Yes - 5Twins and Marlin are correct - and so is Brassneck. You should always break loose and lubricate fasteners that have been in-place for a long time.
In my case - the bike was apart 10 days ago and everything was totally cleaned up and then reassembled with lubrication on the threads - so there is no need to break them loose or apply more lube at this stage of the game.
Remember that when you torque a fastener (nut / bolt - whatever....) you don't really
want torque on the bolt. What you are
actually trying to do is generate a
clamping force between the two things that the fastener is holding together (in this case the XS650 engine cam cover, cylinder head and barrel). The nut advancing down the threads of the stud is merely a good way to create that axial clamping force.
Unfortunately, we cannot measure the axial clamping force in the bolt or stud
directly - so we use the applied torque as a
surrogate measurement. For the applied torque to
translate fully, predictably and accurately into an axial clamping force, you need to reduce the friction in the threads and under the head of the bolt or nut to the lowest possible level.
If you're having trouble with that concept - imagine the following:
- the acorn nut on the end of a cylinder stud is not tightened and is just sitting loose (so the cylinder head is NOT held down onto the top of the barrel);
- you weld the acorn nut and the stud together so that they are rigidly connected to each other;
- NOW - you use your torque wrench on the acorn nut;
>>> how much clamping force are you generating to hold the cylinder head down onto the top of the barrel?
NONE - bupkiss / nada / SFA!!!
>>> all of the applied torque on the nut is going into trying break the weld so that the nut can turn on the threads.
Until that nut turns relative to the stud -
NO clamping force is generated. This is the same situation you would have trying to torque a fastener that is sticking or even seized on the stud. You can make your torque wrench click - but you are not generating any axial clamping force that will compress the gaskets to keep the oil in.
The other key point is that for a cylinder head-barrel joint you want all of the studs and bolts to apply the same axial clamping force evenly across the entire surface so that the gaskets are uniformly compressed and the engine is sealed and doesn't leak. Soooo...you want the applied torque to create the
same clamping force in each stud - rather than having one stud that is loose, while the others are tight.
I have attached a .pdf of a slide from a presentation I gave to our local vintage bike group (
www.CVMG.org) that shows the effect on the axial clamping force from a given level of applied torque and also the consistency of force for a given applied torque for lubricated versus non-lubricated fastener threads.
For
unlubricated fasteners, the axial force (in the slide it is called "preload") varies from a low of 23 kN to more than 40 kN for the same applied torque - a huge variation. For the lubricated fasteners, the variation is much less - and so if that was data for the studs on a cylinder head, the force holding the head on would be much more consistent and the gasket would be less likely to leak. Interesting - eh?
Pete