As a structural steel detailer, I have always used a figure of 91.9 Kn for the single shear value of a M20 bolt. This is in a plate of 10mm, the load can go down if you use a thinner plate , but will never be more than 91.9 Kn This is from the BS5950 tables that we use in the real world.
As for your description of a " preloaded " bolt, can I assume that you mean a High Strength Friction Grip bolt (ususally grade 10.9 with load indicating washer and hardened round washer) H.S.F.G
If so then the following text applies :-
The principle of HSFG bolted connections relies upon tensioning
each bolt in the connection to a specified minimum tension so that
the desired clamping force will be induced in the connection interface.
Shear loads are then transferred by frictional resistance at the
joint interface rather than by bearing on the bolt shanks and hole
faces. In this type of connection there will be no movement of the connected
materials when the connection is subjected to these loads.
Movement in many types of joints is highly undesirable, hence the
development of the friction grip connection.
The bolted faces (the faying surfaces) are left unpainted to allow the maximum frictional resistance. Load undicating washers that have tabs on are used and the tabs crush when the required torque is applied, although you ca often catch steel erectors hammering these down prior to installtion to save themselves work (although totally f**king up the principal of how these connections work) – although you never heard that from me !!
