Flywheel bolts

[Bonzo]

The changing of the bolts is a safety issue on a critical application.

Basic theory is : When a bolt is tightened to the max, it will stretch a fraction.
When the bolt is removed it will stay stretched If the old bolt is then re-used it is likely to stretch again.
This in theory may result in a weaker bolt that is just within its yeild zone & in theory is more likely to fail ( Shear )

More science than that involved but that is the basic theory

[mr henderson]

Originally Posted by Bonzo

The changing of the bolts is a safety issue on a critical application.

Basic theory is : When a bolt is tightened to the max, it will stretch a fraction.
When the bolt is removed it will stay stretched If the old bolt is then re-used it is likely to stretch again.
This in theory may result in a weaker bolt that is just within its yeild zone & in theory is more likely to fail ( Shear )

More science than that involved but that is the basic theory

I see exactly what you mean there, and if I was to unbolt a flywheel I would definitely replace the bolts with new, but my question is that if the bolt stretches, and stays stretched (in other words it's plastic) then where is the clamping force coming from? Wouldn't it want to spring back (elastic) in order for the correct amount of grip to be applied?

[Bonzo]

When I say stretch a fraction, I am only talking microns.

With the bolt under tension when cold the bolt would retain any elasticity it has.
Once the bolt has been subjected to many hours, within a relatively high tempature enviroment, the material of the bolt will become " Normalised " thus the original few microns of stretch will be lost.

The bolt will allways maintain a good mechanical grip as all things are taken into account when the optimum torque setting is calculated.

Re-using the same bolt, in theory, would be like using a bolt that is a bit of an unknown quality. Different engines will have been subjected to different levels of heat during their lifetime.

This may explain why such bolts should be changed as a matter of routine.

Cylinder head bolts also work in a very similar enviroment & very few are re-useable for similar reasons.

[flyerncle]

Simply,all bolts have a Tensile strength ie 8.8 8.9 or higher,stamped on the hex end. Tensile = tension or stretch,if you think of pulling the bolt north and south from each end, sooner or later it will exceed its tensile strength and fail/break/fracture. The stretching of a bolt under torque imparts a load on it and stops it from coming loose without any use of Loctite etc.
The other end of the scale is overheating bolts like the ones used to secure cylinder heads and when this happens they lose their tension and at extremes may even break causing failure of the gasket.
You beat me to it Ronne.
Hope this makes a little sense.

[les g]

umm i was always told that the ford flywheel bolts with the fancy pattern in the bolthead are self locking and once used loose the selflocking ability
and so should be replaced
cheers les g

[flyerncle]

Some bolts do come coated with a coating of a locking fluid,usually activated by the heat caused by tightening them up. And as stated previously they should be replaced as per manufacturers instructions.

[snapper]

ARP bolts on ebay are £14 well worth it IMHO