What are fasteners

Usually, it is a bolt, used to link several components together in order to form a construction.
It can be made out of a variety of specified materials to meet many specifications. Common alloys and various super-alloys are made from steel, nickel, titanium or aluminium.
Which one to choose depends on the bolt geometry, fatigue-strength, tensile-strength, stress, corrosion, weight, conductivity, temperature and many other design specs.
There are only a hand full of serious manufacturers on the fasteners’ market, some earned the trust and became preferred suppliers for High-end racing series worldwide, such as Formula One, Moto GP and NASCAR.
The Starting Grid recommends the following well-known brands:
A-1 Fastener&Components (lisi group)
bat, Blanc Aero Technologies (lisi group)

There are multiple parts categorized as fasteners, each one with their own specifications and uses. Multiple parts are listed together with their respective uses and specifications.

Why cylinder head studs?

Using head bolts or head studs in an internal combustion engine can be an important decision that has an effect on the way an engine operates.  Both are fasteners with a similar function, nevertheless, there are a few differences: Bolts are usually partially or fully threaded.  Studs are typically threaded on both ends and often have an unthreaded middle section of the shaft. Studs do not have a head, where bolts always do.

Looking at the various benefits of studs and bolts, one must overthink following: The difference in engine assembly and disassembly. Torque pressure. Gasket alignmentOverall engine performance. 

Head Fastener Designs:
Cylinder head bolts are designed with stability and convenience in mind. Bolts can be cold-fabricated, to increase their integrity, they are often thermally treated before machining, giving relatively high strength and durability. Studs are carefully shaped to make them concentric, typically rolled to produce a straight, balanced component, either before or after heat treatment. A thermal treatment afterwards provides a higher mechanical strength and stress tolerance in the final product. 

Torque Efficiency:
During engine assembly or maintenance, a bolt must be installed by torquing it into place. Due to the head bolt’s design, it has to be rotated into the block creating both twisting force and a vertical clamping force, forcing the bolt to both stretch and twist. These two different forces reduce the capacity to secure the head and it forms a less reliable seal in high-powered engines. The head-stud can be tightened into place without any direct clamping force applied through the tightening. A stud can be threaded into the block. Afterwards, the cylinder head is installed and a nut is torqued into place. The nut torque provides the clamping force only, there is no rotational force at all. The pressure from the nut will make the stud stretch, only along the vertical axis without any twisting load, with a more accurate torque load as a result. This eventually leads to higher reliability and a lower chance of head gasket failure and therefore a more durable construction. 

Engine Assembly:
Another main difference between head bolts and head studs are how to put an engine together. Specially designed high-end head studs are able to securely positioning the head gasket and cylinders with almost perfect alignment. This feature makes it easier to assemble an engine using head studs.  However, head bolts are far more convenient for maintenance and part replacement of the vehicle. Many cars have master cylinders or other components that extend into the engine compartment. Rear wheel drive cars sometimes have engines blocks hidden under the firewall. Under these specifications, head bolts allow the cylinders to be removed sidewards without removing the entire engine from the car, as would be necessary with head studs. Some brands have an Allen key in the head stud, which not only allows you to install the stud with a tool but in some cases also the remove them if needed with the head still in its position.

Head bolts are more practical.
Head studs are better for high-performance engines when reliability and durability is required. 

One type of fastener isn't categorically superior to the other, the potential of an engine or the performance one strived for, often dictates the type of head fastener. A racing engine requires different engine fasteners the preference depends on the purpose of the engine your building. 
Do not forget to check the homologation! 

Conecting rod bolts

Designing Connecting bolts is a complex process.
The first step in connecting rod bolt design is to determine the load, or forces that it must carry.

The oscillating mass, which is the piston assembly plus a part of the connecting rod, times acceleration, equals the load, also known as oscillating force, this is a scientific rule.
The nature of crankshaft and connecting rod mechanisms’ motion, results in acceleration and
deceleration, which is at its maximum value when the piston is at top dead centre and bottom dead
This is proportional to the stroke and the square of the engine speed. 
(Piston Assembly Weight + 1/3 Connecting Rod weight) x Stroke x RPM2
This means that if the speed is doubled, the load has increased by a factor 4. 
This relationship is shown in the graph below for random rod and piston combination.