I measured one from a 2N and it's 1 3/8".
1951 8N is 1 3/8"
The adjustable front axle on many of the Ford tractor models were mounted to a special deep drawn steel stamping on the front of the engine utilizing a single large diameter pin that allows the axle to sort of teeter on the lateral axis. That pin and the two radius rods are the only supports so they take a hell of a beating when crossing over furrows or road ruts. If it weren't for the suspension on the seat, the ride will beat the operator pretty soundly on a rough patch of ground. I've had to repair cracks in that front support before as well as replace the pin and it's support plates so I can attest to the torture that they endure over time and abuse. Those radius rods can take about anything an operator can throw at them. The tractors with the front loaders take even more abuse due to the extra weight on the front end.
All steel also, no rubber liner.
Can you drill and tap the assembly for a grease fitting?
Those ball/socket joints on Ford tractors are intended to operate metal to metal, with no rubber socket interface. I wondered myself why there was no grease fitting on that joint as the one I removed and looked at had noticible wear. It has operated that way for 70 years, tho. This is my bush hog tractor that my grandad bought new in the fall of '53.
Al Hook