Spec for Bosch Distributor O-Ring?

[m2500tvr]

The Ford 2.8i engine in my '85 280i is weeping quite a bit of oil at the distributor pedastal. I'm guessing this is the o-ring seal on the Bosch distributor shaft. The engine otherwise is pretty much free of nusance oil leaks. Does anyone know the spec of this o-ring so I can source one and have it on hand ready to go when I'm ready to pull the distributor?

Thanks.

- SJ