Its 1 1/8"

If you want a fancy plug you can get it from Uro Tuning (I know you like cleanliness)

Uro-Wiper-Delete Rear Wiper Delete Plug, Mk4/Mk5/Mk6/Mk7 Golf/GTi and B5/B6/B7/B8 Wagons

I actually got a free sample from www.caplugs.com a couple years back which worked perfectly, you can try to get a sample from them as well. No silicone was needed as it was a snug fit and snapped right in place.

https://www.caplugs.com/series/Butto...eads/1046.html

BPF-1 1/8-6 - If you can get a sample order they will send you 2.

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards