J.L. Norton introduced his first OHV engine in 1922 using the same bore and stroke of the earlier SV incarnations including the 16H, 79mm x 100mm.
The new machines were tested at Brooklands where it established a 500cc world record, achieving 89.22 mph over a flying kilometre, before being fielded at the IOM races. Success was forthcoming when, in 1924, Alec Bennett won the Senior TT.
The 1923 catalogue offered the Model 18 which before long became very popular indeed. It appeared in the winners circle in no end of events including the gruelling Maudes Trophy, which Norton dominated from 1923 to 1926.
Initially the magneto was situated in front of the engine but was later moved to the rear. A four-speed CAV gearbox was fitted in 1925 and the chain drive moved to the right, allowing easier access for sidecar riders.
The top end featured well spaced finning on both the barrel and the comparively large cylinder head. The rockers were placed high on the head further enhancing cooling.
Saddle tanks were employed from the early 1930s along with better oil pumps, and the exhaust system was moved to the right side of the machine.
From the 1928 Catalogue:
1. First appeared in the 1923 catalogue, which would have been issued in the latter half of 1922. It did not appear in the 1954 catalogue.
Sources: nortonownersclub.org, nzmuseums.co.nz