Gents and Ladies,
On eBay, there is a Dog-Bone radiator cap that is drilled for a MotoMeter.
It appears to need re-plating, and is a tad expensive, but finding one to fit a Pierce-Arrow (male threads) is extremely difficult.
The description indicates the OD is 2″ and has a 16 thread pitch.
I am confident that it will fit a Series 80 / 81 radiator pipe, as I have a number of them in that size, however I am currently disinclined to add to my already extensive collection of said Dog-Bone caps.
Below I post the eBay link.