Plug spanner size

Here's a question for classic car types. Which arose from a question on a FB group. And I was surprised no one seemed to know.
You've lost your dedicated 14mm plug socket - but the engine design would allow a ring spanner to be used.
What is the correct ring spanner size?
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On 09/12/2018 11:08, Dave Plowman (News) wrote:

The standard spark plug (not the small ones some early Vauxhalls used) is a 21mm ring spanner. I have just tried it.
Jim
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21mm will sort of fit, but isn't the actual size.
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On 11/12/2018 00:27, Dave Plowman (News) wrote:

I confess that the socket I tested was hexagonal rather than the customary 12-point ring, so it looked and felt a good fit. Perhaps a traditional ring isn't as good.
I have an AF set of traditional rings in the garage. I will try those later and let you know if I can get a match with those.
Jim
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Getting closer, but you need to go back further than AF spanners. ;-)
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On 11/12/2018 07:13, Indy Jess John wrote:

I have now tested my AF spanner set. A proper fit with a proper ring needs a 13/16 AF spanner.
(I don't think you can now find Whitworth ring spanners).
Jim
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They're probably still made somewhere. I've certainly got some, and sockets too.
You are right, though. The nut size on a 14mm sparking plug is actually 7/16ths Whit (as marked on the spanner) About 20.8mm AF.
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On 11/12/2018 13:29, Dave Plowman (News) wrote:

You are right - they are still around, though many are used sets, eg https://www.ebay.co.uk/itm/Set-of-6-vintage-Gordon-tools-whitworth-offset-ring-spanners-LOTWD7GRD/183578702095
Jim
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Still quite a few cars around that need Whit etc spanners. Many basically pre-war designs made after WW2 carried on using them.
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