http://www.osengines.com/faq/faq-q687.html
"Some people believe that their engine is leaking air through the front bearing. This is not really possible. Almost all glow engines do not use the front bearing as a seal. Even if you see a metal shield on the front of the front bearing, this is not a seal, but a cover to keep dirt out. For years, many model engines had no shield on the front bearing at all.
Some fuel leakage from the front bearing is normal. That's how the front bearing gets lubricated. The fuel that works its way through the front bearing will be thrown away by the rotation of the drive washer while the engine's running. Some of this fuel will accumulate on the engine and may even form small 'bubbles' of fuel in areas of low airflow, such as behind the needle valve. We've seen engines do this that were returned to us because of a leaking needle valve. If the needle valve leaks, it usually won't leak fuel, but suck air. You'd need a high-pressure fuel system, like pumped engines have, to cause fuel to leak out of a badly-fitting needle valve. Most fuel found on the needle valve comes from somewhere else.
The actual seal is made by a thin film of oil between the crankcase and the crankshaft forward of the carburetor, and just behind the front bearing. If the engine is run too much in dirty conditions, dirt can get into the engine and erode the crankcase forward of the carb and loosen things up. The sigh of a worn fit is fuel spray from the front bearing. The fuel spray will be enough to cause the nose of the model to become wet with raw fuel. You may also be able to feel wetness on your hand if you put it next to the cylinder, behind the propeller. You don't have to get close to the prop at all to feel the fuel spray."
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HoBao Hyper 9 - Novarossi Toro Nero 21T