What tool is required to get in the top of the gearbox

[spudboy6]

I want to remove the big nut in the top of the gear box to reveal the timing marks , anyone know what size it is or what took I need ?

Doubt it makes a difference but 1.8 8v 1996 mk3

[gazareth]

a 27mm bolt with a pair of molegrips clamped to the threads usually gets them moving. or a hammer and a fairly blunt chisel to knock it free. once the seal breaks they usually come out no probs.