I read through the cylinder head stud discussion and everyone seemed to miss the issue of bore distortion.
Most modern water cooled engines use aluminum blocks and heads. The head is supported by the cylinder AND the block walls. You can see how this works in the water cooled Porsche engine in the photo below. The cylinders are free standing (open deck), but there are also block walls to support the cylinder head. Further, being water cooled, the block, heads, cylinder and bolts are all maintained at nearly the same temperature by the cooling fluid.
Our immortal beloved air cooled engine on the other hand carries the full clamp load on the cylinders.
Additionally, the distribution of the cooling air is such that the side facing the fan (intake side) gets more direct airflow than the exhaust side, so the temperatures are different across the various components.
A water cooled engine can handle very high clamp load because of the block support. It uses a multi-layer steel head gasket and torque to yield head bolts (not reusable).
The classic air cooled engine has only the cylinder to support the clamp load. It occasionally uses some type of sealing ring/gasket, and the head studs are reusable.
These differences change the requirements for the head fasteners.
The diagram below shows a typical stress-strain curve. Stress is proportional to tension, and strain is proportional to stretch. Young’s modulus is a measure of elastic behavior. In this region the fastener will return to its original length when un-torqued. The yield point is where the fastener takes a 0.2% stretch. The ultimate tensile strength (UTS) is the maximum tension that the fastener can maintain. After this point, the fastener will keep stretching and stretching, it will noticeably “neck down” and finally break. If you have ever over-torqued a bolt, this is where it starts to get easier and easier to torque, and then breaks.
For the water cooled engine, the structure supports enough clamping force that the head is always clamped firmly to the block and cylinder. It can never be allowed to separate under any condition. If it does, the coolant leaks out. To get the most out of the fastener the torque turn (torque to yield) method is used. This method takes the stress (tensile load) to the blue dot. You have to be careful though. If you go over the UTS it will fail. For this reason, the final torque is given as a number of degrees of turn in order to eliminate the uncertainty of thread and bolt or nut friction associated with using torque. Because we have moved out of the elastic region (over the yield strength point) the bolt is permanently stretched and cannot be reused.
Water cooled engine cylinders deform under from the clamping load. One method of correcting this deformation is to finish the cylinders with a simulated clamp load (torque plate).
Our air cooled engine, with only the cylinder as the structure also can’t take that kind of load without deforming. We have to be careful with how much load we apply to the cylinder in order to keep it from bending out of shape. The sketch below shows in a greatly exaggerated way how our cylinders deform when clamped. The point is, we can’t just increase our load higher and higher without consequences.
To deal with this problem Porsche only tightened the fastener in the elastic range (red dot shown above, below the yield strength point). The expansion of the cylinder supplies the rest of the clamping load, though the stud still remains in the elastic range. This is not a perfect arrangement, as the cylinder at a lower temperature has less clamping load than at a higher temperature. This is where the various sealing concepts came into play, in an effort to stop oil leakage and combustion gas leakage when the cylinders are not very hot
This whole scheme first fell apart with the magnesium block. On shutdown, the steel studs do not expand as much as the aluminum cylinders. The expansion of the cylinders added too much pulling force on the hot magnesium case and the studs gradually pulled out.
The engine bore size was also getting bigger and bigger. This caused the cylinder walls to get thinner and thinner. At 95mm the bores were getting too deformed with steel studs.
The solution to both of these problems was to find a material for the fastener with a thermal expansion coefficient that more closely matched aluminum.
Dilavar fit that requirment reaspnably well, but had its famous problems of stress corrosion failure. Various coatings were tried, with uneven results. Finally, they decided to roll the entire surface of the stud, and used a rolled thread form. Here is a great animation of that process.
https://youtu.be/rwArBBcUNr4
Rolling the thread on the surface crushes all the exposed grain boundaries closed, which prevents salt from getting into the grain structure and corroding the alloy.
This has proven to be outstandingly successful. Now you can have the benefit of low cylinder distortion, and reliability, even at high power levels.