I had my '80 300SD up on a friends lift to install a block heater. The car did not originally have one. ( I don't know why) That pipe plug was one tough mother. We broke a couple of 1/2" drive components before using a standard "L" shaped allen wrench (3/4" I believe) and about a 4 foot pipe. Both me and my buddy were applying as much force as we could on that plug before it finally broke loose.

The flip side is that it sure is nice having a working block heater.

SteveM.

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner

Im still leaving that for later. I have to go buy a big enough socket!

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I think the heater itself is 1 3/4 inch (1 and 3/4 inch) socket. The adaptor is another story...its huge.

Dad and I were working on getting the plug out of my engine.
we already had the manifolds off for installing a pyrometer and whatnot.
Holy HELL that was really tight in there...had to break out the oxy-acetelene and all...it was not a happy baby...
The adaptor is steel I believe and the heater is brass..

Atleast the new ones are