In 1946, Armstrong rode a pre-war Norton Manx at the Bangor Castle races in Northern Ireland. His was fifth in his second race, a handicap event, the Mid-Antrim 150, and then failed to finish at the Skerries 100. He put in an entry for the 1946 Manx but was refused as his 18th birthday was on the same day as the race. He borrowed a 500 cc engine and put it into his Norton, and entered the 1947 Senior Manx race without success.