Ihc engine dating
The 3 & 6HP had no major design changes, but did change the cylinder head and block casting to accommodate the ignitor or sparkplug ignition system.The early engines had ignitors (to 1921), then a switch to sparkplug with a rotary Bosch mag, and in 1924 Wico "EK" magnetos were introduced. changed the same as the larger sizes, but there was an additional change with the 1-1/2HP when IHC switched from Bosch to Wico EK mags.The cylinder block had a larger bore (3-5/8" compared to the smaller bore of 3-1/4" for the earlier blocks).
Also from England, the Leyland 262 introduced in 1976 matched the power of the International, but the earlier Leyland 255 was somewhat lower powered.
The German-built Deutz 4-cylinder D5506 was slightly more powerful and less expensive than the International in 1971.