MAGNETIC PLUG

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The heavy wear is usually a result of letting the gearbox drop below the friction torque setting (shimming) on the dog to gear pressure.  It is also the case from not adjusting the idle speed high enough to damp out the resonance from the dog to gear trying to absorb the torsional movement of crank to gears.  Heavy mass props also contribute to heavy wear, these are the ones that exceed the prop mass inertia specifications.  

The best way to judge if you are in need or resetting the shims, normal gearbox maintenance needed, is to simply listen to your engine.  Once it is warmed up and you taxi to the run up area reduce to your idle speed.  (I like to always be above 1600 RPM and never at the min 1400 shown)  If you have a torsional "dance" going on from your gearbox (rattle shake, rattle shake)  it suggests you have to reshim to protect the parts inside the gearbox.  You can tell it needs it if the rattle goes away as soon as you pick up the idle speed a few hundred RPM and it gets smooth.  This is a sure sign it is under shimmed.  I don't believe in waiting until the prescribed times listed by the MML, that is usually far too long.  

Cheers

I have checked the oil filter, it is normal, there is not too much metal  debris  , thank you very much for your suggestion, I will carry it out.