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Bolt Clamping Force

Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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I am rebuilding a 2016 Yamaha R1 motor due to a failure in the gearbox, I am also going to refresh the engine while it's apart. The bike is a race bike and only used on track.

The sump and engine covers are bolted down with one-time use M6 aluminium bolts by Yamaha. In the service manual, it states to replace the bolts once the cover has been removed. The torquing procedure is as follows:

1. Tighten each bolt to 6 Nm

2. One at a time, back each bolt off, retorque to 3 Nm and tighten a further 180 degrees

My plan was to replace these bolts with titanium bolts because this engine will be coming apart fairly regularly. My thought was to just tighten each titanium bolt to 6 Nm and be done with it, but I'm not sure if that is correct. My main concern is damaging the aluminium threads with a titanium bolt, potentially first go or perhaps over time.

So, do we think the Ti bolts are OK to use and if so is 6 Nm the right torque specification?


I would expect the 6 N.m to be to ensure the threads are clean and not rough, with the 180 degrees a stretch to yield figure.

Normally, I would suggest picking up a new aluminium bolt, or three, and using a dial type torque wrench follow the normal procedure and see what the actual torque at the 180 point actually is, and use that. Not many have that type torque guage, so perhaps take it to the 3 N.m and increase the torque by a unit, or two, until it is tightened to the 180 degree point?

I am a little puzed by the use of the aluminium fastener in an aluminium casting' thread, as that would seem to be asking for trouble with galling* - if you look carefully, you 'may' find the casing is fitted with heli-coils, or some other form of thread insert? The other means of reducing galling is to hard anodise, or plate, the alloy fasteners.

Regardless, a suitable anti-galling lubricant will almost certainly be specified.

*The term for when two materials transfer material - in this case 'pick up' the thread and strip it. There is a lot on-line that should better inform you - such as https://www.thefabricator.com/thefabricator/article/assembly/preventing-thread-galling-for-optimized-fastener-performance Personally, unless the weight really is that significant, I'd probably just use steel.

Anyone? This is an interesting question.

Gord, thanks for the response.

Re torquing up from 3 Nm to 180 degrees, if the material of the fastener has changed would that not mean that the 180 degree is no longer suitable. My thinking is the aluminium bolt would stretch from the initial 6 Nm and then retightening to 3 Nm plus a further 180 degree ensures the right torque is applied. If the bolt doesn't stretch the same from the initial 6 Nm then when 3 Nm + 180 is applied the clamping force would be greater with a Ti bolt than an aluminium bolt. Not sure if my understanding is correct though.

With regards to using Ti over SS, that's just my brain knowing SS would be a larger increase in weight over the stock aluminium bolts compared to Ti. The fact that I have probably had heavy morning constitutions doesn't factor into my thinking!!! ;-)

Yes, the different tensile strength of Ti over Al would mean the torque, and hence loading, would be MUCH higher if taken to 180 degrees.

That's why I said to see what the required torque value was for the aluminium bolt(s) to take them to the 180 degree' yield spec', then consider using that torque value for the Ti bolts - it will be MUCH less angle, but that would be irrelevant as it would no longer be used.

The bolts will certainly be capable of much more torque, but this way the castings and, most important, the threads will be subject to about the same amount of stress, which is more important.

If you're planning frequent strip-downs of the engine, I'd suggest picking up a suitably sized heli-coil kit so, if you do pull a thread, you're already prepared to deal with it. If you find you've had to heli-coil several threads, I'd suggest just biting the bullet and doing them all - they're much stronger (if done correctly) and many high-end alloy parts will come fully heli-coiled from the factory.

Oh, I was forgetting, Ti is potentially very strong, but some alloys can be notch sensitive - flaws can result in rapid crack propagation and fastener failure, as they will be loaded in a steady state, I don't see this being much of a problem, but talk to your supplier about this.

Hi Gord,

Excellent, thank you very much for that detailed response its really very helpful.

Interesting point about heli-coils I had always assumed they were a bit of a bodge rather than a valid solution to a problem.

I think my approach now will be to order a handful of the OE aluminium bolts and try your suggestion and take the average of them all.

Thanks again,


Wurth also make a product that is supposedly superior, they call it Time-Sert - https://eshop.wuerth.de/Product-categories/Thread-repair-and-reinforcement/14016006.cyid/1401.cgid/en/US/EUR/

I can't comment on using it, but it is German quality...

I came across it here, with a chap trying to repair a Lambo' block and girdle - from ~4:20 if you don't want the rest, he goes into the use and fitting proceedure rather well - https://www.youtube.com/watch?v=-GpeAPZM1hE

Another chaps thoughts - https://www.motorcyclecruiser.com/thread-repair-time-sert-vs-helicoil/

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