I've toasted two non-drive side external bottom bracket bearings in a very short amount of time. The first went after about 1,000 miles. The second lasted less than 200 miles. In both cases the drive side bearing is fine.

As I write this, I'm looking at the cup that lasted less than 200 miles. What's weird is that the cartridge bearing itself seems fine. It has no play and it spins freely. But the protective plastic cover broke apart in two pieces. It looks as though the left bearing wasn't spinning with the crank and the friction of crank assembly rubbing against the cover caused the cover to break apart.

Specifically, the flat disk that protects the side of the cartridge bearing separated from the ring that sits between the crank shaft and the inside of the bearing. I don't have my camera handy, but this picture of a Shimano BB should help. I'm talking about the black plastic part...

When this bike was put together, the BB shell was professionally faced prior to installing the cranks.

But now I'm wondering if it in fact wasn't faced or if it was faced improperly. According to the Park Tool site... "If a bearing cup is secured to a shell with miss aligned faces, the two bearings will not align properly with each other. Unlike conventional cartridge bottom brackets, these bearings use the shell face as a reference for alignment. Shell faces that are not parallel may cause a twist or load to the bearings."

Could this be my problem? If so, anyone care to recommend a shop to perform said service? I'm all for purchasing the proper tools to do the job myself, but this appears to be a $450 tool that I might use twice in my lifetime.

If this isn't the problem, what could hypothetically cause two left side BB cup assemblies to disintegrate?

Thanks in advance for your ideas and suggestions.