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.