Most of the older classic bikes have iron sleeves inside an aluminum cylinder. Over time, and with higher mileages, these liners will become oval and the piston-to-bore clearance will become too large to maintain performance. Both of these situations can be corrected with a rebore.
During an engine rebuild the mechanic will typically measure the piston to bore clearance (the running clearance) and the ovality of the cylinder liner. However, if the motorcycle is running, there are a number of ways to check the cylinder condition without disassembling the engine.
The first indication that a motorcycle engine requires a rebore, and/or new rings, is when the rider or mechanic notices the engine emitting smoke. This applies primarily to 4-strokes. On 2-strokes the rider will notice a drop in performance and difficulty in starting.
As the pistons and/or rings begin to wear, oil will get past them into the combustion chamber where it will be burnt during the combustion phase. The oil will give off a telltale blue color from the exhaust system which will get progressively worse as the engine speed is increased.
To confirm the engine requires a rebore, the mechanic can conduct two tests to check the condition of an individual cylinder. The easiest test is a cranking pressure test. This test will typically inform the mechanic of the general internal condition of the various engine parts. However, as carbon can build up over time inside the combustion chamber and on the valves, the compression may still be relatively high, giving something of a false reading.
By far the most accurate test of a cylinder’s condition is the leak-down test. This test involves applying compressed air into a cylinder (via the spark plug hole, at TDC on the compression stroke) and monitoring the amount of leak on a gauge. Besides being able to note the percentage leak, the mechanic can listen for air escaping from the crankcase (caused by worn rings and pistons), the exhaust (caused by a worn exhaust valve guide) and through the carburetor (which indicates a worn inlet valve guide).
The piston rings in a 2-stroke have a much harder time than their 4-stroke counterparts. On the 2-stroke, the rings must pass over various ports in the cylinder wall: the inlet port, the exhaust port, and the transfer ports.
In addition, on a 2-stroke, the combustion process takes place twice as often as that of the 4-stroke which creates addition heat and ultimately wear.
Similar checks as those performed on a 4-stroke can be conducted on a 2-stroke (cranking pressure and leak-down tests). Although these tests will give an indication of the internal condition, it is generally best to take the head and cylinder off the engine (a relatively easy task) and measure the various components carefully.
Measuring the Internal Components
The following items should all be measured to compare them to the manufacturer’s specifications:
Measuring the piston to bore clearance is simply a case of sliding the piston (in its correct orientation) into the cylinder with a feeler gauge between it and the cylinder wall. It is best to start with a relatively small feeler gauge, such as one measuring 0.001” (0.00004 – mm), then gradually increase the size until the piston will barely slide in. This measurement will be double the running clearance.
The piston ring end gap will increase as they wear. The mechanic must place then into the cylinder approximately ½” below the top. (Note: It is important to keep the rings parallel with the top of the cylinder when doing this check). A feeler gauge can again be used to measure the end gap.
Typically, cylinder bores wear because the piston tips as it traverses up and down. The result is that the cylinder bore becomes slightly oval. The mechanic must therefore compare the diameter from side to side with that of the front to rear of the cylinder. In general the piston and rings will wear more than the cylinder, but reboring and fitting new rings/piston will ensure a good seal, and by extension, good compression.