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32 of this class were rebuilt from 4-cylinder compoundClass B between 1906 and 1917. The outside high-pressure cylinders were removed and the inside low-pressure cylinders were re-used, in their original position, to make a two-cylinder simple expansion engine. The boiler pressure was reduced from 200 psi to 160 psi to keep the tractive effort approximately the same.[1] The rebuilt engines retained their old numbers. Additionally, 60 new Gs were built in 1910. The rebuilt engines were easily distinguished from the new builds by having "piano fronts".
Rebuilding[]
The first of the class, LNWR No. 2653, was rebuilt to Class G1 in 1912. The remaining 91 engines were inherited by the LMS in 1923. LMS numbers were 9077-9144. The LMS rebuilt them all to Class G1 between May 1924 and 1937, thus rendering the class extinct.