If the mating surfaces join tightly, the high temp silicone should last indefinitely.

I have done this many times. best to put a bead on, assembly lightly prior to tightening (to let the silicone 'skin'), then tighten.

So where do you get the "trap oxidizer replacement pipe" ?

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.