The engines were simples, i.e. not compound, with two inside cylinders. Their boilers were rated at and were fitted with Schmidt superheaters. All eighteen engines, though differing slightly in appearance had, until 1945, the same power output. The only dimensional difference was caused by six boilers, identical with the others, except that the opportunity had been taken for the first time with a Derby boiler to use the broader Irish gauge to full advantage for a wider firebox. The first two of these G7S boilers were fitted to Class A "Heavy Compounds" Nos.67 and 59 when they were renewed as Class U2 in 1934.