On the drivers side. white plastic plug.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

yep either use a big screwdriver or a penny. You don't want to break it.

On the driver's side, yes. It seals with an o-ring so you don't need to tighten it past snug. The head is very easy to twist off.

__________________
'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."

On the opposite side on the bottom there's a black plactic plug but with a Mercedes star (not meant to use any kind of driver on). What is that for?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

It's the mounting lug on the bottom of the radiator.

__________________
'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."