I have replaced all OM603 injection pump seals / gaskets / o-rings leaving the pump in the car EXCEPT the o-ring between pump and engine. Not fun, need a mirror and lots of patience, in retrospect it would be faster/easier to remove the pump and then do them all.

On the OM603, the screws on the bottom plate are Torx, T-20 or T-15 I think.

Never ever ever use RTV to seal anything related to engine oil on a Mercedes diesel engine that has the piston-cooling oil jets. It is possible for a small piece of RTV to break off and flow with the oil to the jet and plug it, and then your engine becomes a doorstop. I suspect that the RTV pieces can go through the oil pump and the oil filter bypass (if your oil filter is old and the primary element is plugged up) and then plug up the jet.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC