Hi,

Does anyone know which ones are stretch bolts and need replacing when doing the timing belt on a 2004 Mk1 1.9 1u5 Octy?

AI is doing my head in today telling me different numbers

Cheers

Vijay

When I did my MK2 I just reused the originals and have had no problems although I believe they are stretch bolts so should have been replaced. Think if it gives a torque and then a degree angle they are stretch. If its just a torque setting then they are not.

Alasdair

Hello bud,

Exactly as I've read, anything 1/4 torn are stretch. Ai tells me they are an extra turn but I'll try and look into it further. Someone on a fb page said he's always just reused them without a worry

Angle tightening is essential for fasteners that are taken into the plastic region, but far more often it is simply a far more accurate way of tensioning a(ny) bolt compared to torque alone.

The impact and variability of friction when tightening only by torque creates wide uncertainty in the achieved pre-tension.

Angle tightening is independent of friction because the bolt extension is directly related to thread pitch and the angle used.

Smaller torque plus angle gives a far more predictable outcome.

So it's just a better way of tightening important fasteners, not a guarantee that the bolt in question is permanently lengthened in each usage.

Cheers Pete.

What do you mean "fasteners that are taken into the plastic region"?

That's the technical term for causing a screw or bolt to stretch to the point that it doesn't return to original length when undone (elastic region is everything before that).

It's actually fairly easy to feel when (if ever) this happens.

You continue to turn the head but without any increase in effort like there normally is.

Weird feeling.

Rare on cars, even on headgasket bolts.

2 minutes ago, Breezy_Pete said:

That's the technical term for causing a screw or bolt to stretch to the point that it doesn't return to original length when undone (elastic region is everything before that).

It's actually fairly easy to feel when (if ever) this happens.

You continue to turn the head but without any increase in effort like there normally is.

Weird feeling.

Rare on cars, even on headgasket bolts.

As @Breezy_Pete says it's a term that refers to the properties of the material. Most materials are linear in that they stretch as a result of a load but when that load is removed they return to Thier original length. That's termed 'elastic' deformation and is a linear relationship that can be measured and is referred to as 'Youngs Modulus'. However they also have a point at the top of that linear relationship, called a 'yield point' and if they are loaded or stretched beyond that point they don't return to thier original length and the load remains constant and doesn't increase if lengthened further until fast fracture occurs and the material breaks. This characteristic beyond the Yield point but before fracture is termed 'plastic' deformation.

It's made use of in in stretch bolts to provide an accurate loading.

Usually there is a slight drop in strength below the Yield point that can be felt, so if you torque a bolt to it's Yield point and beyond you feel it almost 'give' a bit.

(Guess who's studied materials and fast fracture mechanics 🤣)

Edited 42 minutes ago42 min by skomaz