I can't decide to to use a thread lubricant or not on the head bolts. The manuals don't mention lubricant so I would guess to install dry. Please let me know what you think.

 

In this particular case, follow the manual because ...

the torquing procedure has been worked out to get the
fastener into the correct state of tension.

The several rounds of tightening preceding the final 90*
is meant to get the fastener to a known base position.

If you use lubricant, you will be starting from a different
base position because the resistance to a torque wrench
is less with the lubricant.

However it is important that you start with absolutely
clean thread holes and fastener threads. Again, for the
same reason as above.

 

Every head gasket change within my 23 years as a technician, I have always either used thick engine oil like 20w/50 or gear oil on the threads and on the underside of the bolt head. These past 4 1/2 years as a Jag technician, I still use either oil on new head bolts. Never had any issues. Just my 2 cents.

 

I'm a lube type as well ... but with these particular
fasteners you can see where every round except the
last is only to setup the staging ground for the last
round. That means getting there accurately, or at
least "as designed " is critical.

Would I lube under the bolt head or wipe the threads
with oil? Likely yes. But I'm not going to deviate from the
published instructions when advising others who may
not have done any reading at fastener vendor sites.

Don't know what the Jaguar "engineers" were thinking.
A more accurate baseline could have been established
using lubed fasteners torqued to the appropriate x,y,z
values before the final 90*. Lubed fasteners are more
consistent. Different values for x,y,z but more repeatable.

I'm used to big honkin' bolts on iron blocks, not these screws
they call bolts on the Jag V8. The turbo sixes on Supras needed
15 ft/lb more than factory spec to survive even stock boost.
The tolerance for that additional torque was established with
some confidence that the bolt would not just snap because
the Supra forums happened to have a couple of metallurgical
engineers as members.

 

Plums and Addicted2boost, Thanks for the good discussion! I decided to go with a very light bit of assembly lube. This was my first time installing this type of bolt. It was kind of freaky turning that last 90 degrees without it feeling like it was getting any tighter. That tells me the bolt was yielding. If I had my old wandering needle torque wrench I would have used that to see what the torque was as the bolt was yeilding then I could have calculated the stress and checked against mfgr specs. I am a geek.

 

It occurs to me that the original specification was developed
for a composition type gasket. The usual substitute is the
4.2L shim gasket. Wonder what the manual says for the
4.2L motor.