Suinroof drain tubes - replacing steel with plastic

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Hi Guys

I´m replacing the rear sunroof drain tubes (steel) on my 76 930 with the later plastic tubes. I think the plastic tubes were used from 86.

I have removed the steel ones and ordered plastic tubes (incl. intermediate rubber tubes and rubber grommets)

I guess the exit holes at the top of the engine bay (below the rear windshield)needs to be drilled to a larger diameter in order for the rubber grommets to fit into them.
The late type plastic tube exit through a rubber grommet, where as the early style steel tube is welded directly onto the exit hole.

My problem is:
The car goes to the painter for a fresh coat of black, before I receive the new parts.
I would prefer to do the drilling now rather than later when the car has been painted.

So, can anyone tell me the diameter of the exit holes in which the rubber grommets should fit?


Best regards
Christian