10mm. mine were pretty tight. my gearbox drain plug didn't have a magnet on it. the magnets actually on top of all the gears. **** spot cause ya cant check it for shavings.
The magnet is actually just on front of the drain plug. I was looking at my gearbox the wrong way but after putting it back together this morning I realised. So you should be able to clean it off by dragging it out with abut of wire.