Piston rings never fully prevent combustion gases from escaping into the crankcase when an engine is running. Leakage happens even when the best cylinder finishing and highest quality piston rings are employed, inevitably getting worse over time, and the issue creates two distinct problems.
One, some amount of power is continually lost to this leakage. And two, the engine designer must find a way to minimize the buildup of blow-by in the crankcase, which creates both corrosive contamination of the oil supply as well as excess pressure that must not be allowed to blow out seals and gaskets.
There are two readily available methods to determine ring sealing: the simple ‘compression test’ and the more accurate ‘leak-down test.’ To perform a compression test, a pressure gauge is screwed into a spark plug hole, and while the throttle are held wide open so as not to restrict airflow, the engine is cranked over for several seconds until the gauge stops rising. Each cylinder is tested in turn and the pressures are compared to one another. We’d like to find them all within 10 percent of one another and close to the maximum pressure specified in the service manual. However, a particularly poor result may indicate valve seat leaks, so a second ‘wet test’ - a squirt of oil into the offending cylinder – will momentarily seat the rings, giving a higher pressure. No change suggest a valve-sealing problem. Of course, gasket failures are also possible.
By contrast, the leak-down test requires an air compressor and a more expensive gauge. Each cylinder must be carefully positioned at top dead center on the compression stroke so the valves will be closed and so the cylinder can be filled with a regulated amount of pressurized air without causing the crank-shaft to turn. An in-line pressure gauge will indicate leakage as a percentage. Because the engine isn’t turning, valve seat leaks can usually be heard hissing at the intake side of the engine, exaust leaks at the muffler outlet, and piston ring leak can be heard at the crankcase’s oil filler opening.
Although top racers will try to keep leak-down to 2 percent or less, 10 percent leak-down isn’t unusual and figures of 20 to 30 percent aren’t surprising on motorcycles with high amount of kilometers engine. But don’t forget, leakage has its greatest effect at low engine rpm, so even 20 percent leak-down may not hurt high-rpm power significantly.
So-called ‘zero-gap’ piston rings, which have half-thickness overlapping ends, can help reduce blow-by, but normal butted rings have gaps that continue to grow larger with wear, until perhaps 20 percent of the engine’s potential power is lost to reduced compression, even when rings gaps are still within specifications. A second reason for decreased performance is that blow-by increases crankcase pressure, which serves to increase windage and pumping losses and also hurts ring sealing. The added pressure also encourage oil leaks, and back in the day, just as steam engines had done before them, it was not uncommon for internal combustion engines to be covered in oil that had seeped past gaskets and seals that were never expected to hold back high crankcase pressures, and to leave small puddles wherever they were parked.
One of the first automotive crankcase ventilation system was called ‘road tube’ or ‘draft tube’ design, but even these were rare on motorcycles. The valve cover’s oil filler cap would be fitted with a metal mesh to keep out bugs and act as a flame arrestor while allowing fresh air into the crankcase, and a high-amounted vent tube attached to the crankcase would have its exit slash cut and mounted low behind the engine to create an aerodynamic draft that drew blow-by out of the engine, hopefully before it had time to mix with the engine oil.
Road tube ventilation systems were claimed to greatly increase engine life but left a classic oily stripe in the center of the lane that was a constant hazard to hapless motorcyclists. And it wasn’t until 1958 that the GM Research Laboratory, led by Dr. Lloyd Withrow, determined that roughly half and automobile’s emissions were coming from its road tube crankcase ventilation system.
In response, the modern Positive Crankcase Ventilation (PCV) system was mandated on cars in 1961, and it had changed very little ever since. The basic design was to provide a controlled vent between the crankcase and the air cleaner, so that oily crankcase gases would be routed through the engine to be burned, perhaps a second time.
In fact, PCV systems were so effective that engines with leaky rings could mysteriously use significant oil without smoking. ‘Where did it do?’ Unfortunately, PCV systems also have a serious drawback that has essentially escaped attention, and it wasn’t until I blue-printed the engine of my motorcycle that I got a sense of its magnitude.
With the addition of a PCV system, the engine is no longer inhaling clean air, but a mixture of ambient air combined with moister, burned and un-burned fuel, combustion gases, ash and atomized engine oil that together readily form thick carbon deposits of the intake valves. And it doesn’t take much imagination to realize the consequences of this buildup: reduced airflow due to rough, narrowed passages caused by crusty surface buildup. Also, as these oily gases add to the carbon buildup on the piston crowns and combustion chamber walls, excessive blow-by tries to make the engine run hotter and also tends to poison catalytic converters.