When undoing some bolts eg clutch slave cylinder it seems a lot of people use a flare nut wrench since the open part allows you to get around say a pipe which a normal Socket would not do.

However it seems to me you could just use the open end of a combination spanner?

Is it be ok to use either an open ended spanner or a flare nut wrench or must you use the latter, if so why?

2 Answers 2


Flare nut spanners are designed to spread the load on the soft flare nuts.

Ordinary nuts are made of a harder material.

If you use the wrong spanner and round the flare nut off then you create more work for yourself - but that may be a useful learning experience...

Oh, just by the way there are some special sockets available with side cut outs... usually in « professional » tool ranges.

  • 1
    a lot of times with even soft nuts on traditional threaded fasteners you can get a closed box wrench around it. But there will be NO way, never to get a box end around a the hydraulic tube nut. Oct 15, 2020 at 22:47
  • 1
    @AndyzSmith - It's easy to get the box wrench on. The trick is getting it off, after you've fastened the fitting.
    – Hot Licks
    Oct 18, 2020 at 0:00
  • 1
    @HotLicks you could plasma cut the frame of the assembly/chassis into little pieces, carbide titatanium drill out the fittings for the hydraulic valve mounting studs, twist the hydraulic valve off by twisting the hydraulic valve off, the remove the wrench, and get a Tesla. Oct 18, 2020 at 14:58
  • Not me learning this the hard way... Sep 19, 2023 at 13:18

An open-ended spanner only engages on two faces of the nut. As you can see in the picture below, the inside of the spanner slot is not shaped to engage with the nut. Open-ended spanners are used where the only possible access to the nut is pushing it across the faces of the nut. The "play" between the spanner and the nut means that the spanner exerts most force on the corners of the nut, which risks rounding the nut off.

Open-ended spanner

A flare nut spanner engages on four faces of the nut, as you can see from the picture below. The spanner has lips at the front which ensure it engages on the inside of the spanner slot. Most of the force is still born by the two opposing faces, but with well-defined engagement with the spanner it is possible to shape the spanner so that it contacts the nut more towards the centre of the nut faces with a wider contact area, in the same way as a good box wrench does, instead of on the corners of the nut as an open-ended spanner does.

Of course the lips on the spanner mean you can't just slide it onto the nut, as you would for a regular open-ended spanner. For plumbing/hydraulic applications, the nut is (almost) always wider than the connected pipe, so you slot the spanner over the thinner pipe and then push it down onto the nut.

Flare nut spanner

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