 |
 |
 |
 |
 |
Cylinder head stretchbolts, no problem or . . .
It's an interesting question how to deal with cylinder head screws (bolts) and how to tighten them. I have some experiences that I would like to tell you about. I also like to use the word screw instead of bolt for the simple reason that screws have threads and bolts per definition may have threads, but they are then rather hefty things for big machine and building constructions and the like aimed to be used with nuts.
To ensure a long lasting sealed union between the cylinder head and engine block you must make sure that the force holding those two parts of the engine together is sufficient and stable over a long period of time. It's not advisable to solely rely on the method where you tighten the screws to a specified torque as this method has a great uncertainty built in. The friction may for instance differ a lot between a dry and a well oiled screw. When tightening a cylinder head screw it's not only the friction between the screw head and the washer that you have to overcome. Normally you have a long screw with threads deep in the cylinder block casting. A long screw in a corroded block makes a considerable resistance and several tenths of Nm torque may be needed just to rotate the screw even before it begins to pull the parts together. Another fact that makes the work difficult is that the quality and precision of available torque wrenches may vary a lot.
However a help on the way to a good and long lasting result can be found in the way the steel material behaves by itself. At load the steel material initially stretches in an elastic way (returning to its original shape when unloaded). When the load is increased, depending on the toughness, the material undergoes a plastic deformation until the limit of breach is reached and the screw snaps off. The trick is to make use of that elastic area of the deformation cycle in an optimal way.
When the screw is tightened the tension increases gradually and the material will first pass its elastic state until it finally reaches its plastic state. After that, further tightening will result in lengthening of the screw (the material is floating) causing a permanent deformation. In this state the required torque does not increase in any degree worth mentioning. Under practical circumstances it is impossible feel this unless the screws are oiled to a degree where the friction is close to zero. As this ideal state can never be reached in reality, the normal way is to state a certain angle to which the screw may be turned to reach the desired tension area, e.g. pass the stretch limit but not more. As a starting point for this recommended angle you normally take the reading when the screw is tightened to a position when it begins to pull or e.g. 20 Nm and after that 100 to 120 degrees further rotation. This method has of course been tried out in the car manufacturers laboratories. A description of the complete procedure can normally be found in the workshop manual.
The principle of this method can in a popular way be described as follows. If a screw is tightened, without passing the stretch limit (going into the elastic state), it acts like a rubber band that can compensate for changes in the material caused by temperature changes, compression of seals etc. To make full use of this property, one chooses to tighten the screw a little bit past the stretch limit. It is impossible to decide when you have reached this critical point by means of a torque wrench or your own feeling. The tightening must be done, not only with a certain force, but with an empirically defined angle as I mentioned earlier. The method is thoroughly tested and there is no reason to invest in an expensive torque wrench.
My own experiences from a number of nights in the garage are rather interesting. When I recently renovated a French light alloy engine I did this the 'normal' way. So when fitting the cylinder head I began to clean the cylinder head screws and treat them with a generous amount of oil. With great patience I turned the screws up and down in the cylinder block until they easily could be turned by the fingers. With gasket and head in place I prepared the screws with grease containing molybdenum disulphide. I tightened the screws with a torque wrench as I was used to until I felt it could be enough and read 85 Nm on the scale. After that I looked in the manual and found that the screws should be turned to a certain specified angle. I loosened some of the screws and tightened them again, this time to the recommended number of degrees according to the manual and at the same time I read the scale of the torque wrench. Here is the clue, believe it or not. I read a torque that was identical with the reading the first time. Could this be a coincidence or is it logical.
The oil manufacturers recommend that screws oiled with molybdenum lubricants should be tightened to a torque setting 20 % lower than normal value in order to compensate for the lower friction. This recommendation is given as there otherwise is a great risk of entering the range of tension that the material can not stand. In my case the result was lucky. I hit the right range of tension just for the reason that I used moly and at the same time I neglected the recommendation of a lower torque setting. This may not always be the case so I recommend you follow the manufacturers instructions in detail.
To me the various moly products has become very useful tool to achieve good results in a variety of works and especially on screw tightening. Treated with moly you can turn the screw in a smooth and well controlled way and the last but not the least important reason is that you can unscrew those bolts after a number of years with the same feeling as if the engine was assembled the other day. The screws are heavy to loosen just as they should be but they are gliding without any protest. To dismantle an engine after ten years is always an adventure that can be rather unpleasant. The screws may break and you are really in trouble. Many of you are surely acquainted with the sound and feeling of a cylinder head screw that comes loose with a sharp sound just before it breaks. Presumably the risk to damage the screws is higher when you try to loosen them rather than when you tighten them. The changes the screws undergo during the time they are mounted in the engine can be left out of consideration except for the fact that small leaks of water passing the head gasket into screw bores often create real problems with corroded threads on the screws as well as in the cylinder block.
The manuals recommend that screws for the cylinder head should not be used more than once. I have never worked according to this advice and surprisingly enough I have never had any problems that could be related to this misbehaviour. Of course there is always a slow creeping in the steel material as it is exposed to tension and heat. This will not be noticeable until you pass a heat region in excess of 500 degrees Celsius. The cylinder head screws will never reach that temperature so the risk for deformation is negligible. A routine to change to new screws is of course the right thing to do if you want to be sure of a good result. I will also stress the importance of screws that turns light and smooth. Use 'moly'! This is important not at the least if you have invested in new screws. They can then be tightened evenly with great precision to the recommended range.(No, I'm not a Molyslip dealer!) Good luck!