 There was some swapping of numbers, so that engines in better condition were given the lower numbers.The leading dimensions of the GWR 3000 Class were the same as the GCR Class 8K except that the GWR increased the boiler pressure to which increased the tractive effort to or .The GWR borrowed 30 Class O4 from the LNER in 1940 but returned them between 1941 and 1943.