I'm planning on replacing my bottom bracket soon and was looking around for a tool to remove the fixed cup and came across this on ebay.

https://www.ebay.com.au/itm/HEADSET-W...item27ca92e871

Do you think it would work? My current BB has a hexagonal adjustable cup so i was thinking I could also use it for that. The spanner is 6mm thick. Is this an all in one wonder tool or am I just dreaming?

It won't work any better than an adjustable wrench. If anything it will be worse because it's a thin flat piece. Not designed for the serious force it takes to remove fixed cups.

Have you ever removed one? It takes quite an arm. My preferred method is to stick the flats in a bench vise and turn the whole frame. I tried Sheldon's nut and bolt method but I didn't use high enough quality stuff because I bent up all my washers and IIRC I broke the bolt I was using too..

Actually it would probably not work work as well as a 12" adjustable wrench because there would be about half the jaw contact with the multi-wrench. With a headset it's not as much an issue because the torque is much less. There is a reason most fixed cup wrenches are closed end, because as Fast Jake said, you often need a great deal of force to loosen one. If the fixed cup has recently been removed, you might be able to get away with it, otherwise it's really not necessary to removed the fixed cup for routine maintenance. If however you are R&R the entire BB for some reason,you will need a closed end tool.

If this works, it's only because the cup was too loose to start with.

IMO this is a poorly designed tool not even adequate for headsets, especially on the larger step. There's simply not enough meat to keep the jaws from spreading under load.

If you look at the tools made for 36mm fixed cups, they all have a completely enclosed ring to ensure that the tool doesn't spread open under load.

If you can't find anything decent locally, I have a few of the Sugino tool sets pictured and can sell you one $21.00 plus postage.