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. shit 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.