Modern oils and manufacturing systems have gone a long way to increasing the effective working life of motorcycle engines. However, many classic motorcycle owners will face the possibility that their engine needs to be rebored and have new pistons and rings fitted at some point.

The wear rate on the internal moving parts of an engine is highest at the piston ring to cylinder bore interface, due to the distance these components travel.

Using basic math, we can estimate this distance over a given period of time.

For example, assuming an engine has covered 20,000 miles at an average rpm of 2,000 and an average speed of 30 mph, with a bore and stroke of 2” x 2”, the pistons and rings will have covered some 5,000 miles.

Besides the distance the piston/s and rings will have covered, they are also operating in an extreme environment of temperature and pressure change during the two or four-stroke cycle within an engine.

Typically, the first sign that an engine needs attention to the cylinders, bores and pistons, is when it starts to smoke. However, some multi-cylinder engines may smoke on one cylinder only – 2-strokes for example – and this may indicate a problem with other components relative to that cylinder only.

To check the condition within a cylinder, without resorting to a tear-down, the mechanic can conduct a compression cranking pressure test and a leak-down test. These tests will determine if a cylinder (and its associated components) requires attention.



Cylinder and Bore Types
Steel Liner

The easiest type of cylinder to recondition or repair is the steel lined type. These liners are typically fitted inside aluminum cylinders and are designed to be rebored two or three times (this varies amongst manufacturers and types). Most local engine machine shops, and some dealers, can rebore a cylinder of this type. However, it is important to follow the manufacturer’s (of the new piston) recommendations for the piston to bore clearance.

After the cylinder has been rebored, it is imperative to thoroughly clean the cylinder. All good machine shops will do this cleaning as a matter of course, but the mechanic or owner should ensure total cleanliness of the components before rebuilding the engine.

If the liner has been rebored a number of times (typically the maximum is three bores), the liner must be replaced. The bore diameter will then go back to the stock size and the new liner should last for another three rebores.

If the engine is a 2-stroke, the mechanic must check the port alignment when a new liner has been fitted. The performance of the engine will be affected if the new liner obstructs any of the ports.

(Note: Although there are many assembly lubricants on the market, the mechanic should build the engine and lubricate the parts during this process with the oils it will be run on).

Chromed or Nikasiled Bores

Generally found on performance 2-strokes, the hard chromed bore variant of cylinders must be inspected on a regular basis. Detailed inspection can be done with miniature scopes and cameras. Wear tends to be the greatest at, or near to, the exhaust port on 2-strokes.

As long as the aluminum cylinder below the chrome has not been damaged (it is possible to get cylinders repaired when damaged but it can be expensive), the chrome or Nikasil finish can be replaced by specialist companies. A web search will locate a number of these companies, alternatively most motorcycle dealers can get this work done on behalf of a customer.