I removed the drain plug no problem on my 87 and it had not been used in 20 years I am sure. I just replaced the rad on it and drained the block that way. So try and see if the plug moves, just don't over do it. Mine came out no problem and I also have a block heater.Block heaters certainly will work better. But this did not really answer your question....No idea if one is available. I assume you have a Mercedes as you mention and show nothing...

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1987 300 DT