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billygildark5007

Head bolts

6 posts in this topic

Yamaha does not consider them a consumable item, like they do seals, for example, that should or must be replaced during a rebuild.  You will find no reference to them needing to be replaced after one or any number of uses.  In general, I wouldn't replace them unless there was some indication they were overused, like the torque necessary to rotate them the required 180 degrees being markedly different from one bolt to the next.

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The overall problem with reusing head bolts is the stretch factor. Torque is already a somewhat unreliable form of measuring bolt stretch, and reusing bolts makes it even more unreliable. Once a bolt is stretched, it has a tendency to remain stretched. For every reusing, it stretches less and less (because it has already stretched and stayed there), and yields less and less reliable clamping forces. It's not a matter of the bolt breaking so much as it is a matter of not forcing the head down as much as it used to for the same given torque.

I'll reuse them once, and then they get replaced. Cheap insurance thing..

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That's what I was reasoning. If they stretch they should have a service life but I couldn't find anything here or in my Manuel specifying replacement. But maybe that's why yamis use angle torque?

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The thing is that steel has an elastic limit.  When stretched beyond that limit, it is permanently deformed.  If not, it can be stretched and relaxed, and stretched again a relatively limitless number of times, same as any spring.  In service, the torqued down head bolts are stretched roughly .6mm, and expected as a set to maintain even and consistent clamping force on the head and cylinder.  Both head and cylinder are aluminum, and because of that are constantly expanding and contracting.  The head bolts, then, are expected to be stretched into a loaded condition and cycled back and forth through heavier and lighter load pressures repeatedly, and because the elastic limit is so well above the operating range, they can do the same thing reliably for years, like the springs in the front of your truck.

 

If you want to worry about something, worry about the threads in your $800 crankcase set. 

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