ehz660 wrote: The engine code is NC13E - 1200 911

That engine number indicates nc30 engine, more info in the model identifier here: http://www.400greybike.com/docs.htm

Modifying the heads to take 10 mm plugs is not uncommon.

Ahhhh cheers guys!

I'm guessing that the Denso plugs i currently have in are probably the wrong heat grade and could ruin performance/do some damage? Is CR10EK the best 10mm plug to use on these engines or are the Y27FER ok to use?

I guess i'll have to shorten the plug caps slightly so they fit flush and don't let moisture in.

I always thought that Y27FER = NGK ER9EH

Sorry guys they're Denso U27FER9 plugs in mine - Equivelant to the NGK CR9EH-9 10mm thread plugs that belong in the NC24

More to the point. I do have some NGK ER10EH plugs now sitting on my windowsill i no longer need.....

http://www.ebay.co.uk/itm/250398677089? ... 1439.l2649

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

These are the ones. Anyone want them For £30 delivered by royal mail recorderd delivery? Save yourselves a tenner

Wouldn't 10 grade plugs be a bit cold for a for a road bike, they may foul really easy.

grade 10 in a race bike is enough of a ballache, let alone on a road bike.

Any pictures of down the plug holes, I think they've probably been tapped out.

No NC30 ever used 10mm plugs as stock according to any documentation I have. My bike is a really early 89 and that uses 8mm plugs, the 1st edition of the parts manual also makes no references to 10mm plugs.

Most likely its been drilled and tapped / drilled and helicoiled out to take larger plugs.

They are cheaper for one, and perhaps it suffered a knackered thread.

The NC24 didnt use resister plugs though, NGK C8EH9 or ND U24FE9 is the stock plug.

For colder climate you might need NGK C7EH9 or ND U22FE9

NGK C9EH9 or ND U27FE9 are really only for hot climate / extended high speed racing.

As above, like those ER10s you mention, they suck on a road bike, foul up too easy.

For 10mm plugs stick with C8EH9 / CR8EH9s.