Toby,
Ideally when you measure the torque of a bolt, it should
be an indication of the clamping force applied to the item
(i.e. cylinder head).
There are lots of internal friction points of the bolt head
getting down into the threads, in addition to the friction of
the bolt head against the cylinder head (I always put a drop
of oil between the head bolt head and washer to reduce that
friction).
When a head BOLT is torqued, you have a residue torque
(twist) to the headbolt.
With a stud, you are pulling the stud and the triction is
at the nut being twisted at the top of the stud. More accurate
reading.
ON the VW Diesel engines they were known for blowing headgaskets
big time. ARP had a stud kit out. Seemed odd, but torque to
20 ft lbs... then 720 degrees of rotation! I thought those studs
were ready to break, but with two full turns... it was tight.
Headgasket lasted another 150,000 miles after that... studs
paid for themselves.
Only pain... Getting the head onto the studs. Tight fit, so you
have to lower the head just right. Not a problem on a roadster,
but with a BIG heavy cast iron head... whew!
Cheers,
Tom
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