HELP, cant undo nut :\'(

[L18M]

How to undo this nut?
[ATTACH]56422[/ATTACH]

I can get a spanner onto it and rotate it about 5 degrees but then the spanner fowls the turbine housing. I cant rotate it enough to turn more.
Are there some other tool i can use to undo this? its still fairly tight on.

Any help appreciated. Thank you!

[davehg]

You may need to grind down an old spanner to allow a little bit More of a turn

[L18M]

thanks dave, i got another thinner spanner from a friend which i hope will do the trick else i\'ll use your suggestion