The cylinder head plays an essential role in the operation of the engine. Its correct position affects the operation of the gas distribution mechanism. Also, together with the cylinder block, it forms the combustion chambers. Therefore, when repairing, the correct tightening of the cylinder head is important.
The structure of the cylinder head
Every internal combustion engine, with the exception of the Stirling engine, has a cylinder head. It serves to accommodate the gas distribution system in it: valves, camshaft and its drive.
The cylinder head is an aluminum part with many holes for installing intake and exhaust valves, channels that form the engine cooling jacket. Depending on the number of cylinders and engine type, the cylinder head may have one or two camshafts.
Also in the head are channels for timing lubrication.
In the upper part there are holes for spark plugs in gasoline engines and holes for injectorsdiesel.
Consequences of incorrect assembly
The head is attached to the cylinder block with steel bolts or studs screwed into the block. It is known that steel and aluminum have different values of thermal expansion. When heated, aluminum expands more than steel, so this point must be taken into account when tightening the cylinder head. That is, if the head expands when the engine is heated, and the fasteners prevent expansion, then microcracks can form in the cylinder head. They will subsequently lead to failure of the entire engine.
For example, coolant can enter the combustion chamber through cracks. Or a crack in the channels of the lubrication system will cause the valve system to operate in the "oil starvation" mode. Also, failure to tighten the cylinder head leads to pinching of the camshaft and wear of its bed.
How to properly assemble the cylinder head
To assemble the head you will need:
- New gasket.
- Torque wrench with head. The key scale, showing the tightening torque, can be either in Newtons or in kilograms. Therefore, you need to remember that one kilogram is equal to approximately ten Newtons.
- Motor oil. It is necessary to lubricate the bolts before installation.
Before starting work, you need to check the new gasket for breaks, as well as the exact correspondence of the holes with those in the engine. Then, if the cylinder head is after repair, you need to check it for compliance adjacent to the enginesides. To do this, the head is applied to a flat cast-iron plate and see if there are any gaps between the plate and the adjacent surface.
After that, the gasket is laid on the upper plane of the cylinder block. The cylinder head is installed on top of it. Then you can start tightening the cylinder head. Before starting work, do not forget to lubricate the bolts with engine oil.
Cylinder head installation features
You can distinguish differences when working with engines that have a different number of cylinders. The tightening order of the cylinder head will also be different. But the principle is the same. It lies in the fact that the bolts are tightened from the center to the periphery of the head. This is necessary so that when tightening, the resulting stresses go away from the center beyond, and not vice versa. This will keep the seal tight and reduce metal stress during engine operation.
Tightening torque is controlled with a torque wrench.
It is a crank with a long flexible handle. On its handle there is an arrow that deviates at the moment of applying force. The arrow points to the scale, which is fixed motionless. There is also an electronic version of the torque wrench. It is more reliable and shows the force more accurately.
After installing the head, all the mounting bolts must be screwed into the holes of the block. Tighten them by hand as far as possible. You don't need to use the key right away. This may cause the boltwill not go along the thread, after which you will have to restore the thread on the cylinder block.
We work according to the scheme
Then, using the diagram, tighten the head. The most common four-cylinder engines have a similar layout. For example, consider the tightening procedure for the VAZ-2106 cylinder head. Bolts are tightened in two stages. The first stage is pre-tightening with a force of 35-41 N/m. Then the bolts are pulled in a second circle. The final tightening torque of the VAZ-2106 cylinder head is 105-115 N/m.
First, the two middle bolts are tightened, which are indicated by the numbers 1 and 2 in the diagram. Then the bolt to the left of the center and diagonally opposite, indicated by the numbers 3 and 5. After that, bolts 4 and 6 are tightened in mirror image. Then come the extreme bolts 7 and 8, behind them on the opposite edge 9 and 10.
Four-cylinder engine models have ten bolts. But not everyone will have the same tightening pattern. There are options when, after the two central bolts, the two lower or upper bolts are tightened in pairs, and not diagonally, as in the case above. It should be noted that the cylinder head tightening schemes for the VAZ-2106 and VAZ-2108 are different. Therefore, you first need to study the circuit for a specific engine.
Common mistakes when installing cylinder head
In the repair of equipment, one should never neglect the tightening torques of threaded connections. Tightening “by hand” or “by eye” leads either to insufficient or excessive force of fixing the part. If this is an assembly unit, suchlike a cylinder head, a careless approach will inevitably lead to breakage, and possibly more expensive repairs. Insufficient tightening will cause exhaust gases to blow through the cooling jacket. And excess can either lead to a violation of the threaded connection in the cylinder block, or it will clamp the camshafts excessively.
The next mistake could be using the wrong tool or bolts that are not designed for this connection. Usually these bolts have a specific fine-toothed thread. Therefore, if you replace them with others, you can damage the threads in the cylinder block. In addition, "native" bolts have high hardness due to heat treatment. This is done so that during operation the thread does not stretch and the gap between the head and the block does not increase.
Incorrectly selected tool can erase the edges of the mounting bolts and this will lead to insufficient tightening torque or problems with subsequent loosening.
Masters advise after some time after the repair to re-tighten the cylinder head. This is done with a run of 500 to 1500 km. During this time, the head of the block finds its optimal position, as a result of which the fixing bolts are weakened. Therefore, they need to be stretched again. This procedure does not take much time, but significantly reduces the risks associated with insufficient tightening torque.