Why do some stems use an allen head and others the button bolt? is there any rhyme or reason for which stem gets which?
Do people swap those out or do they come stock like that?
i think i have seen the same model stem with two different types, but maybe not.

are the button bolts more aero?

hanks for any insight!

Aesthetics, high end model for those willing to pay vs low end model for those on a budget, cost and availability, effort and assembly times, maybe they got stuck with a bunch of bolts/screws for something else and this already being in inventory worked and etc.

All sorts of reasons.

Aero, you really had to go there! <grin>

Not only aero, possibly a (very marginal) safety issue as the left hand photo has some sharp edges on the bolt head. I’ve mostly seen only recessed / smooth bolts on these components.

The "button" head is a keyed bolt, the stem has a slot in the hole that you cannot see when the bolt is in, that (is supposed to) keeps it from rotating when you tighten the nut.

The socket-head bolt likely has a shoulder on the inner face to prevent the nut from rotating. If it does, it doesn't; that is, it doesn't very well - I frequently see these with the shoulder damaged from the nut turning. Use a spanner.

And "aero"? Don't make me laugh, a human on a diamond frame bicycle is an aerodynamic disaster, the shape of the bar-clamp-bolt-head matters about as much as whether your mother had curly hair.

I cannot say I have ever been a fan of the buttons, nicely styled on one side and then a basic nut in full view on the other.

However, I will qualify that by stating that on some of my old folders they have a lever instead of a hex nut to allow the handlebars/seat to be dropped/rotated when folding. They are nice!

I've seen it mentioned on another write up about dating bikes, that the Allen head bolts did not start showing up till around 1980, unfortunately I cannot find the article, others may have better filing system. If its NOS remake then its I guess purely costing

I agree - Allen-head fasteners are a relatively recent addition to bike tech - I’m trying to recall if any of my bikes from the 70s or even early 80s had Allen fasteners - I don’t think so. The other advantage of the fastener on the left is that it avoids threading a steel fastener into relatively soft aluminum - a recipe for stripping the threads for the ham-handed- or novice mechanic in a time before widespread use of torque wrenches. I don’t know what the setup is with the stem on the right, but I had at least one Cinelli stem in which the fastener threaded directly into the Al

you mean like this pic? yes i like the nice clean look,

wonder who got to bottom tap all those stems? hopefully some machine did it, wonder how many stems were re-melted due to broken taps?


notice the drilled out bars for the Dia Compe AGC levers to improve ferrule life, a trick i learned at this forum of course, full props to whoever that was,

those bells do not work good in the rain. instead of ding ding, they go dink, dink,

yes those Nitto bars have the collar for extra strength,

looks like some tricky machining underneath that button head bolt,

seems like a better system than the tapped out hole in the stem, have seen some stems with what looks to be excessive bending/warpage along the front of the binder aluminum section, as that is where it bends when spreading it out for new bars,

i almost bought the stem on the right till i noticed an oblong shape in the handlebar hole, you can see that it was not making contact with the bars from the lack of grooves,

I have similar problems with old pressed steel clamps as well. They tend to make the handlebars go oval, and then you have the problem with an ovalised bar that is not going to do anything good for another stem clamp, and a stem clamp that is not going to do anything good for another handlebar. If you have chopper-style handlebars then it also becomes difficult to adjust them forward or backward to suit the rider.

I had much fewer issues with motorcycles, with beefier handlebars and a pair of cap-type clamps.