I replaced my primer pump on my 81 300D. I suspect its the same as on your 240D. It was trivial to replace but I don't remember the wrench size. Just unscrew it and put the new one in. I got mine from Phil and the new one is FAR supperior to the old one. The new one does not require you to unscrew the top to start pumping. All that you do is push the pump...very easy.

clint

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81 300D N/A 130K miles (Nelly)
87 300D Turbo 80K miles (The Scalded Dog...because it runs like a scalded dog)
95 e320 Wagon 104K
2003 Sprinter 47K miles.
1999 Tandem Bicycle
One beautiful low miles wife (who likes diesels) (that's my wife holding the sign)

My son and I fixing Nelly's odometer