This example shows a tyre width of 3.5 mm for Gauge 0 Fine.
For Scale7 remove more metal from the back of the wheel to give a tyre thickness of 3.2 mm instead of 3.5 mm.
6′ 8½″ diam., 22 oval spokes, rectangular inner rim, 4¼″ apparent tread.
Hub 17″ face dia., 3″ out from the tyre, pin between at 13″.
Sources: Swindon drg 96784; Side elevation drg, NRM No GW/664 showing wheel reinforcements, Russell vol. 2 p. 154, drg of 7007 'Great Western'; Haresnape, Collett & Hawksworth Locomotives, p. 29, photo of 5073
'Blenheim Castle'
Photo on web at http://www.warwickshirerailways.com/gwr/gwrls163.htm of 4037 in 1937 with reinforced drivers.
Casting: 47.0 mm tread dia., 41.7 mm i/d., hub 1.75 mm out from the tyre, pin at 7.6 mm.
Casting prices and stock . GWR loco sets, price list
Scan of the castings in sets for a GWR Castle
27 Mar 2013