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MG Midget and Sprite Technical - Torque setting - NMC

On my ZS180 racecar I have a 2-part brake disc assembly with cast iron discs and aluminium bell centres. The fastenings are 8mm Allen screws with slim-wall nuts. Could the engineers among you suggest a correct torque setting please?

David Smith

Phew, I'm glad it's not the rear hub nut! ;o)
Dave O'Neill 2

David,

my dad always advises tightening anything until it shears and then take it back a quarter of a turn

you're welcome

Alan
a reid

Bump
hoping that someone has a sensible answer?
David Billington?
David Smith

What grade are the fastening ?

I assume the ZS is metric, in which case they are 8.8 or 10.9 or very rarely 12.9.

Malc
Malc Gilliver

If you look at Unbrako website they have a pdf which lists torque settings for both alloy and stainless steel capscrews. It's a guide at least although there is a lot going for Alan Reid's suggestion!
Graeme Williams

Graeme, thanks, looks like just what I wanted :-)
David Smith

Just be aware that Unbrako are top quality items so may be stronger than many out there. Also - and I didn't check this through - if you have a thin nut then the nut will be the weakest link and may not be the same quality as the capscrew.
Graeme Williams

Just for completeness they are marked YFS, who AFAIK only make high tensile 12.9 Allen setscrews so I'll tighten to 39Nm. (Malc- these are not ZS standard brakes but Wilwoods, metric nevertheless).
David Smith

If there's a range of torque settings between which your fixings should be tightened, don't go for the high end of the range because the tensile strength limit includes the tension produced by tightening; in other words, the tighter the fastening the more likely it is to fail. On the other hand, tensile strengths of steel increase with tension up to a point. So about a third of the way up the scale is about right. If the scale is (say)25 to 35, then 35-25 = 10, 1/3 of 10 is 3.3 so a good setting would be 28.3

The 'just before it shears' is, I know, a joke, but some take it seriously. The shear strength is not what most fastenings are about because they are designed to clamp and it's the clamping friction that provides the mechanical grip, not the shear strength of the fastening - unless you're talking about swivel pins and they are components not fastenings which is why bolts should never be used as pins - their shear strength is usually not very high.
Nick Nakorn

This thread was discussed between 13/07/2015 and 17/07/2015

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