1. Can anyone tell me what the inside diameter is? (If you don't know for sure, guessing won't help.)

2. Anyone know of a stock radiator hose from which a short section could be cut to use as a substitute for the bypass hose?

'82 300D

Use I.D. 1-1/2" or 38mm. When I looked for the by-pass hose from the thermostat to the water pump I bought a straight hose made by Gates #24024 Vulco 1-1/2" I.D. (38mm). Where are you located? Good luck.

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Roman
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87 300SDL 152K
86 420SEL 228K
83 280SL 145K (5 spd)
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That's a great source. The dealer wants to sell you one meter of that hose.

Thanks, Romansek.