I've posted my solution before, but here it is. I used a dremel tool and a standard allen wrench over 14mm (I can't remember what size). I cut a 1" piece off of it, and then ground 1/2" of it down to 14mm using a 14mm wrench as a guide. The big end fits into a standard socket and voila, I have a 14mm allen wrench. As someone above noted, the other plus with this setup is when I have a tight fit (like the filler on my 190SL transmission), I can use a box wrench and the 1" piece of allen wrench.

I also found a 14mm some time back at Autozone, but didn't buy it 'cause I already had one that worked! Maybe PartsShop or Fastlane should sell these...

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Bill Streep
San Antonio
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)