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Cylinder head bolts are critical bolts used to fasten the cylinder head to the engine block.

Their role is therefore that of keeping the cylinder head pressed hard against the engine block with a gasket.

The engine combusts between this component and the piston, precisely inside the combustion chamber inside the crankcase.

In the four-stroke engine, combustion is the third phase, following intake and compression, and the last before exhaust.

Combustion occurs with every revolution of the engine, and the cylinder head bolts are fundamental for keeping the engine compressed in this phase.

Every combustion is a small explosion in the combustion chamber, so good operation of the cylinder head bolts allows the combustion to be confined to the combustion chamber.

Thanks to these fasteners, the cylinders stay sealed, preventing drops in pressure in the combustion chamber and seepage of combusted gases. In other words, the cylinder head bolts are downright cornerstones of the engine and in addition to connecting its parts, they allow it to reach the required performance, so a very high quality of execution is necessary.

Mechanical strength

Small range of tolerance

Surface coating

Defined and constant coefficient of friction

Strength class

Up to 15.9


Cylinder head bolts are critical bolts used to fasten the cylinder head to the engine block.
Thanks to these fasteners, the cylinders stay sealed, preventing drops in pressure in the combustion chamber and seepage of combusted gases.


Cylinder head bolts are equipped with a washer or flange. Partially threaded, they are somewhat long bolts with a diameter to length ratio greater than 10.

#Brugola OEB

The Brugola OEB cylinder head bolts are the result of many years of experience and utmost attention paid to the unique and specific needs of every customer. Every fastener that we produce comes up to the Total Quality and Zero Defects principles.

The cylinder head bolts are among the most stressed fasteners.

The main characteristics of Brugola OEB cylinder head bolts
The cylinder head bolts found in the engine usually number 2 x qty. cylinders + 2 and are tightened to torque + angle.
They can be either the internal wrench or external wrench type. They are never bolts with the head only – otherwise, they would risk eroding the engine – but instead, have a washer or a flange.
Partially threaded, they are rather long bolts with a diameter-to-length ratio greater than 10.
The length, diameter, and shape of the head usually vary based on the engine type, its architecture, and the dimensions contemplated.
In most cases, the cylinder head bolts are made of steel and have to be tightened onto the engine, which is instead made of aluminum.
This is why they need to be perfectly straight. A crooked cylinder head bolt would risk scraping against thealuminum of the engine block, damaging it.
Making a long and straight bolt is a critical step. What determines the difficulty is especially the diameter-to-length ratio.
Our production of cylinder head bolts goes all the way up to strength class 15.9. Depending on the strength class, the materials with which the cylinder head bolts can be made may change. This occurs, for example, starting from class 12.9 and up, where more superior materials in fewer quantities are used. According to the same principle, also heat treatment changes (as for class 10.9).

Since they are critical bolts, cylinder head bolts are among the most highly stressed fastening elements.

More specifically, they are subject to thermal problems.
As it heats up, the aluminum engine block expands, and so do the bolts, which however are mostly made of steel. The engine block and cylinder head bolts, therefore, have different expansions, meaning that they expand and shrink in different ways.
These characteristics lead the bolts to becoming loose. Up until about 20 years ago, it was easy to have to resort to controlling the tightening of the cylinder head bolts after the first 1000 km.
Today, technology has instead taken great leaps forward in tightening.
>By using better-performing materials and improved tightening strategies, cylinder head bolts no longer become loose, and it is unnecessary to resort to checks that were necessary for the past, despite the thermal expansions to which they are subject.
Finally, cylinder head bolts must not exceed in loads, under penalty of the cylinders ovalizing, resulting in engine malfunctioning.
This is why their mechanical strength characteristic must have a small range of tolerance and their surface coating must have well-defined and constant coefficients of friction. The friction windows are also critical parameters in which maximum precision is required.

Brugola OEB: decades of experience

Quality, accuracy, and knowledge of the sector allow us to present ourselves as experts on the side of our customers, advising them on the characteristics of the cylinder head bolts for their engines, data ready.

Our upstream consultancy service, therefore, saves time and work for customers, while ensuring that the cylinder head bolt produced is the best suited for their specific case.

Why choose
Brugola OEB cylinder head bolts

The Brugola OEB cylinder head bolts are the result of many years of experience and the utmost attention paid to the unique and specific needs of every customer.

Every fastener that we produce comes up to the Total Quality and Zero Defect principles.

Based on the configuration of the engine and needs, we treat the cylinder head bolts with different coatings.

For every situation we assess which lubricant or dilutions to use, how long the bolts are to be left in the solution, and at which speed to centrifuge them to remove the product. Depending on the products and processes used for the coating, basis characteristics are determined, such as the coefficient of friction or the corrosion resistance.

This aspect is an important part of the know-how of those who manufacture bolts, and Brugola OEB knows that.

For this, we accompany the customer with customised consultancy able to point out the most appropriate choices for the type of cylinder head bolt necessary to produce based on the application.

Our customised consultancy on the production of the cylinder head bolts starts immediately when we receive the customer’s specifications.

When we know a few parameters such as the basic requirements, type of load, the space where the bolt is to be mounted, and the cylinder head material, we perform a study based on which we design custom cylinder head bolts.

Furthermore, we can mount cylinder head bolts on a complete engine and run tests (for example, thermal tests or verification of the axial load by ultrasound) to understand how the bolt will behave even before it is produced.

In this way, the customer will own a record of the performance and capabilities of the bolt even before it goes into pre-series.

The software that simulates the real behaviour of a cylinder head bolt in a given application was developed and implemented by Brugola OEB.

To ensure the best quality, we check 100% of the bolts we make, not only the critical ones such as the cylinder head bolts. Using an ultrasound instrument, we are also able to check the loads on the customer’s engine.