Gresp15C

I used a 9 mm nut driver on a Shimano "Acera" derailer yesterday, to affix the cable.

cny-bikeman

Well, now you have a multitude of opinions on an vague question - not very useful. It appears you may be looking at Y wrenches from your later post, though at no point to you come out and say that. So I don't know if the info below will even be relevant (garbage in, garbage out) but it may be useful to others.

A Y wrench is merely a convenience, not a necessity. When I wrenched in a shop we had generally only one Y wrench - an 8/9/10. It was used most typically for brake and derailleur work, where one could come across any one of the three sizes on a single component, so you always had the right one at hand instead of having to reach for a different wrench. Later on we had 4/5/6 allen wrench versions, which would are much more useful with recent vintage bikes. A person who does not do a good bit of work on older bikes would be wasting their money on the older style Y wrench in my opinion. The allen wrench version would be a wiser choice.

I would not bother with other hex sizes at all, as there is no advantage to them and they may not offer proper leverage or clearance for working on some parts, but instead just use a regular combination wrench . Wrenches are made in lengths that correspond to the size of the nut for a reason, and a Y wrench defeats that utility.

Wilfred Laurier

As cny said, your question is too vague to know if the information given is relevant. And when pressed for details you present an internet image of a bolt?

Anyhoo, on bicycles, 9mm are far more common as that is the standard for derailleur bolts going back over 30 years. Most other hex bolts are 10mm (brake cable fixing bolts, brake pivot bolts), although there are some 11mm bolts I have seen, but mainly on older bikes and department store bikes.

Reynolds 

8 and 10 are the most common IME.

Nick The Beard

8,9,&10mm nuts and bolts are somewhat common binders for cables on cheaper bikes or you might need one for brake or derailleur position adjustment. We keep a 3 way Y wrench with these sizes at all of the work stands at the shop.

I would guess that if you are asking the question it is because you haven't needed one yet, or you would know the answer. If you haven't needed one yet, you probably wont need one unless you're working on older bikes or kids bikes.

If you are talking about allen wrenches then an 8 is the biggest you are likely to need for anything outside of working on a bike's freehub.

anga

For those who offered constructive and useful comments, thank you--you know who you are.

For all the expertise this forum has, there are some consistent and negative traits. Let me address some of those.

1. Where is it used?
This is a forum about bicycles--so the reasonable conclusion is that it is about bicycles unless otherwise specified.

2. What do you mean by a hex bolt?
There are different types of hex bolts. But the common one (in bicycles) is an external hex bolt. If it was something else, it is common sense to specify it.

3. 9mm could be of the size of anything. Is it the head, the body or the length of the bolt?
It could be anything, but again the most reasonable conclusion is the obvious one--it is the size of the head (or whatever you call it) of the (external) hex bolt.

4. If you have to ask, you don't need it.
Let me decide what I need.

5. Buy it when you need it.
Let me decide when to buy. Not everyone has a bike shop across the road.

6. Buy it if you need it for your bike.
Again, let me decide.

7. Depends on your bike.
How does this help?

8. You don't need this--buy something else.
Unless you have good reasons why one should go for the alternative, do not make this comment.

Gresp15C

In post #26 , I was talking about a hex nut, not a hex bolt. Also, it's more common to specify the body size than the head size. Please accept my deepest apologies for the offense.

cny-bikeman

1. It's polite to thank people specifically, rather than as part of an insult to others.

2. On that we will agree, and I'll give you my list:

1. Posing vague questions that prevent accurate answers
2. Failure to clarify with requested info when asked
3. Claims of unfair or impolite treatment while taking no responsibility for one's own actions.
4. Failure to acknowledge that it is the poster's choice to ask for help in an anonymous, free forum when in-person help is almost always available.

I can only speak for myself, but I gave a detailed response that fully included my rationale for the advice I gave. You don't have to accept the advice, and to ask a vague and somewhat uninformed (fact, not insult) question, and then expect very knowledgeable respondents to ignore the relevant info of which you may not be aware, in order to honor the constraints of the exact wording of the question, is self-defeating.

rydabent

If you buy a metric wrench set you seldom get an 8mm wrench.

HillRider

What? Every metric wrench set I've ever purchased, combination wrenches, open end wrenches, 1/4"-drive sockets, 1/4"-drive deep well sockets, Allen keys and even 3/8"-drive sockets always included an 8 mm.

howeeee

Most but not all metric open end wrench sets will not come with a 9mm wrench. The question wasnt vague at all.

LesterOfPuppets

9 is more common than 11 by far.

off the top of my head:

8 is somewhat common on 70s, 80s, early 90s bikes. And fenders, racks.
9,10 quite common, lotsa things
11 not much call.
12 rare.
13 is on a lot of stem bolts for bmx bikes, older cheaper road and mtb. Occasionally on front axle locknuts.
14 mostly locknuts, some front axle bolts, seat rail clamps, crank bolts
15 axle nuts, crank bolts

bicyclelove

^^^This has been what I have found. You could safely skip 11. If you need one you can use a 7/16 as it is almost exactly the same size.