Cylinder Head Bolts with Angular Tightening

Posted by Ajusa: June 22th, 2018

The new technologies used in the engines, as well as the new angular tightening systems, make it essential replace the bolts every time the cylinder head gasket is replaced.



AJUSA introduced in 1993 the head bolt product line as an excellent complement to the production of cylinder head gaskets. As with its complete range of gaskets, AJUSA offers complete head bolt coverage for all engines requiring angular tightening. At present, it has more than 400 references of head bolt setw.


The structure of the head bolts in general is made of two well differentiated parts: the head and the stem.



Over the years, cylinder head bolts have been changing both in design and in properties to be able to adapt to the increasingly demanding needs of today’s engines. The main function of the cylinder head bolt is to apply the clamping force uniformly between the various parts of the engine involved, usually block and cylinder head. The bolts ensure correct gas and liquid tightness in the cylinder head gasket, which has to withstand the thermal variations, vibrations and changes in the engine.


Angular tightening

The cylinder head bolts always require an angular tightening, by kilos and by degrees. The main difference between angular tightening and that achieved only by kilos is that in the angular the elastic area of the material is exceeded and the bolts undergo permanent deformations. For this reason, it is important to know that they cannot be reused.

In the case of the bolt with tightening only by kilos, only the elastic zone is worked and the deformations are reversible when the force ceases. Therefore, they can be assembled again after checking their good condition. Of course, with the angular tightening we avoid retightening of the cylinder head bolts. 




Angular tightening characteristics

In the angular tightening we apply a small initial torque per kilos to the bolts, continuing with a tightening by degrees, so that all the bolts are tightened in a similar way almost excluding the friction of the final result, which results in a greater uniformity of the closure between the block and the cylinder head.

For proper operation of the screw, and therefore a suitable cylinder head closure, it is necessary to apply the tightening procedure as accurately as possible, paying special attention to the sequence, steps and tightenings specified by the manufacturer. AJUSA offers tightening sequence and specs in all the head bolts sets. 



Tightening torque

The cylinder head bolts (eg 1 - 10 on the 4 - cylinder engine, illustration) must be tightened in a precisely defined tightening order (see manufacturer ’s data). This order, like the tightening torque and tightening angles, is specified by Ajusa, and has been adapted to the design of the engine in each case. Each cylinder head gasket and each Ajusa gasket set are accompanied by specific tightening instructions for the engine.



 Bolt tightening is performed in several stages, which can be as follows:

• 1st stage 20 Nm (ie tighten bolts 1 - 10 with tightening torque of 20 Nm)

• 2nd stage 60 Nm (ie tighten bolts 1 - 10 with tightening torque of 60 Nm)

 • 3rd step 90 ° (ie tighten bolts 1 - 10 with 90 ° clamping angle)

• 4th stage 90 ° (ie re-tighten bolts 1 - 10 with 90 ° clamping angle) 

Each succession of tightening stages is based on the following rule: The different stages of bolt tightening always start at the center of the engine (between cylinder 2 and cylinder 3, see example) and continue in the form of a spiral or a cross towards Both sides, towards the outer screws of cil. 1 and cil. Four.


This ensures optimum positioning of the cylinder head and cylinder head gasket on the crankcase at all times.


Failure to tighten the bolt implies insufficient sealing forces and therefore loss of liquids and gases in the cylinder head gasket; While if it is excessive, the plastic area of the screw will be exceeded, reaching the breakage of the screw. Likewise, reusing angular tightening bolts can lead to insufficient closure in the cylinder head and even the bolt failure unexpectedly before the tightening is completed. During assembly, it is recommended to lubricate both the head and the threaded área. 


At AJUSA we have a test bench for bolts capable of checking characteristics and conformity of the cylinder head bolts. Each engine has certain requirements for the cylinder head bolts which must be met in order to guarantee the efficiency of the sealing system as a whole. For this reason, in AJUSA we thoroughly analyze for each type of screw the drawings, the homologation reports, as well as the corresponding chemical and dimensional certificates.

Measurements performed on a test bench for bolts as well as hardness tests prior to authorization ensure the required quality of our product. 

More information about our products: web AJUSA


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