Assuming you're talking about the rubber fuel hose to the lift pump and return? Original size is 7mm which is just a hair smaller than 5/16". A lot of people use 5/16, I had bad luck with air ingress when I tried it and ordered the correct 7mm size and have had 0 issues since.
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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)
Black Sheep:
1985 524TD 167K (TotalDumpster™)
Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)
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