Honda shaft drive oil leak

I have a Honda CB550SC Nighthawk. Yesterday, after a ride of about 300
miles at speeds between 70-90 mph, I noticed a bit of oil around the
final drive breather cap (silver cap on top of the bevel drive unit).
The oil level is fine - to the bottom of the filler cap threads - and
the oil is the correct grade of Castrol gear oil. I haven't drained or
filled the drive since last summer, and have since put about 5000 miles
on the bike. Should I be worried, or is this what the breather cap is
there for (getting rid of excess oil so seals don't blow)? Maybe the
fast ride caused the oil to get a bit hotter than normal and leak?

The ring gear in the final drive looks fine and the oil seems to be free
of metal particles. I don't think that the final drive could be filling
with engine oil due to a blown seal, since, unlike in old BMWs, the
shaft housing is "dry" not oil-filled, so there's an air space
separating the two components.

-Drew

Andrew Szafran wrote:

 > is this what the breather cap is there for (getting rid of excess oil
so seals don't blow)?

No, a breather cap allows whatever container it's installed on to
*equalize internal pressure* with ambient air pressure. If you
overfilled the final drive gear unit with oil, some oil might be pushed
out by excess pressure, or by climbing to a higher altitude.
Trapped air has to go somewhere. It would push excess oil out of the
unit, but you're supposed to be aware of the capacity of the unit and
not overfill it...

And, the spinning of the ring gear might be throwing a bit of oil up
inside the case and air pressure changes might carry a few drops of oil
out of the case through the breather...

I remember one guy who installed a simple chain oiling system on his
handlebar and filled it up in the San Francisco bay area. He was
absolutely amazed that the oiler puked all of its contents all over his
rear wheel when he rode over a high mountain pass on his Seymour Rhodes
Tour to Death Valley. Air pressure differences caused that oily mess...

 > Maybe the fast ride caused the oil to get a bit hotter than
 > normal and leak?

You could do some research on the boiling point of various lube oils
and weight of gear oil, or you could just accept the fact that
mechanical items need to breathe and sometimes the air moving through
the breather carries a little oil mist with it and that unsightly gooey
stuff will deposit itself when it cools off...

Forty years ago, cars had oil filler breather caps on their valve
covers. The cap was a handy size that fit in your hand, and the cap had
some wire mesh screen inside it, since there was room for the screen...

It was a de-mister cap, as well as a breather and an oil filler cap.
Car engines also had a road draft tube with a tapered end going down
behind the engine. The tapered end would cause turbulence behind the
tube and air would be sucked into the engine through the breather/oil
filler/demister cap when the car was moving...

(Hey, there's an idea! Maybe you could get some copper tubing from
your local hardware store and bent it in a curve so the oil drops got
sucked out and fell onto the highway. And other bikers would ask what
that funny tube was and you could explain how road draft tubes worked
and they'd ask, "Are *you* that krusty kritter dude that goes on and on
about carburetors on rec.motorcycles.tech?")

But when the engine was shut off and it was still hot, oil vapors would
rise up to the breather/oil filler/demister cap and the wire mesh was
supposed to condense the oil vapors back to liquid and the liquid oil
was supposed to flow back to the engine...

In prectice though, you'd see some liquid oil on top of your engine's
valve cover. And oil that condensed in the cap would turn to sludge and
that sludge would block the desired flow of air through the engine and
out the road draft tube and you'd start seeing valve cover oil leaks,
and the oil would run down onto the exhaust manifolds of some engines
and even cause oil fires under the hood of the car on a really hot
day...

Been there, seen all that, had an oil fire too. What a mess...

There's very little room for any demister mesh inside that little
chrome breather cap on your final drive unit. It may seem unpleasant
and unesthetic for a little oil to appear there after a long fast ride,
but what the heck, it's just a little breather cap...

(Maybe you could get one of the bigger 1.5 inch diameter K&N oil
breather caps made of fabric and wire mesh and mount that in place of
the stock breather cap. And when other bikers saw that, you could say,
"Oh, yeah. Well, I *raced* my Honda at Bonneville salt flats last week,
and the gearbox got really, really hot as I was going 200 miles per
hour, so it needed a really big breather filter...")

Or, you could just leave the little chrome doodad on there and wipe the
oil off occasionally...<!-- ~MESSAGE_AFTER~ -->