I keep getting this nearly every day. People, if the thing between the inner and outer races of the bearing is metal, it is a shield, not a seal! There is a gap between the shield and inner race (about .005") otherwise you would have metal to metal contact. Sealed front bearings have rubber, Teflon, polyamide, or other flexible seals to keep lube in and dirt out. Except for a few particular cases, the front bearing does NOT seal the front of your engine! For most two strokes, the crankcase is sealed by the oil film in a close fit between the crankcase and crankshaft.
Here are some cases where sealed front bearings are normally used:
YS Four Strokes
Some RC Car engines
Some engines with poor manufacturing tolerances (usually cheap engines)
Most engines come with a shielded front bearing. Webra and Fox engines use open bearings on the front of their engines! This shield isn't even necessary because most dirt is kept out by the prop drive washer. RC cars don't have that so that is one reason they use sealed bearings.
The only other reason to use a sealed bearing on an ordinary two stroke is if the fit between the crankcase and crankshaft is too big to sustain the oil film sealing. When this happens, you can see an excess of oil coming out the front of the engine, or it gets difficult to shut down the engine with the throttle.