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I have done 18-20 # birds in the past.
Cook using the indirect method using the plate setter with legs up and a aluminum drip directly under the bird to catch the drippings. I baste the bird every hour or so using the drippings.
As mentioned above, move the lid thermostat out from the lid using the spring clip for the clearance of the probe to the bird.
I just did a trial spatchcock turkey yesterday (1st time with a turkey) at 15 lbs., raised grill to the top, direct. It fit easily. I think I could confidently get a 17-18 lb turkey on there doing it that way.However, I wasn't really impressed with the results. I suspect it was my lack of experience, but I don't really know what to do to confidently correct it other than start experimenting. Since Thanksgiving is upon us, I'll just resign to to it in the oven again this year and experiment down the road. (FWIW, I did it about 400-425+ and the temps came into range at about 1.5 hrs. The turkey was dryer than I would have liked and the breast skin was not crisp - legs were.)