Why are cylinder head bolts loosened and tightened in a specified sequence?

Why do cylinder head bolts need to be tightened in specific sequence?

Tightening a bolt stretches it, while pulling the head and block together. These bolts elastically deform and return to their original dimensions when loosened. Friction between the bolt head, its washer, and the cylinder head isn’t consistent.

What is the sequence of cylinder head bolt tightening?

Normally you would tighten conventional head bolts in a crisscross pattern in three steps, to one-third, two-thirds, and then to the final torque. Stretch bolts are tightened in a torque-plus-angle sequence.

Why is uniform tightening of cylinder head bolts important?

The bolt tightening force is an important factor for the mutual sealing of combustion chamber, lubricant and coolant channels, and the ambient air. … When tightening (torquing) the cylinder-head bolts, the bolt force generates the so-called surface pressure, which is essential for achieving a perfect seal.

When tightening the cylinder head the bolts installing sequence usually begin?

Each bolt tightening procedure always begins in the middle of the engine (between cylinder 2 and cylinder 3 – see example), moving in a spiral or crosswise direction outwards along both sides until the outer bolts on cylinder 1 and cylin- der 4 have been tightened.

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Why are cylinder head bolts tightened to a specific torque?

Cylinder heads must be tightened to specific torque loads. … Too much torque can cause bolts to become loaded past their limits when cylinder pressures in the engine rise, resulting in catastrophic failure of the bolts and thus the seal between the cylinder head and the engine block.

What is the torque sequence for head bolts?

Torque sequence refers ro the pattern in which you tighten the bolts. The overall goal is to spread the torque evenly as you are tightening the bolts. Usually, you start at one bolt, and you work your way around in a circular pattern. Doing so ensures that your head won’t be warped during the tightening sequence.

What is Torque sequence?

Bolt tightening sequence, or torque sequence, is defined in the torque tightening procedure. Flange joints are torqued tight with the help of a torque wrench (manual or hydraulic). … Torque bolts and nuts in a “CRISS-CROSS” sequence using a minimum of three torquing passes and the maximum bolt stress as defined.

What is cylinder head bolts?

Cylinder-head bolts are the connecting elements between engine block, cylinder-head gasket, and cylinder head. The bolt tightening force is an important factor for the mutual sealing of combustion chamber, lubricant and coolant channels, and the ambient air.

What are cylinder head bolts made of?

The cylinder head engine bolts used in automotive. Automotive fasteners are a collection of products that are used in automotive and vehicle assembly. They are usually designed for specific makes/models of vehicles. The fasteners are made of SCM435 with heat treating to reaches grade 10.9/12.9 and then phosphate.

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Why should aluminum heads be tightened only when cold?

Aluminum heads should only be torqued when cold. Iron heads can be re-torqued when hot. It seems like the head bolts may expand in length when warm more than the iron head. Otherwise the head gasket may settle some when it gets warm.

Why do head bolts break?

Fatigue failure happens when the bolts have not been tightened properly, or have loosened up during its service life. If enough force is acting on the loosened joint during use of the product, bending stresses can weaken the fastener, eventually causing it to fail.

What causes head lift?

Causes of Cylinder Head Lift

Cylinder head lift is caused by excessive cylinder pressure, which can result from several things, including detonation and/or pre-ignition; improper fuel type; over-advanced ignition timing; improper air-fuel ratio; vacuum leaks; malfunctioning sensors; and increased boost pressure.