CARSONS

Reconditioning Service for Motor Vehicle Engine Components

 

 

 

Reboring Block

Cylinder Block Reboring


The cylinder block comprises the main body of the engine. Due to the constant, reciprocating action of the pistons working rapidly and under load, the cylinders are subject to wear over time. The wear is not uniform and is greatest on the thrust side of the cylinders, near the top of the piston travel, eventually leading to the upper surfaces of the cylinder bores becoming non-circular in cross section. When wear and tear on the pistons and rings are added to this, the resultant excessive clearance will cause a loss of compression in the engine. The wear may not be readily apparent on inspecting visually but can be easily detected by inserting a feeler gauge between the pistons and cylinders; the rings will have to be removed to perform this test! The extent of wear can be measured accurately with an inside micrometer and the unevenness of wear can be clearly demonstrated with a dial gauge.

The poor seal between the pistons and cylinders will cause some of the fuel / air mixture to leak below past the piston rings, impairing engine performance and causing the unburnt fuel to contaminate and dilute the engine oil. Moreover, engine oil can get past the pistons and rings into the combustion chamber, fouling the spark plugs and resulting in consumption of oil that is emitted as grey smoke from the exhaust. When this happens, it is time to overhaul the engine; reboring the cylinders would be a basic part of this exercise.

A piston is not perfectly circular when cold but will assume that shape once it reaches the correct operating temperature and is under dynamic loading. Due to the considerable expansion that occurs as the engine warms up, sufficient clearance must be provided between the pistons and cylinders to allow the parts to work freely. Clearances must be checked while the engine is cold. Diesel engines require bigger clearances than petrol engines in respect of the bearings and pistons.

All pistons have machined into them channel shaped grooves to accommodate the piston rings that provide the actual gas-tight seal so essential for maintaining high compression. Two or three compression rings made of cast iron are usually fitted above the gudgeon pin and an oil control ring below it. The latter could be of cast iron or steel. The rings are supplied with the pistons and the gaps set correctly when they leave the factory so as to match the specified bore diameter.

All new engines, whether stationary or vehicle mounted, are shipped with standard size pistons. After reboring, the diameter of the cylinders will increase and so oversize pistons will have to be fitted. These are available in a range of sizes, varying from 0.25mm O/S to 1.5mm O/S. It makes sense to use the smallest possible O/S piston so as to leave sufficient material in the block to enable subsequent reboring operations. This operation is performed on a high precision machine designed specially for this work. It must be performed with great care by a skilled operator because of the need to adhere to tight tolerances. The difference between precision work and sloppy work is a few hundredths of a millimeter!

After reboring, the engine block is transferred to a honing machine to polish the slightly rough finish on the cylinders. What is desired is not a mirror finish but a micro textured surface with a cross hatch pattern that will retain a layer of oil to aid lubrication during the running-in period that has to follow an engine overhaul. The rings play an important part in controlling lubrication of the cylinder walls. Despite all this, it will take some time for the pistons and rings to seat properly on the cylinder walls to get perfect sealing, and until then some oil consumption will be unavoidable. When assembling, care must be taken to space out the gaps in the rings equally around each piston to minimize leakage of gases past them.

There is usually a limit to the number of reboring operations that can be performed on cylinder blocks because pistons are not made in sizes larger than 1.5mm O/S. It is possible though to give a new lease of life to an engine block by fitting sleeves into the bores. A resleeving operation involves removing extra material from the cylinder walls and pressing in tightly a thin walled sleeve by means of a hydraulic press. Original equipment sleeves can be used if available or else sleeves can be machined out on a lathe from cast iron material. Thereafter, it is quite feasible to rebore the sleeves multiple times, using progressively larger oversize pistons.

 

 



Cylinder Layouts


Some engines make use of wet liners as the cylinders. These are thick removable sleeves, with the coolant circulating directly in contact with their outer surfaces. Sealing is by means of the head gasket at the top and polymer 'O' rings at the bottom. Certain Diesel engines may be fitted with thin-walled dry liners that are a light push fit in the bores. These are usually hard chromium plated, with flanges that fit in recesses at the top of the block. In most engines though, whether petrol or Diesel, the cylinder is formed as an integral part of the engine block.

There are so many ways in which the cylinders of a multi-cylinder engine could be disposed. They are usually set up in an inline or 'V' pattern and sometimes in a horizontally opposed layout. The cylinders of old aircraft engines used to be arranged in a radial pattern. Shown below are a few common types of multi-cylinder engine blocks.

Due to the rapid motion of the rotating and reciprocating components, various modes of vibration can be set up in piston engines by the action of the unbalanced dynamic forces generated. Whereas the magnitude of the vibrations is basically dependent on the weights of the moving parts, it is also influenced significantly by the number and disposition of the cylinders. A horizontally opposed engine layout, such as the one shown at the bottom has the best balance, whereas in other types of engines such as certain V4 and V6 designs, the unbalanced forces can be so disturbing that one or two counter-rotating balance shafts may need to be deployed to keep vibration within acceptable limits.


  3 cyl block
     3 cylinder block

4 cyl block
  4 cylinder block

 

  5 cyl block

     5 cylinder block

 6 cyl block

      
6 cylinder block

 

    V6 Block

     V6 cylinder block

V8 Block

    V8 cylinder block

Boxer Engine
Horizontally opposed 4 cylinder layout

 




Other Engine Configurations


The so called VR type engines are an ingenious blend of Inline and V type designs that were first popularized by Volkswagen. These narrow angle V engines do not have separate banks of cylinders like traditional V engines. Instead, the cylinders are arranged in a staggered pattern of two closely spaced rows inclined with respect to each other, at an included angle of 15° or an even narrower angle. There is only a single cylinder head for both rows of cylinders. The advantages offered by this configuration are that they allow a multi-cylinder power unit to be designed very compactly and manufactured with significant cost savings.

VR6 Engine Block
A VR6 cylinder block


A 'W' or 'double V' type engine is made by casting as one unit a pair of narrow angle VR type engines joined together at the bottom with a wider included angle between the two sets of cylinders. This angle is usually 60 or 90 degrees. Some Audi models and the Bugatti Veyron come equipped with this type of engine. The former come with W12 engines and the latter with W16 engines.


W12 Engine Block

A W12 cylinder block

 

Engine Layouts

Comparison of Inline, V and VR engines

 

Motocycle Block

An air cooled 45° V2 motorcycle block


The engine blocks of most motorcycles and a few cars are cooled by the passage of air through fins, which by presenting a large surface area to the natural or forced airflow, facilitate efficient heat dissipation. While this results in a lighter construction it also proves to be noisier in operation, a trait common to all air cooled engines. There was a version of the Porsche Flat-6 engine that came with water cooled cylinder heads bolted onto air cooled cylinder blocks!

 

 

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