Dumb spark plug question: Metric or not?

But it fit OK in the other three cylinder holes

Try to clean that hole with a steam cleaner. If that does not work... you can try some cunning plan with epoxy and a use-once tool that you can make (good luck with thay!), or you will have to remove the head to have a look from below :(

I don't recomend any of the above though. Just keep on cleaning that hole, it can´t be that hard to get it out!

There may not be enough room to use a straight wrench extension. Try a u-joint or flexible socket extension. My 11mm plug socket has an outside diameter of 21.5mm. It's hard to believe that the opening around the plug is smaller than 25mm.

3 out 4 of isn't bad. Maybe a piece of the connector is stuck on the plug, look down in there with a CSI lite.

Apologies for this not threading properly; my newsreader has purged the older posts.

Pic of NGK BKR6EIX spark plug removed from 1997 NA (BP 1839cc engine, standard tune). Don't worry - the centre electrode - the iridium one - is supposed to look like that. Just to show it's in good condition and the right plug for the engine.

Pic of above spark plug in 10mm (British terminology) spark plug socket:

Pic of spark plug showing model number:

Pic of calipers showing both plug and socket are 16mm across flats:

Close-up of caliper measurement:

They're good plugs; the service life is probably 50,000 miles or more and there is a tiny improvement in miles per gallon. It's best to take them out occasionally to ensure they don't get seized, as 10 years installed in a low-mileage engine might be pushing your luck.

Well thanks, but the question remains unanswered. That question being "What feature of the subject spark plug corresponds to 10mm?" The thread size of the BKR6 spark plug is 14mm.

Here's what you previously wrote.

My only guess is that the BKR6xx plugs have a non-standard thread-to-flats relationship. Normally 16mm across flats would indicate a 10mm thread, but here it obviously doesn't. At the end of the day I have all three common plug socket sizes, so it doesn't matter.

To the above statement. Whitworth and British Standard wrenches and sockets are sized by thread diameter. This is likely a rather clumsy carry-over to spark plug sockets.

16mm or 5/8 has been a standard hex size for 14mm threads for decades. I have no knowledge of a 10mm thread sparkplug being used in a Mazda. Most 10mm plugs are found in motorcycles.

It's strange you should say that; I bought those iridium plugs from a motorcycle dealer.