Hi,
I should have been clear about the sizes; those are the names that Arrand gave the different models. Based around the maximum bore using a side tool. From the catalogue the outside diameters of the heads are:
the 3” head 1.3/8” or 35mm
the 4.5” 1.3/4” or 44mm
The 5.25” 2.1/8” or 54mm
the 6” 2.1/2” or 63mm
the 6.5” 2.3/4” or 70mm
8” boring and facing 3.1/2” or 89mm
10.5” 4” or 102mm
The catalogue I have does not mention a 3.25” head.
AFAIK Arrand does not manufacture any more but still trades selling from stock. I purchased my 3” head a few weeks ago direct from Arrand.I think rotagrip have some of the heads in stock according to their website.
steve