Crud in gear box

Just replaced the gearbox oil and found just a few bits of loose metal on the magnetic drain plug... Bentley recommends 'periodically' checking and cleaning this, I wonder why?

 

It's fine/normal. You've got steel gears that tend to shave bits off each other, which the magnet catches. Nowt to worry about.

 

I was expecting some on there, was just a surprise when I saw them looking like that.

 

If you rub that between your fingers in some paper, you'll find that it's more like dust than large flakes

 

It's like primary school science experiments! Except with gearboxes.