How remove incorrect spark plug i.e. no clearance

[markw996]

Some fkwit has put the wrong sized spark plugs in my Aprilia RST1000 motorcycle engine.
I only bought it recently from a private seller (half way across the country) a little while ago and discovered this after attempting a basic service. The plugs should have a 16mm hex but the outer plugs on both cylinders (twin-spark) are approx 18mm. Anyone know any tricks to removing the plugs? I suspect that the threads are correct or I'm really bksed.

Problem is the clearance between the spark plug hex and the cylinder head, grinding away the head is an option but space is limited and it's got a lot of potential to go wrong, hammering on the thinnest wall socket I can find is another option.

I've got an 18mm box wrench on order which I could grind down a bit... am I missing an obvious solution?

[Bennachie]

Cheapo tube plug spanner should get that. I guess it is what they put it in with. As you rightly say, the thread MUST be correct or they would have been fitted with a hammer..............

[shtu]

You should be able to take a normal plug socket and grind it thin\away where it clashes with the head.

You don't need the socket to be perfectly round, just clearance enough to get it into place and turned a fraction. Once it turns a little you should be able undo it with pliers, put a piece of fuel hose over the ceramic to get a grip.

[markw996]

Thanks, my dad just phoned to say he's found a few old AF box spanners that look approx 18mm so I'm going round for a visit!

[illmonkey]

I've got one like this: https://www.google.co.uk/shopping/product/15425522...

But mine is flat metal to the outside of the tool, so it's hex shaped, taking up minimum space.


ETA: Like this, but longer: https://www.google.co.uk/shopping/product/43634959...

[markw996]

One of the box spanners just about fitted over the top section of hex and luckily the plug wasn't very tight.

Quite amazed at what I found...

Both plugs are actually the same 18mm incorrect size, but someone had filed down the inner plug to the correct 16mm as there is even less clearance on the inner!
I didn't even notice that when I removed it.

Frightening really, if I'd done that it would have given me sleepless nights





The good thing is that the threads are ok

[Bennachie]

Hope it was cheap.............

If that is the state of two spark plugs................


What else has been disturbed?

Brakes?

Oil

[Snake the Sniper]

Assuming the correct plugs weren't available at the time, why would that be frightening? That's not to say I would recommend fitting anything other than the right plug, but if push comes to shove I'd be tempted. It's only cutting the flats back to a smaller size, it won't have any effect on the plug itself.

[mr.man]

Just so you can sleep at nights I'm letting you know that every nut and bolt in the land had their hexagons reduced by 1/16"
at the start of World War 2. Why ? To save material so more steel could be available for the war effort.
They didn't bother changing the markings on spanners so to this day it's why you need a 7/16" W spanner to fit a 1/2" W nut.
and so forth.