I'm getting a little skittish about Rotax's 20Nm lubricated thread spec for spark plug torque. I cannot find a single source which supports that. NGK and Denso publish their accepted torques for 12mm plugs, e.g. https://ngksparkplugs.com/en/resources/spark-plug-installationhttps://am.denso.com/plug-global/basic/torque/, and those are spec'ed dry. (See screenshot from https://www.youtube.com/watch?v=XtzpzsPfsYE). However, our plugs are installed wet, because they're lubricated with silicone grease (the thermal paste).

In general, thread lubrication reduces torque values needed for proper preload by 10-30%, and I notice Rotax's new spark plug is spec'ed for 16N-m lubricated. This is an almost exact 25% reduction in torque, as one would expect for lubricated threads.

While there's no danger to the plug by over-torquing, since they are designed to withstand the forces for installation into a cast-iron head, it certainly seems like there is global agreement that plugs into aluminum heads shouldn't go past 20Nm dry. Other spark plug manufacturers, such as Champion, publish almost identical stats.

So what's the smart money here?

1) Trust that Rotax knows what it's talking about because people have been doing it like this for decades?


2) Use the available data, combined with the knowledge of how engine manufacturers are extremely reluctant to admit fault or change, and that spark plug torque is almost always a range and not a specific number, to aim for a more conservative 16-18Nm, thereby reducing risk of damaging the threads or the plug?

10077_1_Screenshot 2023-09-26 at 1.50.10 PM.png (You do not have access to download this file.)
  • Re: Is Rotax's 20Nm spec for spark plugs correct?

    by » 8 months ago

    Hello Kenz

    I think that you answered your question very well.  With the thread heat transfer paste the 16 NM is fine.  The issue is you have an aluminium head, not cast iron, so do not put it to the max value.  20 NM is just too much.

    Rotax no longer uses NGK, due to NGKs position of not selling to aviation applications.  The specs listed now only show the 16 NM value and have done so since they switched plugs to the Rotax branded spark plugs.  These are not NGK plugs, we can't assume that the shell of the plug is exactly the same as the NGK.  



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