Model Engine Company of America
Fuel leaking out the front bearing???

A big misconception of this problem is thinking the bearing seal is no good. The seal on ball bearings is not designed to retain pressure, their only purpose to to keep dirt and moisture out of the bearing. The pressure developed inside a model engine's crankcase is much higher than this seal is able to withstand.

The way a model engine prevents the pressure in the crankcase from leaking out is the precision fit between the crankshaft and the crankcase, mainly just behind the front bearing. If this fit is loose the engine will blow fuel out the front bearing. Many engines do not have seals or shields on the front bearing. The balls and cage are completely open, however no fuel leaks out.

Besides the precision fit a groove is usually cut in the crankcase just behind the front bearing. Another groove or hole connects this groove to the inlet passage just below the carburetor. The intake stroke suction of the engine is directed to the groove so any fuel is sucked back into the engine. Many K&B engines use a spiral on the crankshaft to pump the fuel back into the intake stream.

Even when the above mentioned is correct your engine can still leak fuel out the front bearing, This is caused from excessive blow by past the piston. This can be because the piston ring is not seated or the engine isn't broken in. If this is occurring in a new engine the problem may disappear as the engine gets more running time on it. See Ringed engine break in

If you have a ringed engine (See ABC, Ringed) with running time on it, the problem may be a stuck ring. If an engine has been sitting for some time the ring can become gummed up and stuck in the groove. This makes its sealing function ineffective. See How to unstick a piston ring

If you have an ABC engine it could be worn out and require a new piston/cylinder assembly.

Note: If fuel is leaking from the front bearing while the engine is not running it probably because fuel is siphoning from the tank and filling the crankcase, not the issues mentioned above. If the crankcase fills with fuel it has no where to go except out the front bearing, back out the carburetor, out the draw bar, and up into the cylinder. This condition may cause "Hydraulic Lock" and this will result in damage to your engine.

The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.