Here is the supplement service manual with torque values. Not listed is the stud values. Yamaha didn't think anyone would remove them I'm guessing.
Haynes list the torque for the 10mm stud at;
13 ft lb for XS650 (uk)
11-14.5 ft lb for XS650 D (us)
No listing for any other newer models from 78 and up.
So my value of 20 ft lbs was close, but looks like 15 ft lbs is enough.
I would use the red loctite as you never remove these unless broken or your doing something out of the ordinary to your engine cases.
Hope the engineer types chime in here.
In layman's terms, as I'm no engineer just a home garage mechanic, the torque value of 27 or 30 ft lbs on the cylinder head nut is to put the right strain and stretch on the stud itself to maintain the proper pressure between the cases, cylinder and head. The stud with all it's threads into the case probably will stand at least 35-40 ft lbs of cylinder head nut torque as a safety margin. How tight the stud is into the case doesn't come into play so much. I'll bet 10 ft lbs on the stud with locktite would be sufficient. The 15-20 ft lbs of torque just insures it will not back out when you install and remove cylinder head nuts.
My 2 cents.
So 15 ft lbs, red locktite, and your good to go.
