How Seals In Motorcycles Make It All Happen

We immediately think of oil and coolant containment—oil seals, O-rings, and gaskets—in this connection. But combustion gas is the fluid that produces power, so we have to add the sealing functions of piston rings, and intake and exhaust valves to our list.

Long-lasting oil seals and O-rings are as recent as the synthetic rubbers that can make them impervious to gasoline, oil, brake fluid, or water. Consult Parker Hannifin’s online Fluid Power Seal Design Guide for compatibility. Retaining oil in motor­cycle engines, forks, and wheel bearings are round dual-lip oil seals with one lip backed by a slender coil garter spring that holds the lip facing the fluid in constant contact with the shaft. Dimensions are the obvious ones—OD, ID, and thickness. The other seal lip is there to exclude dust from the sealing zone.

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Most seals place the seal material in direct contact with the rotating shaft, but in cases of high speed or pressure, the contact lip may be coated with the slippery, heavy-­duty reddish plastic Rulon, or the lip may bear an embossed chevron pattern to give it an inward-pumping action. Special materials must be used in contact with brake fluid, which also attacks painted surfaces. Some synthetic oils shrink engine/gearbox seals of earlier decades. Modern gearbox or crankshaft seals for use in engines with horizontally split cases have ribs on their ODs, which fit into corresponding grooves in the cases to prevent seal push-out. No gasket compound should ever be applied to oil seals.

O-rings are used to seal in static or slow-moving situations, like the slight back-and-forth bending of the links of an O-ring final drive chain. O-rings for moderate pressures are fitted into roughly square-section grooves, whose proper dimensions can be found online or in industrial handbooks. To ease their installation, they can be lightly oiled, but need no help from gasket cement.

Gaskets were called into existence to compensate for poorly machined surfaces, the idea being that a soft material such as paper, compressed between mating metal surfaces, will be forced into irregularities to prevent them from leaking. Many years ago, a British rider, on the way down to Brooklands Speedway for a weekend of sport, took along the Times of London for its possible use as gasket material should need arise. The old way was to shellac the paper in place. Removing such a mess before reassembly required patient scraping. Every engine builder had a favorite gasket goo. Then the Japanese made it all unnecessary by introducing their wonderful self-bonding black gaskets that could often be lightly stuck in place on one side and thereafter reused again and again.

Crankcase halves are usually assembled without gaskets, but are instead thinly coated with case sealer. Sealing surfaces must be degreased before sealer will stick to them. I once saw a Harley Hummer engine whose owner had overdone the case sealer so tremendously that it squeezed inward to glue the crankshaft to the cases.

Head gaskets span a range from simple—stamped from copper sheet for old-time two-strokes—to complex modern affairs of embossed multilayer steel, bearing printed-in-place synthetic rubber to seal oil and coolant passages.

Piston rings are springy split rings of iron (old tech) or steel coated for wear resistance, fitted into machined grooves near the tops of pistons. Rings are split so they can spring out against the cylinder wall, compressing the end gap to just a few thousandths of an inch. Does the ring gap leak? Yes, but very little because at 6,000 rpm on the highway, the high-pressure part of each power stroke exists for just 0.0025 second. Piston rings have become steadily narrower in the interest of lower friction and lighter weight. As pistons decelerate nearing TDC at the end of the exhaust stroke, inertia force from a thick ring can unstick it from the bottom of its groove, thereby losing its seal. The real force of sealing comes not from the ring’s springiness, but from combustion pressure that flows in above and behind the ring and pushes it out against the cylinder wall. Below the two gas sealing rings is an oil-scraper ring, usually in three-piece form, whose edges scrape oil back to the crankcase, leaving just enough on cylinder walls to lubricate the piston and rings.

Also sealing against combustion pressure are an engine’s intake and exhaust valves. Each resembles a tiny manhole cover (the valve head) on a central stick (the valve stem, which guides the head by sliding back and forth in a tubular valve guide). The edge of the valve head and its seat in the port are ground at a 45-degree angle to seal against one another.

Modern motorcycle engines stay dry and nicely sealed for many thousands of miles thanks to these mature technologies.