anyone know the size allen wrench that goes in the fuel strainer on a 123 300TD?

Can't tell you precisely what the metric size is but I found what was I believe a 7/8 in. (or was it 3/4 in?) hex key at a local HW store and ground each face down a few thou and its a perfect fit. Lacking that I've seen shirt sleeve mechanics use a pair of large vise grips or slip-lock wrench on the outer lip, its just thick enough to grab a hold of. It's not the tightest thing in the world to get off so this method works although it leaves marks around the rim.
PS Be sure you have abig enough bucket to catch the fuel that is left in the tank

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

is the size of the bolt- get 2 nuts so that you can stick the bolt in the hole, and the top nut will press against the bottom one.

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1983 300TD, white, 240k
PAY TAXES ON ELECTION DAY!

You mean jam two nuts together and gronk on the top one to get the filter out. Some red loctite or crazy glue might be a good idea also.
Yes that would require the nuts be 19 mm across the hex, which is 0.748 inches so that's why my 3/4 hex key had to be ground down a little.
Anyhow you do it I guess is fine as long as it comes out.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!