Bore Honing

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Cylinder Bore Honing

 During the reboring operation a small amount of material is left in the cylinder bore for honing.  The honing operation achieves both the final surface finish and also the exact size required for the correct piston to cylinder clearance.  Removal of this material is achieved using abrasive stones mounted in a special carrier which is rotated within the cylinder while at the same time a pressure is exerted against the cylinder wall.

Correct sizing of the bore is important as exact clearances are needed to achieve quiet running without having the piston too tight that it might be prone to seizure.  Also, the final bore diameter determines the fitted ring gap which if too tight can also lead to seizure and subsequent engine failure but if too loose can result in excessive oil consumption and/or poor compression and loss of performance.

Correct surface finish is critical to the sealing properties of the piston rings and is extremely important in the engine rebuild process.  The boring operation leaves a surface which under a microscope would have a torn and fractured appearance.  The honing operation smooths out this roughness to create a “cross hatch” finish which aids both the ring seal and oil film retention.  When the honing head is rotated within the cylinder, whilst simultaneously being drawn up and down the length of the bore, a series of intersecting lines is created on the surface.  The relationship between speed of rotation and stroke determines the angle at which these lines intersect.  An angle of approximately 45 degrees is the optimum for piston ring performance.

A steeper angle encourages oil to drain down the bore resulting in too thin an oil film which can lead to ring and cylinder scufflng, whereas a flatter cross hatch angle will hold excess oil and cause thicker oil films which the piston rings will ride over causing oil consumption to be increased.

Cylinder roughness is also critical to the correct sealing of the piston rings.  To achieve the optimum finish for ring sealing the correct grade of abrasive stones and honing procedure must be used to produce the most ring friendly finish possible.

A ring friendly surface is a finish that provides good support for the rings, retains oil and does not require lengthy running-in.  The best way to achieve this is with plateau honing.

Plateau Honing

Even a newly honed cylinder will still have many microscopic peaks and troughs in its surface. The troughs are cut into the cylinder wall by the abrasives during the honing process.  The peaks are the high points which if left on the surface would make contact with the rings.  Large, sharp peaks will gradually be worn off by the rings as the engine runs.   As they are worn away, the rough points are flattened out creating a “plateau” effect. This action increases the surface area for the rings to run on and makes it smoother for them rings to run over the surface on a film of oil that is retained in the valleys.

Once an engine is run in and the rings have fully seated bore wear is virtually eliminated because the rings are then supported by a thin film of oil and as a result don’t actually make contact with the cylinder wall.

Piston rings eventually produce the optimum plateau finish during service so the nearer we can get to this during the honing procedure the less the rings and cylinders will wear during the running in process.  The correct plateau finish will enable the rings to form their seal more quickly and help them to last longer in service.

The ideal cylinder bore surface should therefore aim to replicate this.  Using the correct honing process a finish can be achieved which allows the rings to bed in quickly with minimal wear and which retains the correct amount of oil so that the rings are properly lubricated.

To reduce the formation of peaks on the surface, the bores are finish honed with stones that have a relatively fine grit size. The finer the grit size, the smoother the finish.

The final plateau effect is achieved using a flexible brush known as a flex hone.  This brush is used to help smooth the minute peaks remaining on the bore surface after the honing operation.   Flex Hones are available in various sizes to suit a wide range of bore diameters.

Following the honing operation the cylinder block is washed using our rotary machine which ensures that all abrasive material is removed from the cylinder walls prior to assembly.

Call us for expert advice and a quotation for your engine machining requirements.