IIRCC I was able to hold that 27 mm nut (on the one valve I had trouble with) using either an adjustable wrench or channel lock pliers. This was a few years ago on my 83 300D so my memory may be off.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked