Rear heat exchanger removal from cylinder head

[DE Type 3]

I need a little advise on removing the rear heat exchanger from the cylinder head. I can get the outside nuts off but having trouble getting to the inside nuts on the inside. Can someone help with the best way to access the nut? I am almost to the point of cutting the heat exchanger off. Any suggestions would be appreciated

[Tram]

A 13mm on a LONG extension will get one of them (can't remember which side) and an S shaped box end is needed on the other.

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[KTPhil]

...and when you reinstall, use the thinner and smaller 12(?)mm copper nuts.

[Slow 1200]