I have a Grohe Roman Tub filler that has replacable cartridges which I need to replace. However the cartridges have a nylon extension on top of them that needs to come off first. It is a round cylinder that is screwed on the top of the cartridge and has holes in it that I assume you are supposed to be able to put a small round bar into to be able to unscrew it.

The problems is that the extension tube distorts when I try to unscrew it and I am leery of breaking it by putting too much pressure. I am looking for any helpful hints (that have worked) on how to get the extensions off so I can replace the cartidges. It is complicated by the fact that only the top 3/4 inch of the extension tube sticks above the tub deck, 2 1/2 inches of it are below the deck inside the valve casing.

My next step is probably to put some thread oil inside to see if that will free them. I really would like another option as I do not know how the oil will work with nylon threads on a brass cartridge top.

If I break the extension spindle then I really have a problem as it is goes down 3 inches into the valve.