The lubrication film in this area between the case and crank is what provides the seal
my 61FX has the Slot in the case just behind the front bearing as Baxter pointed out, this is were the case seals, I pulled the crank and the crank and case look good, no marks and no noticeable wear, the crank had the normal dark spots on it from fuel so I cleaned them off,
but> the gallery that runs from the Carb Port to the Slot had a tiny bit of debris in it, and the Slot did not have a lot of oil in it, so I am now wondering if the Slot did not have enough oil in it to seal the engine.
I will put it back together but I wont run it till the spring,
yes I clean my bearings well before installing them with CRC brake cleaner, oil them too,, I would bet the front bearing had 2 shields, if it did I removed one, it's been a while and I can't remember.
When bearings get loose it allows some movement of the crankshaft that can rub on the nose of the case..
I would think that if the crank rubbed on the case you would see where it rubbed, besides that the new bearings are tighter than the old ones (well they should be anyways) and yet while the old one was ugly it didn't leak, I'm thinking that the Gallery became plugged on that 2nd tank and the Slot didn't get enough oil to seal the engine
Jim