My 999S crank has been apart for some fiddling and I need to obviously put the conrods back on. In the olden days I learned never to use conrod bolts twice as they torque up to a specific stretch and if they are Ti bolts in particular, re-torqueing used bolts can led to fatigue and later failure when the bike is running = bike engine rebuild costs. The question is whether I should replace the conrod Ti bolts on my 998S (8000 mile engine) or can they nowadays be re-used with some confidence. At £130 a bolt it's not a cheap solution if I need to buy new! Any engine tuners know the score out there?
Whilst I can't comment on your specific question I can say that in the company I work for we do use titanium in certain applications specifically because it is very resistant to fatigue. My guess would be that a titanium conrod bolt would have an infinite design life and occasional retorqueing would not affect that. I will be very interested in what anyone else has to say on this.
I always re-use torque-to-yield bolts. Although some are happy reusing them. Ask Nelly, he'll see you right.
If they are torque to yield bolts then they probably do require replacing each time. What does the manual say ?
There is specific process to follow when torqueing these bolts, if you follow the process it will be obvious if the bolts need replacement or not, its really quite simple You will note that the shoulder of the bolt is cut down, so that it is slightly less than the original diameter of the bolt. There is a reason for this. All bolts must be able to stretch - but only a predetermined amount. Without stretch the bolt will not properly clamp the cap to the body of the rod. If the bolt stretches too much, or fails to return to its original length when loosened, DISCARD IT. It is also important "where" the bolt stretches. The reason for the shoulder reduction is this is the area where the stretch is supposed to take place. The diameter of the bolt in this area will be slightly less than the root, or bottom, of the threads. If this shank reduction were not there, then the bolt would have a tendency to stretch in the thread area. This would not be a good idea. In general an 8mm bolt should stretch between .005-.006 thousands of an inch at or below indicated torque. Each manufacturer will have his own specifications, and it is important that their recommendation be followed. Step 1 15nm Step 2 30nm Step 3 50 degrees Step 4 Measure stretch - btw .155 and .175
The ti bolts can be used 3 times as mentioned above. The steel ones are throw away. Obviously, if you know this is the first time they've been off then you're ok. If there's doubt, replace them.
That's great news thanks Nelly - first time the engine has been apart at 8k miles so I will re-use them. Great news becasue they are £205 each!!
