The SD diesels are based on this series of motors (bore spacing and basic block layout) The 1H is not related to the later H engines.
With a bore and stroke of 73 mm × 89 mm (2.87 in × 3.50 in) it displaced 1.5 L (1,489 cc), power was 50 PS (37 kW; 49 hp) at 4,400 rpm in 1956, but this increased to 57 PS (42 kW; 56 hp) in August 1958.
[2][3] Applications: The basic H is a 1.9 L (1,883 cc) engine produced from 1962 for Nissan's Cedric.
Displacing 2.0 L (1,982 cc) thanks to a larger 87.2 mm (3.43 in) bore, H20 engines produced around 99 hp (74 kW; 100 PS) and 167 N⋅m (123 lb⋅ft).
The R engine used essentially the same block as the H20, but a 17 mm (0.67 in) shorter piston stroke resulted in a capacity reduction of 387 cc (23.6 cu in).
The "R" motor made the switch from 3 to 5 main bearings in 1967 for improved reliability, and the H20 was developed from this arrangement.
The U20 was produced in two forms: a stock 135 hp (137 PS) version with twin SU carburetors, or a 150 hp (152 PS) version with twin Mikuni/Solex carburetors and a "B" model camshaft.