I noticed that the air cleaner bracket is loose on my 1979 300sd w116.

Two 13mm bolts hold the bracket from the top.
I noticed that there is a 3rd bolt that is missing and this is allowing the bracket to move around.

Not sure about the thread pitch or bolt size.

Anyone here know the size of the bolt that secures the bottom of the air cleaner bracket to the side of the engine?

Are you talking about the bolt below the bracket in the middle? The one that goes to the engine? It should be a 13mm the same size as the two bolts on the top.

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Nosce Te Ipsum "Know thyself"

Yes, the middle bolt.

I tried inserting one of the 13mm bolts and it was too small.

13mm bolt fits. Threads start deeper in the opening

That bolt is a *****. I had to replace mine because it was so loose it wore a flat spot on the manifold. Definitely get a jointed socket extension to put it back in.

Yeah, I hear you.
Replaced it today and hit a few knuckles.
Just getting the bolt positioned was hard let alone threading and turning it
Had to use a swivel socket.

Not sure if the bracket is going to last.