He worked for AL Adams repairing boilers on ships and locomotives, although he didn't tell his mother about their secret work building rocket launch barges for use in the D-Day landings.
Harold remembers the air raid alerts as the Germans dropped bombs over south Wales - and trying to get up for work at 7am after a sleepless night. The bombs had more of an impact on his little sister Rosemary (pictured here together).
He recalls the sense of relief at the end of the war, although rationing continued for many years after.
But Harold says one novelty was being able to cycle with lights on again after years of riding around in near darkness.
Read extracts from Fred Simon's diary