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Critical fasteners’ bolts are fastening components used to mount the engine’s vital parts. They are subject to high performance, unusual stresses such as thermal expansions, or critical application problems.

They are called “critical” because attaining the engine’s performance depends on them.
This means that no errors are allowed in the making and tightening of critical fasteners.
The malfunctioning of even just one of these bolts would cause significant engine damage.

Critical fastener bolts are not to be confused with safety bolts, such as wheel bolts and nuts.

The making of these is, in fact, strictly tied to the safety of those riding in the car. Their malfunctioning would first of all create an inconvenience or an actual danger for drivers and passengers.

Critical bolts, on the other hand, are linked to the operation and the performance of the engine. They must perfectly fit in with their components, in this way becoming an integral part of the engine and guaranteeing the dynamic structural integrity of the engine itself with the axial force they supply.

Type of critical fasteners

Out of about 70 types of fastening elements in a car, the critical bolts make up a highly limited set. To be precise, 7 screws have critical characteristics, which therefore are part of this group of fasteners:

Limiting weight is essential when an engine has to reach high levels.

The main characteristics of critical bolts


As specified, the characteristic that the 7 critical fastener bolts of the engine have in common is precisely the “critical nature” of the role they play.
In addition to this, each one has its own peculiar characteristics, depending on its position in the engine and its specific task.Making critical fastener bolts is nevertheless an undertaking that involves custom design capability, since besides the general characteristics that these fasteners may have, every manufacturer has its own particular requirements and precise performance to achieve, which are different case by case.
One practical example is this: there are critical fastener bolts  in both economy cars and racing cars, but the two vehicle categories are very different in terms of performance!
In this regard, limiting the overall weight of the bolts in a car, including the critical fasteners, becomes essential when an engine has to reach high levels.
This necessity leads to making choices of compromise concerning dimensions and strength class of the critical fastener bolts, at times removing material from the fasteners without, however, impairing the engine’s performance.
Going into the details of the critical bolts, the cylinder head bolts are a cornerstone of the engine, as they fasten the head to the engine block and have to respond to considerable thermal expansions.
The main bearing cap bolts keep the engine main bearing secure, and are not subject to the same thermal stresses, but are tightened twice, so it is necessary to pay attention to their dimensioning.
The con rod bolts tighten the con rod caps and are one of the critical bolts of the engine most highly stressed.
In addition to the same thermal expansions of the cylinder head bolts, they are subjected to forces in three dimensions and are, for all practical purposes, moving components.
The flywheel bolts, on the other hand, transmit the torque from the crankshaft to the gearbox and have to respond to important space limitations that highly influence their design.
The camshaft cap bolts tighten the camshaft caps and must retain their precise mechanical and strength characteristics for the entire lifetime of the engine.
The crankshaft bolts instead tighten the crankshaft.
They are critical fasteners because they must always ensure perfect synchrony between the components that they fasten.
The  camshaft sprocket and VCT bolts also must retain the exact timing between the gears or phase transformers and the crankshaft.
However, they are somewhat short elements, so they have to be treated with micro-encapsulated bonding agents to perform perfectly.

The role of the field material

Depending on the type, and also on the manufacturer’s will, the critical fasteners can work in the plastic field or the elastic field.
If they are tightened in the plastic field, they are subjected to a “stretching” that exceeds the elastic load of the bolt, bringing about a final and irreversible plastic deformation.

This usually ends up in the bolt becoming longer than its initial conformation, so it is necessary to take it into account during the design stage.
If they are tightened in the elastic field, the bolts will instead always return to their original conformation, despite the stresses to which they are subjected.

Why choose
Brugola OEB critical bolts

Critical fasteners’ bolts are Brugola OEB’s true core business.

These special components are developed in close collaboration with the customers and scaled to the characteristic specifications of their engines.

Consulting begins with the specifications: depending on the dimensions, materials, and all the other data indicated, we design and make custom critical bolts according to our Total Quality and Zero Defects objectives.

The care and precision that we put into making each one of our bolts is indeed maximum.

During the preparation of the wire, we don’t use the classic chemical pickling to clean and lubricate it, instead, we use a sand-blasting process. In this way, we avoid the accumulation of hydrogen in the steel and the resulting embrittlement of the bolt starting from the production stage.

Thanks to the know-how fine-tuned over years of experience, our critical fasteners’ bolts in this way arrive at the end of the process containing no hydrogen, which is a huge advantage even by the crucial role they will play.

Depending on the application requirements, we use different types of coating.

We assess the products to be used and how to make the coating in order to achieve the desired result case by case.

It is precise because of the coating that the critical bolts achieve characteristics such as the friction coefficient or corrosion resistance, which are different based on the precise tasks of the bolt and the customer’s requirements.

Lastly, we test 100% of the critical fasteners’ bolts that we produce.

Using a software program developed in-house, we can carry out simulations on the behaviour of the bolt even before it is produced.

Having certain and reliable data on how the screws will react to stresses, expansions, forces and the type of tightening to which they will be subjected ahead of time is a precious aid for the customer.

In this way, the customer will already have precious information during the preliminary phase and can choose to send only the critical fasteners’ bolts that best meet its requirements into production.