Fatter longer screw with some kind of gunk on the threads or a piece of hose of the same diameter slid over the fitting, bent over and held in place with a small hose clamp or rubber band. You can even buy vacuum cap plugs that I bet would slide over the fitting. If it still leaks, buy a long piece of hose, put it on the fitting, feed it out of the engine compartment at the cowl, over the roof and into the fuel tank.

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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