Guiness keg and tri-clamps

[atticpc]

Hi All

My boiler is a guiness keg. I want to update things to use a tri-clamp to attach the head. The ferrule on a guiness keg is just a little larger than 2 7/8 inches (its 2.884 inches, 73.3 mm)

So my option is to go for a 2" tri-clamp which according to St Pats is good for 2.516" OR going for a 2.5" tri-clamp which is good for 3.047".

Any thoughts and comments would be appreciated. Just wondering if the 2.5 tri-clamp would work with a ferrule which is smaller than recommended.