In looking at my 615 FSM engine manual, there appears to be a pressure relief valve in the general vicinity of that bolt, but I cannot see a picture of the bolt labeled in the FSM. It it is the pressure relief valve then I think it would result in poor oil flow to the rest of the engine if it fell out. Are you getting a healthy flow of oil at the camshaft currently? What is your oil pressure?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13