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Stuck 24mm union nut(Audi autobox drain) - Mapmaker
Lovely morning, so I took myself out at some unearthly hour to change the gearbox oil.

Unfortunately the union nut (whoever designed it like that - you drain it by undoing the union nut connecting the dipstick/filler pipe to the box) won't shift. Obviously I'd like to get a ring spanner round it, but that just isn't going to be possible (!). What is the best tool for this job that's hopefully going to grip the nut on more than 2 corners, and so not round it?
Stuck 24mm union nut(Audi autobox drain) - Cliff Pope
You can get sort of open ring spanners. They have a slot, so that you can get them over brake pipes etc. I don't know whether that would work here. I suppose you could make one.
Stuck 24mm union nut(Audi autobox drain) - Aprilia
Get a 24mm ring spanner and grind a slot in it - not perfect, but better than open-ended.
Also put some penetrating oil round it and try doing it when hot - the expansion might help it free easier. Some very gentle taps on the faces of the nut might also help free it up.
Stuck 24mm union nut(Audi autobox drain) - henry k
Get a 24mm ring spanner and grind a slot in it
- not perfect, but better than open-ended.

Borrow a crows foot spanner.
(They seem to come in sets unfortunately.)

only picture I could quickly find.
www.find1001.com/acatalog/info_LT3282.html
Stuck 24mm union nut(Audi autobox drain) - greaser pv
Try water-pump pliers , mole-grips or a small pair of stilsons . You could ply a bit of heat onto it , Ive never had much luck with pipe spanners unless they are the best quality.
Alternatively thrash it down the motorway for a few miles get it really hot , you might find it will then undo. HTH
Stuck 24mm union nut(Audi autobox drain) - Mapmaker
Thanks, all good ideas. Otherwise, buy a suction machine as suggested by ST if I completely round it off. £60 seems like a lot for a simple pump - I might try a piece of pipe and a siphon first.

Blowtorch on the nut? Or is that asking for it?
Stuck 24mm union nut(Audi autobox drain) - Mondaywoe
When all else fails (and the nuts have already been well rounded!) I usually resort to a Stilson. I have two - a small one which I bought a few years back and a (very) old one that belonged to my grandfather (pre-war I would think!) There's not much they won't shift, provided access is good enough.

You can always replace the nut once it's off.

Graeme
Stuck 24mm union nut(Audi autobox drain) - Cliff Pope
Good old Stilsons - I love them! An old quality Stilson and 6 feet of scaffolding pipe will move anything, or crack the entire machine!

Seriously though, IME the secret of shifting any tight nut is to get a really solid grip on it. You need a good spanner that is a tight fit, or file two flats on the nut so that the next size down will fit. Also useful sometimes, space permitting, is tapping the nut round with a sharp cold chisel.