Did your mechanic take a look to see if there was a groove worn on the surface that the seal rides against?

When I replaced the front main seal on my m117 engine in the SEC, I found a groove where the seal's lip rode on the harmonic balancer hub.
The local Napa Auto parts shop had a repair sleeve, which was just a thin bronze sleeve that you press over the bearing surface after putting some sealant down under it, and smeared inside it.

If I remember correctly, it was a $10 fix, and I haven't had any front main seal issues since.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...