What ....? 
The bolt plug is 11mm or 7/16in.
I managed to strip the bolt plug when reinstalling after an oil change and purchased these replacement bolts:
Are you
crazy .... uhem,
serious?
This bolt is
surely not 7/16 in or even 11 mm.
It's a Mercedes, it's European, it's metric,
and as far as I remember it has a 17 mm hexagon head and thus should be M12 (metric, 12mm thread).
The Mercedes parts number (please look under group 50 radiator and not group 18 oil distribution system)
for this bolt is:
N00
7604 012105,
that means it is a
DIN 7604 bolt (DIN = German industry norm, mostly today some ISO norm) with a 12 mm thread (the "12" in the second half of the parts number)
and will look like this:
https://eshop.wuerth.de/Produktkategorien/DIN-7604-Stahl-verzinkt-Form-A/14013511190102.cyid/1401.cgid/de/DE/EUR/Unfortunately, without the Mercedes-Benz norm parts number conversion catalog you cannot say from the 105 ending within the second half of the parts number which thread incline it has and how long the plug's thread would be.
But most likely the original MB part (actual cost is about $ 2.00) looks very similar if not identical to the Würth part.
So, why did you use such an non-original inch part ... on a Mercedes? Mike, please....!
With that you clearly messed up the original thread of your radiator.
Well, whatever...; if it worked out for you.
Oh... I forgot, Mike ... sorry about that.
Your car is/was a (former) US version car now running in France, correct?
So, maybe one of the preowners back in the US messed that already up because inches hardware are so much more common over there...
Oh well....
Whatever...,
Erick,
your question was,
is the original thread M12 x 1.5 or 1.75.Well, I haven't taken out my plug now from my 114's oil cooler (which is by the way absolutely identical to the 280 SL's oil-air heat exchanger)...,
but the Würth catalog (see link above)
says
M12 x 1.5 (mm).
Try to get it from your local Mercedes dealership.
It's not expensive ....
Good luck,
Achim
