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Cylinder Block Boring

When do I need a Re-bore?

Cylinder bores will require re-boring if they become damaged or worn to the point where the piston rings will not be able to achieve a good seal.


Very often, this is purely as a result of wear in the bore after considerable mileages have been covered although since the advent of modern oils with additives which prevent such wear this is a less common problem.  For older engines which have previously been run on less efficient oils a wear ridge at the top of the bore just at the point where the top compression ring is at its maximum travel is a common indicator.  This is often more pronounced on engines which feature a longer piston stroke as this design transmits more side thrust forces from the piston to the bores.


More common in today’s engines is damage due to some external force or failure.  This can include seizure caused by excessive heat often resulting from coolant loss, scoring from contaminants entering the bore and damage resulting from the piston itself failing in service.


Any cylinder being re-bored will require over-size pistons to be fitted.


How is it done? 

Cylinder Blocks which require boring are set up on our BERCO AC650M vertical boring and re-facing machine.   The block will be mounted on the machine using the sump pan face as a datum so this is first cleaned to remove any debris or burrs which might affect the accuracy of the set up.  Depending on the working height of the piece, the block is either placed directly onto the machine bed or on to parallels before being fixed in place using special quick release clamps or alternatively machine clamps which fit into the “T” slots in the machine bed.


Once fixed, the block can be positioned with the first of the bores below the boring column.   Once in position a special stylus is fitted and the column lowered into the first bore.  The stylus is permitted to touch the bore wall and in turn is connected to a dial indicator gauge at the top of the boring column.  The stylus can be rotated within the bore and the concentricity in relation to the bore diameter shown on the gauge.  Using hand wheels, the machine bed can be moved both laterally and longitudinally and using this facility the block position can be adjusted so that the boring column is exactly concentric with the cylinder bore itself.  Once in position, the machine bed can be locked down to prevent any movement during the machining operation.  The boring head is retracted and the centering stylus removed.


Before the boring operation can begin the final size of the finished bore must be determined.


Sometimes specifications for this are found in technical manuals supplied by piston manufacturers but more commonly today we use a method whereby the piston running clearance is used to determine the finished bore size.  To achieve this the piston itself must first be measured before the relevant clearance is added to calculate the final size.  Both piston and vehicle manufacturers will supply information as to the optimum running clearance for any given application which means that using this method precise and accurate clearances can be achieved resulting in quieter and better performing engines.


As part of this calculation an allowance is made to machine the bore slightly undersize which means that some material can still be removed during the final honing operation without exceeding the optimum measurement.


Piston to cylinder clearances are critical.  If they are too tight the engine will fail prematurely through seizure or other related problems but if they are too loose the engine can be noisy and suffer piston ring related issues.  These can cause smoking and loss of compression which are both symptoms of excessive ring gaps and are directly linked to the bore diameter.


Once the size of the bore is determined, the correct cutting tip is selected and fixed into the tool holder.  The tool is set for size using a dedicated micrometer.  Once set, the tool is inserted into the boring column and locked in position.  The boring column is again lowered to within a millimeter or so of the block deck and the machine safety guards closed.


The block is now ready to be bored and the automatic phase of the operation can begin.  The machine has options for several different speed and feed rates to be used and the most appropriate one for the job in hand is selected.  The boring head motor is started and the automatic feed engaged so that the cutter rotates whilst simultaneously travelling through the bore.  At the end of the cut, a safety switch can be set to automatically end the cutting cycle which stops the tool head rotation and feed rate simultaneously.  The cutting tool can then be withdrawn from the bore using the rapid column feed lever.


When roughing out stock material (before fitting cylinder liners or large oversize pistons for example) large cuts can be made however in every case, the final cut prior to the honing operation will be of no more than 0.5mm.  This ensures that the final finish is not too rough for the hone to achieve the correct surface finish with the minimum amount of material removal.


Once the last cut has been made the size is checked using an internal bore gauge to ensure that the final cut has achieved correct dimensions with no significant ovality or taper.


Once bored, the cylinder block can be refaced on the BERCO AC650M without the need for a different set up or transferred to our dedicated BERCO STC361 head and block re-facing machine.


Cylinder Bore Geometry

Accurate cylinder bores are essential for proper piston ring sealing which prevents problems including smoking and high oil consumption.  This is especially critical on later engines equipped with oxygen sensors and catalytic converters which can be damaged in such conditions.


When at operating temperature, each bore should be as round as possible with little or no ovality in diameter or vertical variation (taper).


Bore distortion prevents the piston rings from conforming to the cylinder walls.  This allows oil to pass through into the combustion chamber where it will be burned resulting in a smoking exhaust, poor emissions and high oil consumption.


Compression gasses are also able to escape past the rings and into the sump causing crankcase pressure to build up which can lead to problems with oil seal failure.


Correct cylinder boring and honing procedures produce bores which are correctly sized and finished and which will eliminate all of these issues resulting in engines which are clean running, powerful and efficient.


At H T Howard & Co we have the machinery and technical knowledge to ensure that your cylinder block is re-bored to exact specifications for reliable performance and long service.

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