I notice on my chinese made Norco that the keys are very loose fitting. I know there are different sets of allens but I seem to remember them either fitting snugly or not at all. The reason this concerns me is it feels like I could strip the hole especially on the brakes which have to be tightened down well.

  • 1
    Are you using a SAE (fractional) or metric set of Allens?
    – mikes
    Dec 12, 2017 at 22:49
  • Not normal. Are you sure you have the correct size?
    – paparazzo
    Dec 13, 2017 at 15:16

4 Answers 4


They all have some play in the fit. The target outside dimensions for the most common bike hex wrench sizes (4, 5, and 6mm) are all around .1mm less than the bores they fit in, which match the nominal sizes with a tolerance of I think +/-0.05mm-ish. That shouldn't lead you to feel they're "very" loose, but they typically aren't exactly snug. Very loose suggests using imperial aka fractional/SAE wrenches, whereas virtually all Allen bolts on bikes are metric. The only exceptions I've encountered are certain 90s BMX stems.

That it's Chinese is here nor there.

  • 1
    There are some very good chinese tools, and the opposite. Pricing is an indication of quality but I've seen some expensive prices for middle of the road tools so its not a great indicator of quality.
    – Criggie
    Dec 13, 2017 at 0:00

In a word - tolerances.

The fastener tends to be oversize and the tool tends to be under the nominal size.

http://www.bikeradar.com/mtb/gear/article/dont-buy-the-cheapest-tools-44468/ is a good run down, but the upshot is that the larger the difference between the fitting and the tool, the more force is focused on a smaller area, being the corners.

A nominal 5mm hex hole should use a 5mm tool, but if it was exactly the same internal and external size, then your tool would be hard to fit and harder to remove.

This assumes your tool is the correct size for the job. Using a 3/16" tool in a 5mm fastener is a recipe for disaster. If you don't own the right tool, buy the right tool, whether it be a workshop P handle driver or a quality multitool for on-bike.


Ball end Allen keys tend to feel looser than square end. While useful you should only really use them when you have to because the contact area is much less so they wear the screws faster. The tools also round off easily, making it easy to wreck screw heads (even in such a way that they feel fine at home, but then fail with your roadside tools)


I was an alarm technician with a couple of other trades thrown in as well. While innovation is essential when it comes to tools and earning a living I agree the best tool is the one suitable to the job.

I use allen keys a fair amount and even fitted one into a battery drill as I had to use it to unscrew and screw one inch long fine threaded screws. Any allen key up to date had a snug fit compared to working on my bike brakes. I went to my l.b.s. and asked if it was normal. My mechanic let me use brand new keys meant for my bike. They had the exact same loose feeling.

It may have to do with the fact I am unused to working on bicycles. Perhaps it is the highly polished screws themselves.

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