Spark plug sockets

[227bhp]

Why is a 10mm spark plug socket called so?


An M14 plug socket correlates to the M14 thread on the plug (21mm spanner flats or socket size).
I have a plug socket marked '10mm plug' for smaller plugs yet nothing on the plug is 10mm (apart from the porcelain, it can't be that), why is this?
The smaller type of plug has an M14 thread and 16mm across the spanner flats.

https://www.bhl.co.uk/laser-spark-plug-socket-10mm...

Edited by 227bhp on Wednesday 27th February 20:55

[Coilspring]

I have to agree, spark plug socket measurements make very little sense, conventional sockets are measureed to fit across the flats of the hexagon.

But there are now 3 main plug sizes for cars.

The old ford ohc/ohv is 19mm, hardly used now. Conventional small plugs (fiesta, focus, vauxhall etc) is 16mm, or 14mm depending where you measure. Then there are the new (relatively) small plugs, mostly renault which are 10mm. From memory i think they are bi hex, not hex.

Still doesnt explain why they measure plugs oddly

[wildoliver]

More to the point when will someone design one that the bloody rubber insert comes out and sticks to the plug every time you use it on a recessed plug. Yet never on an exposed plug.

[GreenV8S]

A 10mm spark plug has a 10mm thread, surely. Are you saying you've got one that doesn't?

[paintman]

wildoliver said:

More to the point when will someone design one that the bloody rubber insert comes out and sticks to the plug every time you use it on a recessed plug. Yet never on an exposed plug.

Lost the inserts on mine years ago (all of them!)

I use a short length of rubber tube that I can push over the end of the plug to lift out when undone.
Same when refitting, once the plug is started in the threads by twisting the rubber tube pull it off & use the socket to tighten. Much less chance of crossthreading too as the tubing will just slip on the plug.

[stevieturbo]

Get a magnetic plug socket or the ones with a little ball/collar to grab the plug.

[thebraketester]

GreenV8S said:

A 10mm spark plug has a 10mm thread, surely. Are you saying you've got one that doesn't?

I think he is saying that the thread diameter and the hex size are not the same which can cause confusion.

[227bhp]

thebraketester said:

GreenV8S said:

A 10mm spark plug has a 10mm thread, surely. Are you saying you've got one that doesn't?

I think he is saying that the thread diameter and the hex size are not the same which can cause confusion.

No and no:

227bhp earlier explicitly said:

Why is a 10mm spark plug socket called so?

I have a plug socket marked '10mm plug' for smaller plugs yet nothing on the plug is 10mm (apart from the porcelain, it can't be that), why is this?

I've wondered for years what the 10mm bit refers to, but haven't found an answer yet.

[Mignon]

Jaysus. It's not rocket surgery. For decades the standard automotive spark plug thread was 14mm and the hex size was 13/16" (20.6 mm). When 10 mm spark plugs for Japanese motorbikes were introduced the hex size was reduced to 5/8" (16 mm). This hex size was later applied to 14 mm thread plugs in recessed locations in 16v car engines to save space so a smaller plug socket could be used.

[227bhp]

No it was never going to be rocket surgery. It just needed someone old enough to have been there as the history of the 10mm spark plug was played out
Well done