Stretch Bolts

February 28, 2022, 09:01:56 AM

There is some debate, mainly due to me, regarding stretch bolts and if they are used on a MK2 cylinder head. Rather than mess up somebody else's thread I have started this one so we can debate stretch bolts and decide if indeed those MK2 head bolts are stretch type.

Audi recommends that almost every bolt removed is replaced with new. This resulted in a belief that Audi bolts were not reusable because they are stretch bolts. After the discussion on the subject I posted this summary for A2 owners:

Order new fasteners or reuse? The Audi manual states that all bolts, screws and nuts, relating to this job, are use once only. It is a good idea to order and collect replacement fasteners before starting work to avoid any risk of work being delayed parts on back-order. However if you are delayed by a fastener back-order or unsure of the need to to replace all fasteners then a summary of an A2OC group discussion on this subject follows:

It is important to replace all self-locking nuts, however some A2OC members suggest that the bolts are not Torque to Yield bolts, more commonly known as stretch bolts, and are therefore safe to reuse. Stretch bolts must not be reused, all bolts stretch but a stretch bolt is stretched beyond the elastic zone into the plastic zone.

The advantage provided is that the stress strain curve is much flatter in the plastic region so the clamping force is better controlled. The clamping force is set by a characteristic that is manufactured into the bolt; it is not variable depending on the friction felt by a torque wrench. The friction felt by a torque wrench is dependent on the condition and lubrication of the threads and so can be quite variable. Finally a stretch bolt provides a greater clamping force for a given bolt size. Stretch bolts when removed cannot return to their original length and strength and therefore must not be reused. If you have any doubts replace your fasteners as advised in the manual. For others I summarise a debate regarding stretch bolts here on A2OC.

A standard bolt stretched within the elastic zone generally has a single torque figure specified, ie 55Nm. The torque wrench can only respond to friction felt which in turn will be influenced by cleanliness of the threads, lubrication etc. The actual amount of bolt stretch achieved therefore can be variable along with the associated clamping force.

If tighter tolerances are required for the clamping force required from a standard bolt stretched within the elastic region then a lower initial torque followed by an amount of turn can be specified, ie 30 Nm + 90 degrees. A portion of the stretch is determined accurately by an angle of turn after a possibly less accurate torque wrench setting. As the stretch is tighter controlled overall then so too is the clamping force. A torque wrench setting followed by an angle of turn does not necessarily confirm a bolt is a stretch bolt. A screw is unlikely to be a stretch fastener because there is no thread free portion where necking can occur.

A torque wrench only setting cannot be specified for a stretch bolt. A torque wrench that takes a bolt to the point of necking will never click. It will continue stretching the bolt to the point of failure. Torque specifications for stretch bolts specify and initial clamping force followed by an angle of turn. A stretch bolt can generally be identified by a slightly reduced diameter unthreaded portion of its length between the head and the thread. This is a head bolt set supplied by Elring:

In the future I will be making a judgment based on this information and only replacing bolts that are likely to be stretch bolts.

That was my conclusion regarding reusing Audi bolts, however the Elring set of bolts above looks a lot like the Elring MK2 bolts that I fit to my Golf. I don't however recall the reduced diameter where yielding occurs. Thoughts?

I'll try and find the original illustrations and post better copies