May I use a new set of pistons from engine 104 (E320) 89.9mm to replace the old one in a 103 engine (300E) 89.5mm? Are the connecting rods the same? Thanks

I'm going to say No..the combustion chambers are undoubtedly very different.so you'll need 103 pistons.

Rods look to be the same on quite a few M104's, namely.980/1 & 990/1/2/4/5

Jonathan

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Jonathan is right, they do not interchange. The 103 is a 2 valve engine and has 2 valve pockets cut into the pistons. The 104 pistons will have 4 smaller valve pockets cut into the piston, hence the lack of interchangablity.