7/16" is what I used, and it worked great.
I couldn't find a 7/16" allen key, so I had to make my own. I went to Sears in the tool section, and found a 1/2" chisel that had a 7/16" hex shaft for like $3. Cut off the chisel part, and made my own allen socket with a 7/16" socket on the end.

Worked great, and only cost me $3.

Steve

 

All-thread coupler for 1/4" has a 3/8' hex (learned the hard way) Yes, I used a double nutted 1/4 bolt for years.

Many "real" parts or hardware stores have them in bolt bins as do bolt and fastener supply stores. Military surplus is another place to look. Note; I said "real" not Advance, AutoZone, Shucks, Fastenal, PepBoys or any "fast-food" type supply outlets. Many punches or chisels have a 7/16" hex that work well if you grind off the end.

If all else fails order one from McMaster-Carr

 

Wish I would have looked here before I paid 10 bucks for a 11mm allen wrench. I just tried a 1/4 bolt head and it fits perfect. Duh on me!

 

Thank you all for your input. Got the 7/16 allen key and worked great. Seems to knock down the temp real nice.

Merry Xmas

HiTech