Dumb spark plug question: Metric or not?

All sparkplug wrenches are metric. Always have been.

"Lanny Chambers" wrote

That's strange...I have spark plug sockets (have a rubber thingy to hold the plug) marked 13/16 and 5/8. I think I bought the 13/16 when I owned a '63 Mini-Cooper.

-- Ken Lyons '97 Brilliant Black/'90 Classic Red Inside the Beltway [Remove the first two digits to reply]

There are a lot of metric wrenches that are almost the same as SAE sizes, for example 13mm and 1/2", 11mm and 3/8", etc.

Both 13/16" and 5/8" correspond closely to metric sizes and since plug wrenches are hex rather than 12 point, they can be a little loose w/o damaging the plugs.

Look here:

Oops, that's 11mm and 7/16", of course. I shouldn't try to do stuff from memory...

The MX-5 needs a 10mm spark plug socket (the smallest of the three common sizes), and I expect the Protege is the same.

Miata uses a 16mm (5/8 ") socket. You will need a socket made for spark plugs.

Spark plug socket dimensions don't refer to the distance across the flats, which could well be 16mm - they refer to the thread size, and the flats are always the same size for a given thread. No idea why.

Spark plug sockets are dimensioned like every other socket-acroos the flats. What dimension does the 10mm you posted correspond to? The threads on a Miata plug are 14mm.

It must be a weird British convention then:

I can always post a picture of one of my car's NGK BKR6E11-IX plugs sitting in a "10mm" socket...

The link you posted has spark plug sockets listed by hex size across the flats, exactly as on this side of the pond. There are 14mm thread spark plugs with either 16mm or 21mm hex sizes. How do you handle that? You seem to be stuck in the old Whitworth system where the wrench size DID correspond (somewhat) to the thread size. As this is not a binaries group, you would need to post the picture on such a group. Please do so if you can.

I'll ask again, what dimension does 10mm correspond to? The BKR6E11 plug has a 16mm hex and 14mm thread. Where, how, does 10mm figure in?

Now that, I would like to see. 10mm ? What is the gap on that plug ? :-)

Bruce Bing '03 LS

BKR6E11

B : 14 mm thread KR: 5/8 hex, Resistor type

This comes from their partnumberkey.pdf file available on NGK's website. Where did you get the 10 mm number from?

It's definitely a "10mm" spark plug socket with 16mm across flats, as per the link I posted. Do I really have to pull out a plug and photograph it in the socket?

The socket I use is engraved "Socket for Spark Plug 10mm". The continental Europeans would call it a 16mm spark plug socket (more sensibly).

I think spark plug sizing is just one of those things which gets lost in transatlantic convention, like "hood", "boot" and "rocker".

If you want to convince me and others, yes, you will need to post photos of the plug and socket.

Will do next week - I'm away for a long weekend.

OK I cant get one of the plugs OUT of my car!

Feels like the socket wont seat on the plug correct.

What now? Advice?

Remove the little rubber inside the socket. Fit the socket on the plug by hand so that you can feel it slide into place. If the plug has corroded in place, you will need to use a piece of pipe as a handle extender on your wrench. Make sure you are attempting to turn the plug counterclockwise. The plug will come out!

Done the above but still no go

My plugs are in a recessed hole abt 6 inches deep

I'm wondering if there is some kind of "crud" down there say wedged in between top of plug and sides of hole that is blocking the wrench form seating on the plug.

Just seems like no matter what I do I cant get the wrench to 'seat" on the hex head of the plug!