The reason for the crankshaft oil seal falling off is this!

A few days ago, I took over the repair of a Thai Honda (local name) model bent beam motorcycle. The owner informed me that the engine was leaking oil and asked for help with the repair. After inspection, I determined that the left crankshaft oil seal (from the magnetic motor) was leaking oil and told the owner that it needed to be replaced.

However, the owner looked unconvinced and said that the oil seal had been replaced less than a week ago, so it couldn't have failed so quickly. I said we should let facts speak for themselves; let's take it apart and see.

Haha, it turned out not to be an oil seal wear issue but rather something had fallen out from the starter plate. At this point, the owner lost his temper and quietly told me that last time there was an oil leak, it was also due to an oil seal falling out, and this same problem had occurred multiple times.

I said you've become quite knowledgeable about this from your long experience; you still don't believe it's an oil seal leak. That being said, work still needs to be done. I thought to myself that if the oil seal keeps falling out, then the issue might not be with the quality of the oil seal but perhaps with a hole in the starter plate being too large...?

However, there was no such starter plate in the shop, and since the owner was urging to use the vehicle, I had no choice but to improvise on-site by applying some 502 glue on the outer ring of a new oil seal and install it. Let's see if it will fall out again.

After finishing installation, the owner took a test ride around but didn't notice any issues for now. I suggested that since there had been an oil leak before, there might not be enough engine oil left; let's change it. The owner thought about it and reluctantly agreed to change the engine oil.

When removing the drain screw, there was a loud 'pop' sound as the screw shot out far away. At this point, I finally understood why there was an oil leak: when the engine is running, pressure builds up in the crankcase which forces out the oil seal.

It seems that there are still problems with this vehicle; it's likely due to either a blown cylinder gasket or leaking piston rings causing excessive pressure in the crankcase due to a blocked ventilation pipe. After inspection, I found that indeed one of the crankcase ventilation pipes was blocked; after clearing it and changing the engine oil, I returned it to him.

Image sourced from online

As for whether there's any air leakage from cylinder gaskets or piston rings, since the owner said his vehicle is too old and he doesn't want further repairs done, we didn't check those either. Therefore it's still uncertain which aspect caused excessive pressure in the crankcase; hopefully this issue won't occur again.