How to Rebore Our Motorcycle Cylinder Barrels

There's a whole lot more to reboring a barrel than simply sticking a cutter down it... With use, there inevitably comes a time when our motorcycles' performance causes concern. It rattles when cold and smokes when hot, you lift the cylinder head off hoping to find the valve guides worn and, maybe at worst, the piston requires a new set of rings but a big lip around the top of the barrel confirms a rebore is necessary. Of course you also have owners who want to rebore the cylinder to enhance performance.

In principal, reboring your engine cylinder barrel and fitting a new, oversize piston is straightforward provided the engineer performing the work is competent and has knowledge regarding the type of engine needed to reborn. Older air-cooled engines, and modern, liquid-cooled engines run tighter piston clearances and finer micron bore finishes that are totally incompatible with another.

When your motorcycle engine was designed, the manufacturer went to great lengths to provide the correct cylinder bore material and surface finish. Consideration was also given to piston design and choice of material that would achieve the optimum balance between performance and longevity. They also had to establish a suitable piston skirt clearance based on the machine's intended use. This essential clearance prevents the engine from seizing.

Touring motorcycle engines normally run cooler with specially designed pistons requiring less clearance than sports and racing engines, although this is not always the case as older ait-cooled trials engines can run as hot as road race engines due to clogged fins and poor air circulation. New pistons are supplied with the necessary technical data to enable the engineer to establish the correct size for the new oversize cylinder bore. This is usually in the form of a specified bore size or a physical skirt clearance. E.g. if the piston measures 76.09mm and the recommended skirt clearance is 0.1016mm the finished bore size would be 76.2mm. It's difficult to quantify when a rebore is required as some older engines can run quite satisfactorily with excessive skirt clearances, but in general if the skirt clearance is twice the recommended figure it's time to consider a rebore.
Before measuring the piston skirt, it's advisable to understand the importance of basic piston geometry. First a piston is not round, but oval. This ovality is required to accommodate thermal expansion of the piston and take care of deflection caused by side loading. Secondly, the piston is not parallel sided but tapered with a barrel form. The piston has a greater diameter at the bottom and becomes progressively smaller towards the crown to accommodate the localized head from combustion. The piston skirt clearance should be measured at the bottom of the skirt and at right angles to the gudgeon pin, however, some piston manufacturers recommend taking the measurement at right angles to the pin but specify a given distance below the oil control ring, if so this procedure must be adhered to. More information regarding pistons can be found here Pistons from your Motorcycle Engine

When reboring the cylinder and fittin genuine, replacement, oversize pistons the skirt clearance quoted by the manufacturer will be satisfactory and no significant gain will be achieved by experimentation. When fitting non-standard, after market or high performance pistons, their design configuration and material specifications affect the way in which the piston expands and some forging alloys expand at a greater rate than casting alloy and vice versa. It is therefore advisable to use th piston manufacturer's recommended clearances in preference to the engine maker's figures.

Some pistons are supplied with the recommended cylinder size shown on the box or listed on the technical data sheet. This is the minimum permissible diameter to which the cylinder should be rebored in conjunction with the piston provided. As regards actual clearances, conditions vary so greatly that it is virtually impossible to quote general clearances.

Rebore Procedure

Cleanliness is a critical aspect of reboring. Before sending your engine cylinder barrel for reboring ensure they are thoroughly clean and free from burrs and that all traces of old gaskets are removed from the faces. A piece of old gasket on the face could mean the correct cylinder axis is compromised. Commercial engine re-conditioners often bore off the largest face, which is usually the cylinder head face. This is bad practice as many motorcycle cylinder barrels have been machines to improve performance or correct warping and any discrepancy between the two faces will mean the newly rebored cylinder axis is not at right angles to the bottom flange. Boring from the bottom face ensures correct axial alignment. One very important aspect is that the boring machine is free from any defects that would compromise the roundness and parallelism of the finished bore.

Cylinder Bore Finish

Quoted and measured in microns (1/1000mm) there are literally hundreds of surface finish parameters, which can be calculated from a cylinder bore trace. From this it's possible to calculate the volume of oil retained, the percentage of contact area available and many other factors. This is a very complicated subject, most of which can't be effectively measured in the average rebore workshop. The general practice is to rebore the cylinder 0.0254mm under the finished size and then produce the required surface finish and bore size with a cylinder honing device.

Cylinder Honing

An abrasive machining process that produces a precision surface on the cylinder bore. It is used primarily to improve the geometric form of the surface, the honing head normally consists of four stones that are held under controlled pressure and feed longitudinally up and down the cylinder bore. It is very difficult to assess the honing process. The three essential components that specified are characterized as the honing angle, depth, surface finish or surface roughness. The honing angle and depth are normally visually assessed. There should be a constant angle between the cross hatch lines over the whole cylinder surface. The angle should be in the region of 35 to 45 degrees. This will give excellent oil retention. Angles greater than 45 degrees will reduce oil retention and promote cylinder wear.

The honing depth should be even for both directions of cut, one set of honing lines are cut as the hone passes down the bore the other as the hone rises in the bore. To achieve the correct cross hatch effect, the hone must rise and fall at the same rotational speed and feed rate. The correct surface finish is achieved by selecting the appropriate grit stones. Stones from 180 grit to 240 grit being the ideal compromise between surface finish and metal removal rate. Flexi plateau hones are also used to finish bores, which provide the ideal surface finish for the new piston and rings.

It provides a good bearing area to support the rings but also has enough cross hatch section to retain oil and provide good lubrication. Cylinder honing is a sophisticated process and must not be confused with glaze-busting, which is a process used to remove the glazing on a previously used bore.

Running in

If the cylinder has been rebored and not surface honed, it will need careful running in to remove any high spots from the machining process and bed in the piston and rings, which will involve keeping the engine revs down for a period of about 800 kilometers and steadily working towards normal throttle openings during the following 800 kilometers. It's also vital that you avoid using engine oil additives during the running in period as these provent the bedding in process taking place effectively. If the cylinder has been correctly rebored and plateau honed to size the period for running in will be greatly reduced, see How to running in you motorcycle engine.

Reboring Two- and Four-Stroke cylinders

The reboring and honing procedure is basically the same for both types of engine however, it is advisable to lightly stone the port edges on two-stroke cylinders to remove any sharp corners. This will prevent undue piston ring wear in the early stages of running in.
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