I used believe earlier, that mild steel was only suitable for building construction. Indeed, this was to distinguish mild- from tool steel, this by the way, is still valid today. However one thing is clear, not every screw must be made of tool steel. Something else that should be clear, is that in the field of construction, e.g., in bridge-building, this material must be used and not mild steel.
The outdated common description 'St', combined, e.g., with '37', is no longer in use. The 'St' has become 'S', which simply means steel in general. The number behind it indicates the upper yield point. This is the strain, when, if not observed, the screw will no longer return to its original form, i.e., it will remain deformed. The replacement is then obligatory.
There is possibly, not much space on the head of the screw. Therefore, the minimum specified tensile strength is shown before the point, divided by 100. This by the way, was the same with the format 'St 37', only here the dividing factor was '10'. Now, after the point, not the upper yield point is shown but it's relationship to the minimum specified tensile strength. Thus, '8.8' means 800 N/mm˛ of minimum specified tensile strength and an upper yield point of 640 N/mm˛ (8*80).
Sometimes, it may be a good idea to use the screws supplied by the manufacturer when replacing them, one should not fall short of, but also not substantially exceed the manufacturers specifications. Particularly as, e.g. in the case of stainless steel, the length and the choice of material must be considered. Only in the case of the special screw-heads used in assembly by robots, can one sometimes deviate from the standard.
It might be wise to have a look at what the screws are to be used for. Cylinder heads and con-rods may require waisted shank bolts. Here, the largest part of the screw has a distinctly smaller diameter, to be able to better absorb impact-like strain. In point of fact, compromising with safety relevant parts is always unwise.
Not every screw can be reused, even though the thread still seems to be in good shape. This is valid for the waisted shank bolt and for the multiple variations of the locking screw. The torque is a chapter of it's own. There are tables, in which the torque is given in relation to the diameter and the description on the head of the screw. This however, is only valid, when it is screwed into a suitably threaded hole.
When, e.g., a cylinder head is to be mounted using dowel-pins, these may also be set in aluminium, which increases the risk during the assembly. Fortunately, there are torque specifications for all new (and a lot of older) vehicles. The condition of the threading is also important. In extreme cases, the specified torque value may be reached before the screw has even reached it's seating.
This simply means, that for some tasks, it may be necessary to take things a bit slower, to have another good look at the job to be done. Having a good feeling about the final result is important. With this feeling, one can take the car out, e.g., for a test drive. The feeling of taking responsibility, this is actually a part of being a craftsman, despite having to rely on all the electronics. 12/13