I replaced mine about 2 weeks ago in my 83 240d.
All four new bosch plugs were around 43 dollars, and found them locally after calling around .
I was stuck at work and wanted to replace them there at night in the cold windy night , but I had the car flatbeded home and did it the next day.
It is easy , I found that a closed end rathet wrench worked best for me, as I left the fuel lines in place.
I figure for the cost it is worth it to replace these as a set.
Good luck.

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A couple of Ford pickups F 150,250, 350 missed.
1983 240 D 256,000 miles ,auto, ac.