Q: For a retrofit, do I drill/screw through the extrusion under the adjustable plate? I don't see any other way, without potentially creating a path for water ingress.
A: Have you already pulled out the existing threshold and if yes can you tell me how it was originally fastened?
Q: I used some short deck screws with washers. I drilled some ~3/16" holes in between the jack screws in the same channel of the extrusion where the jack screws go, then screwed it down snug. The threshold itself sits firmly in place.
A: Are you saying that you first adjusted the adjustable portion of the threshold to its final position and then screwed down through it to secure the threshold knowing that in the future you would not need or would forgo the ability to adjust the adjustable top portion of the threshold?
Q: First I unscrewed the jackscrews until I was able to remove the top plate. Then I installed the threshold, screwing it down snugly as described above. Then I reinstalled the top plate. I checked and the screw caps were seated firmly against the top flanges on the jack screws. The tolerances are just so loose that the caps push the seal upward as I described above. And the top plate wobbles. I don't doubt that you have seen people do some dumb stuff... but I'm a mechanical engineer and while not immune from dumb mistakes I'm not that dumb. :)?
A: Thank you very much for your reply and I will tell you that I will forward your success onto every other customer who asks this question.
I very much appreciate you taking the time to respond.