is it a good idea to use a cornflake box as a gasket for this , if not why!

Kev, There's a bloke on you tube does them for Triumph primary casings, so why not. In fact I just bought a set of hollow punches for the same reason

http://www.youtube.com/watch?feature=player_detailpage&v=NDOcR592-Fw

I,ve used cardboard before for gaskets, coated both sides in grease first

your supposed to back me up,,

i just told my lad he's a fool , and to use the correct stuff.

also told him to torque down the cylinder head , has his mate as just elbo torqued it

Dammit, sucked me in

I do mostly, but complicated stuff i CBA doing although I have bought punches do do this.

sorry Kev

Yeah like why not i used to do it all the time as a temporary repair on cars and bikes.

As Above a temp fix sure but "instead of" that's a no in my book.