Main engine bolt torque?

[iansoady]

I've just fitted a pair of crash bungs (these: http://www.ebay.co.uk/itm/351009442876? ... EBIDX%3AIT) which seem well made.

I was a bit alarmed by the torque figure for the main engine bolt / nut that I found in the manual (this is the one at the bottom of the V shaped arms on either side). The manual gives 93N-m which is b****y tight - the one that came off was nowhere near that. I've done it up to 55 which seems much more likely.

Any views on this? The bungs have a stainless 12mm stud with coarse threads both ends and Nyloc nuts.

The "standard" figures I can find for 4.8 material is only 45Nm.

The nut is item C on the attached:

[greywolf]

Like the rear axle, I've had success with applying anti seize to the threads to prevent galling and using 80% of the specified torque to account for the lubrication effect.

[Juvecu]

It does seem a bit high, that bolt must be roughly the same size as E & H and they are showing 55.

[greywolf]

The other bolts are not only smaller in diameter, but also considerably shorter. A bolt stays in place by stretching from the torque on the nut. A longer bolt can need more torque for the proper stretch. I have seen no reports of problems from using the book spec on those parts.

[iansoady]

But the rear spindle has a specified torque of 100Nm (only slightly above what is suggested for this one) and it's both longer and (much) thicker.

I always respect what you say Pat but am puzzled that this particular figure should be so far out of kilter with other similar fastenings.

[MartinW]

This confuses me too as I am a mechanical numpty, that's why I don't do much work on my bike.

I don't get the "do it up with lubricant so it comes off easier and then don't do it up so tight either" theory. They seem counter-intuitive when done together. Hopefully Pat will enlighten us both.

[iansoady]

The lubricant is essentially to stop the male & female threads galling or microwelding themselves together. The torque you apply is, as Pat says, to stretch the fastener and this is what stops it coming undone. Lubrication achieves the required stretch at lower torque values.

[greywolf]

There you go. As to the comparison with the rear axle, it is hollow for lightness. That has to be taken into account.

[iansoady]

I didn't know the spindle was hollow - it felt pretty heavy to me!

[greywolf]

[iansoady]

You've just prompted me to go and have a look - and of course you're perfectly correct!

[Juvecu]

Imagine how heavy it would be without being hollow, then it makes sense why it is