Help with unusual tap with non-standard cylinder

[suntzu7777]

I have a leaky tap, so I thought I could fix this with replacing the washer, however, when I took the cap off, I could not get the wrench in there to grab hold of the nut to turn! This is because the nut part of the cylinder is sunk into the casing (please see photo in the attached links)! I measured the size of the nut, and it is about 17mm; yes, I only have a ruler to work with! So, I went and bought a set of box spanners, however, the actual size sits somewhere between 17mm & 18mm; one is too small, and the other one is too big (loose)! So if somebody can advise me on the actual size spanner I need, and where to get one, I am unable to get 17.5 mm one. I will be eternally grateful!

 

[ShaunCorbs]

11/16 probably

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[suntzu7777]

11/16 probably

11/16 Deep Socket for sale | eBay

Find great deals on eBay for 11/16 Deep Socket. Shop with confidence.

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Thank you so much, I will have a look!

 

[RayW]

Just bevel the inner faces of the 17mm . Sorted.