Hi folks. The number one question I get from people installing our bearings into model aircraft engines has to do with shields and seals. Most manufacturers remove the inner shield from the front bearing before installation. There is absolutely no reason to do this! A little engine anatomy will show why. Nearly all glow style engines use a very close fit between the crankshaft and crankcase ahead of the carb to seal the crankcase. A small amount of oil is all that is needed for this. The gap between them is tiny, usually around 0.001 inches. A small amount of fresh oil is constantly being pushed through this gap by crankcase pressure while the engine is running. The shields on the bearings are not seals, and the gap between the inner race and edge of the shield is normally 2-3 times larger than the crankcase gap. This means that any oil that is pushed past the crankcase seal will easily get into the bearing. We have been installing bearings with the inner shield in place for over 40 years and have yet to see one fail due to lack of lubrication.
Some engine makers use sealed bearings on the front for various reasons. Saito engines do not have the crankcase/crankshaft close fit sealing due to the design of their crankcase. YS supercharged engines have sealed crankcases where no oil could get to the front bearing to lubricate it. And, some manufacturers do not want to invest in the tight machining tolerances needed to produce the crankcase sealing. That is where a sealed bearing is used. You should never remove the inner seal of these bearings. They have a high quality lube pre-installed that will last the life of the bearing.
If you have an engine that normally used a shielded bearing but are getting a lot of oil leaking out the front, that is not always the sign of a bad bearing. It can be the RESULT of a bad bearing. What has likely happened is one or both bearings have worn excessively and worn out the front of the crankcase. Fear not! This can usually be remedied by fitting a sealed bearing in the front to lessen the amount of leakage. Still, leave the inner seal in.
Send me an email if you have any other questions