Torque To Yield Head Bolts
First service requirements on reconditioned engines, in chassis rebuilds and head gasket replacement.
Traditionally in the past all first engine services included a cylinder head retension. The reason behind this stipulation was very simple and straightforward. Cylinder head gaskets by their nature of construction change thickness due to clamping load changes that occur from a cold engine to a normal running temperature situation.
E.g. If the thickness of the unfitted gasket in a relaxed state measures 2mm. the thickness in its cold assembled state could be decreased to as low as 1.5mm. Once the engine is run up to normal operating temperature extra clamping load will be applied due to heat expansion of the head against the head bolts. After the gasket has been subject to this changed increase in clamping load for a number of times the relaxed state of the compressed gasket will measure considerably less than when first tensioned. This reduced thickness that has developed during the first 800km has now reduced the cold clamping pressure to a point that is unacceptable. To add to the condition we have to also consider a small amount of bolt stretch and recession……………….So the justification for a cylinder head retension has never been in doubt.
Slowly all future production engines will change from using head bolts that are tensioned to a predetermined torque setting to the use of “torque to yield” head bolts.
The reasons behind the change are:
Engine design has reduced the number of head bolts.
Engine design has increased the length of head bolts.
Higher performance outputs require higher clamping loads.
Lighter castings require more consistent clamping loads.
So torque to yield bolts and angled tightening achieve almost all the requirements for a modern engine.
A simple explanation of the difference between “torque bolts” and “torque to yield” head bolts.
Traditional head bolts are tensioned to a predetermined torque measured by a torque wrench. This torque reading is not an accurate measurement of the downward clamping load but rather an accumulated measurement of friction resistance between the two threads and the spot face and bolt head and the clamping pressure. With this system the accuracy and consistency of clamping loads are very unreliable. Each head bolt could have varied amounts of friction created due to differing contact surfaces, thread conditions and bolt contact surface finishes. This method often results in inconsistent and inadequate gasket clamping load.
These bolts are always tensioned up to specifications while still in the elastic phase of tension. (That is where the relaxed bolt still returns back to the original free length.)
As this allows the clamping load to decrease as the head gasket crushes during service a head re-tension at 800km. is always recommended. (alloy cold & cast iron hot.) Some manufacturers stipulate a head retention at every 20,000km. interval due to loss of clamping pressure during service.
Angle tensioning is even a good practice with head bolts that are tensioned within the elastic phase of tensioning. This will eliminate any shortfall caused by friction differences.
Angle tensioning uses a tension wrench to establish a snug torque position for all the head bolts. Snug torque is the term given to the torque applied to establish even direct contact between two components being assembled together. In our industry the head, the gasket and the block face. This position is established at a low torque of around 35ft.lbf. From this equal position, clamping loads can be established equally on every bolt by tightening each bolt a specified number of degrees. This is usually done in a series of steps. This method eliminates any influence on clamping load created by differing amounts of friction present at each head bolt.
This method like the first method reaches the required tension while the bolt is still in the elastic stage. (That is where the relaxed bolt still returns back to the original free length.) As also this allows the clamping load to decrease as the head gasket crushes during service a head re-tension at 800km. is again always recommended.
Now lets talk about the third method that does not look that much different from the angle tensioning method. This method is called tension to yield. (TTY) This method uses a low torque application to establish an equal snug torque position. As in angle tensioning. The main difference in the tension to yield method is the composition of the head bolt and the fact that the required clamping load is established when the bolt is in the plastic stage not the elastic stage.
This is the stage of tightening when you think the bolt is about to break or snap. The difference with this plastic stage is that at this point the bolt may not return to the original length when removed. In the plastic stage not a lot of extra clamping tension is reached but the bolt will tend to hold the required clamping tension even as the gasket crushes in service. This feature has allowed manufacturers to suggest that a head re-tension is not necessary.
If the bolts are replaced or within given specifications a first service head re-tension can be considered to keep the clamping load further into the plastic range of the bolt maintaining maximum clamping pressure for much longer.
Some manufacturers also insist upon TTY head bolts being replaced on every removal while others only require the used TTY bolt to meet a relaxed length specification. While other manufacturers insist upon replacement yet supply a relaxed length specification. A fair amount of confusion still exists in this area.
If in doubt replace is a safe method but be warned these bolts are not always available. The after-market parts suppliers have just upgraded their range in 2002 to a level that covers the most common applications.
Considering all of the above and after consulting some of the largest re-manufacturers in Australia our conclusions are that UMR ENGINES will remove the head re-tension as a requirement of the first service on engines with TTY head bolts. Every other UMR Engine application where the head bolts are tensioned in the elastic stage only will still require head re-tensioning.