Hey all
Currently doing a service on the bike and going by the service manual I'm suppose to loosen the cylinder head nuts and then retighten them. I'm curios what is the point of loosening them first instead of just making sure they are tight? Also couldn't this cause a gasket to leak? Cheers
I would "think" the logical reason for this is in case any of them were over-tightened at some time in the past.
2 reasons........
1) As Rich says, the fastening could be overtight. How will you know unless its backed off and retightened.
2) Static friction (stiction). If the fastening is already tight static friction will be present. The threads will be hard up against each other and the face of the nut hard up against the washer. A proportion of the torque you apply will needed to overcome this friction and to get the nut moving. Upshot being the actual applied value will be fudged.
Think about pushing a heavy weight across a polished floor, once it starts moving the amount of effort needed to keep it moving drops............same thing :thumb:
I would also think that if the nut "seized" on the stud because of age, heat, rust, etc you could get a false torque reading. Pretty much what sledge said...
Better to break it loose and start from zero.
But the bottom line is the people who wrote the manual didn't put in misinformation. If it's there, there's a good reason for it, even if we don't understand exactly why.
So if the book says back 'em off and re-torque them, that's what you do.
I don't think you have to worry about the gasket unless you actually pull the head off and damage the seal. I'd imagine you loosen and retighten the bolts one at a time anyway, I wouldn't worry about it.
Ok thanks for your replies