Bicycle Mechanics - 5mm allen key fits loose, is that normal?

On my stem bolts which are 5mm I guess, my 5mm turns a little around before catching. On the 4mm like on my shoe cleat it fits snug.

5.5mm is too big and the 3/16 ones is about as snug as the 5mm. What gives?

These are new tools with new bolts. But I always remembered the 5mm to give me trouble.

I find the 5mm to almost always be the loosest fit between tools/bolts, too. Not sure why that is. I have several allen wrench sets and use the one that feels the tightest but have still rounded out [5mm] key holes because of this.

I find the 5mm to almost always be the loosest fit between tools/bolts, too. Not sure why that is. I have several allen wrench sets and use the one that feels the tightest but have still rounded out [5mm] key holes because of this.

+1. Especially chain ring bolts.

I find the 5mm to almost always be the loosest fit between tools/bolts, too. Not sure why that is. I have several allen wrench sets and use the one that feels the tightest but have still rounded out [5mm] key holes because of this.

+1

+1. Especially chain ring bolts.

Yes. Does anyone know why? Have we been using the wrong sizes all along? Crappy tools?

Chainring bolts are very thin and tend to get spread out over time with use.

Also the tips of the allen keys can get rounded off with use. Grind about 0.5mm off the end of the allen-key and you'll get a new clean tip. The sharp corners really bite into the bolt and gives incredible grip. :)

Also the tips of the allen keys can get rounded off with use. Grind about 0.5mm off the end of the allen-key and you'll get a new clean tip. The sharp corners really bite into the bolt and gives incredible grip. :)

+1. It's surprising how often an allen bolt that has been rounded out will come loose with a brand new allen wrench. I'm wondering if the 5mm seems to fit more loosely because it's the most frequently used and gets the most wear.

I wonder if the cheaper allen key sets are substituting 3/16" standard sized keys in place of a true 5mm key. It is close at 4.76 mm. Just a thought.

The point about worn vs new allen keys is a good one. Also the keys with a ball-head, that allow the key to be used off access place more stress on the bolt head, since they engage a smaller area of the flats.

Ed

I'm wondering if the 5mm seems to fit more loosely because it's the most frequently used and gets the most wear.

Absolutely. 3, 4 and 6 mm hex wrenches last a long time because they aren't used very much. The 5mm does the majority of the work and wears the quickest. It also get called on in a lot of high-stress applications like chainring bolts, cable clamp bolts, seat post clamps, etc.

I "resurrect" 5 mm hex wrenches with a Dremel tool and a cut-off wheel. Slice about 1/4" off the end of the hex and it's amazing how much better it works.

I have to suspect that it's a combination of wrench and bolt wear. Those bolts are ones you tighten most often.