Should be 28 like all the rest...You could take the key out but the crank is pretty small and has a big taper, be sure you use some Loctite and get it TIGHT...Watch out, I had one here last week with a left hand thread...I stripped it trying to get it off...Left hand 5/16 nuts are hard to find in Flagstaff, but Copperstate has 3 left in stock..
Use the thinnest base gasket you can find or make, the Q50s run best with the least piston to cylinder clearance at the top...