Torque-To-Yield Head Bolts

As technology improves, more efficient methods are utilized in the design and construction of modern engines.  Many vehicle manufacturers now use torque to yield bolts, or commonly known as stretch bolts, in engine assembly. Torque to yield bolts have many advantages over conventional bolts.  The specialised method of tightening these bolts allows them to be more efficient at clamping.  Engine reliability is achieved because the assembly of the engine is far more accurate.  Manufacturers also prefer these types of bolts as fewer head bolts are required to achieve the desired clamping loads.  This leads to reductions in component costs and greater flexibility in cylinder head and block design.

Engine manufacturers which currently use torque to yield bolts are:

  • Chrysler
  • Daewoo
  • Ford
  • Holden
  • Hyundai
  • Isuzu
  • Lexus
  • Mazda
  • Mitsubishi
  • Nissan
  • Rover
  • Subaru
  • Toyota

The use of torque to yield bolts in engine manufacturing creates quite a disadvantage for engine repairers.  Conventional type bolts are tightened using a different procedure and therefore don't generally need replacing when rebuilding an engine unless they are damaged. (eg. stripped thread, bolt head rounded off or severely corroded or pitted).  However, due to design of torque to yield bolts, replacement is absolutely necessary when rebuilding the engine as they can never be reused.

When load is applied to any bolt, it undertakes four stages or phases.

A  Elastic Phase - Under tension, a bolt will stretch.  When this tension is removed, the bolt will return to its original length.
B  Yield Point - This is the defining point between the elastic phase and the plastic phase.
C  Plastic Phase - If we continue to apply tension to the bolt, the bolt becomes permanently stretched and cannot return to its original length when the tension is removed.
D  Shear Point - This the point at which the bolt cannot take anymore load or tension and breaks.


Conventional bolts are tightended in a series of stages, in a certain sequence, to a predetermined torque setting.  As each bolt type is unique, a different loading is applied and the torque wrench clicks off when the desired setting is reached.  This tension is well within the elastic phase of the bolt.  However, the torque setting produced is a combination of the tensioning of the bolt and the friction produced between the bolt and the engine surface.  There is no allowance made for friction resistance and therefore the process is inaccurate.  Only 10 - 15% of the tightening force produces tension as the remaining 85 - 90% is lost to friction.

Torque to yield bolts are also tightened through a series of stages, but the process is different as it includes a sequence of specified angles.  The process starts with a pre-load or snug torque which is a torque figure set to product a baseline from which the true load is applied.  The bolt is then tightened in angles to specified degrees.  This prevents surface contact which creates friction and therefore the load is applied directly to the tightening of the bolt.  Less friction resistance means that a greater load is able to be applied to the bolt and the bolt is tightened past the yield point into the elastic phase of the bolt.  The bolt is permanently stretched and should not be reused as retensioning a used bolt will bring it closer to its shear point.  Reusing torque to yield bolts is unreliable as they are prone to breaking and can cause improper clamping of the head gasket - both of which cause engine failure.

Galloway's engines are completely reconditioned to comply with original manufacturers specifications and therefore contain new torque to yield bolts where applicable.  Galloways also supply an extensive range of head bolt kits to compliment our range of exchange and new heads.

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Frequently Asked Questions

How do I know if my engine needs reconditioning?

Tell-tale signs that your engine requires attention are that it has lost performance or efficiency, it blows excessive smoke or it has developed an unusual noise.  However, a vehicle needn't have these symptoms to benefit from a reconditoned engine.  A complete and comprehensive recondition of your engine will increase power, fuel efficiency and extend the life of your vehicle.

Do you remove and refit engines?

Galloway Engines is a precision workshop which deals in machining and reconditioning engines.  This ensures we are specialists in our field and only a quality job of the highest standard will be delivered.  We can arrange engine removal and fitment or contact your regular vehicle servicing centre for this service.

Can I supply my own parts?

Yes. You are quite welcome to supply your own new parts should you have a preference for a particular brand name.  However, our parts prices are very competitive and our high standards ensure that only quality parts are fitted to any job.  We have liaisons with many Australian and international companies allowing us to have access to world-wide sourcing of parts.

Do you offer a warranty?

Any reputable product will come with a warranty as proof of its quality.  Galloway Engines are no exception.  We offer a full warranty on all of our work.  Contact us for more information on the terms and conditions of our warranty.

Why should I use Galloway Engines?

Galloway Engines supply a quality engine which is completely reconditioned to comply with original manufacturers specifications and are in accordance with Australian Standards AS4182 for petrol engines and AS4427 for diesel engines.  Our up-to-date equipment produces quality precision work and our team of specialised technicians are dedicated to detail.  This ensures you are getting the best job possible.

Can you recondition any engine?

Yes. Whether it be a Caterpillar engine from a giant earthmoving machine or a small three cylinder Suzuki engine, we have the equipment to be able to meet your needs.  Contact us to see how we can meet your requirements.

Do you offer engines which suit LPG?

As LPG has different properties to unleaded fuel, an engine can experience problems when converted to LPG.  We can offer options which can enhance the performance and reliability of an LPG engine.  Galloway Engine Reconditioning also stock a range of exchange heads which are modified to run on LPG.  

Do you have EFTPOS?

We have full EFTPOS facilities available and are able to accept payment via Credit or Debit Card.


  • Address:

    Galloway Engine Reconditioning,
    25 Baker St,
    Pinjarra WA 6208
  • Phone / Fax:

    08 9531 1366
    08 9531 2321
  • Email: