Spark plug size and torque values

Definitely 18mm - just returned a 16mm I bought on Amazon. This one works:

https://www.amazon.com/dp/B06XNN7NT1?psc=1&ref=ppx_yo2ov_dt_b_product_details

Thin-wall is best. 

Or if you don't want to click on links from someone you don't know (best practice actually), just search Amazon for this title:

Kawasaki OEM Thin Wall Spark Plug Wrench 18mm 92110-1111

The spark plug hole size is 12mm.  The size of the socket depends on the brand of spark plug you use.  (shell size) Most are 16mm but some are 18mm.  it is not one socket fits all. 

Cheers

I would sure like to know the reason for the torque value change for the new plugs.  Or, does Rotax mean for the new 16nm torque to be applied to any and all plugs? I can’t think of a reason one would be different than the other - but I’m willing to learn.  What would make more sense to me is that Rotax decided the lower torque was better for some reason and should be applied across the board.  

All

NGK specs for 12mm plug in aluminium head read as (from NGK web pages

Result: 10.8 foot-pound = 14.642833842 newton meter

Result: 14.5 foot-pound = 19.65936025 newton meter

Rotax setting new plugs read as: 

"Apply a small amount of SILICONE heat

compound WACKER P12 from the 3rd to

6th thread of each spark plug and install

them (when the engine is cold) in the corresponding

cylinder head.

Tightening torque 16 Nm (142 in. lb)." Info from MML latest edition. I would guess that increasing a bit would still be in range.  Care needs to be taken not to overtorque the shell (outer case) as it can stretch the shell and weaken the thread from the inner parts.  

From this it appears that the setting is middle of the road of the min max settings for a 12mm plug thread into aluminum head. 

Cheers