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finger_jockey
I think I may be in a similar position...I did the 4 turns and twat method which did sweet fa. I've left it alone and dreading going back to it.
I saw this as a guide when searching on the interwebz:
Releasing a stuck Brompton Stem – a solution!!
• Loosen the stem bolt by using an allen key to undo the bolt by 4 or 5 turns
• Cover the bolt head with a piece of wood and whack it hard to drive the bolt down and release the wedge from the bottom of the stem.
• Remove the front wheel, the front mudguard and the front brake assembly. Let them hang loose without stretching the brake or lighting cables. Secure with PVC tape if necessary.
• Unscrew the bolt and remove. (This is not required if the stem rotates and can be removed in the normal way).
• Using a long screwdriver or a suitable drift, drive the wedge out of the bottom of the header tube. The wedge will be a tight fit at the bottom of the tube and you will need to drive it slowly.
• Secure the bike upside down, say in a Workmate or similar.
• Now you need a socket that is a tight fit to the inside of the bottom of the header tube. The best choice seems to be a 16mm plug socket. A standard (shorter) socket will do but the added length of the plug socket seems to help. If you don’t have metric sockets, 5/8” imperial sockets are good as well.
• Now insert a 150mm (6”) extension bar on to the socket.
• Snug the socket into the bottom of the header tube and allow it to rest on the bottom of the stem. A few goes with penetrating oil on to the bottom of the stem before this will be helpful.
• With the extension bar protruding out of the bottom of the header tube, give the end of the bar some sharp whacks with a standard claw hammer.
• Surprisingly the stem will start to move after a few whacks. As the stem emerges it will drop down at increasing speed. Take care as you get to the end of the stem otherwise you will end up driving the socket into the end of the stem.
• Remove everything and clean up.
• Use a 38mm spanner to remove the headset, clean and grease.
• Reassemble, grease the stem tube before reinserting it. You will probably need a few hammer taps on a wooden block to reinsert the stem.
• Reassemble brake, mudguard and wheel.