure. Socket heads have considerably more material in them then the heads of a taper bolt.
On my Ritchey stem the weight savings of the Ti socket head bolts averaged 10.5g, with the taper head bolts it was 12.5g saved.
I just bought some socket head type bolts because I thougth the taper heads didnt have the strength needed, but I recently learned Zipp uses them on their stems, so I think they are safe to use and save a few more grams.
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