The original design for what was to become the G series engine came from a 'new' OHV engine design which replaced the long running sidevalve engine used in a range of Rootes Group (UK) cars.
The Rootes Group made cars under the Hillman, Singer, Sunbeam, Alpine and Humber brands. The post WW2 Hillman Minx MK1 through MK7 cars used the existing pre war side valve engine from 1945 through to 1954 then in 1955 they released MK8 the all new OHV engine design, the beginning of the G series engine as we know it. These first 1955 Hillman OHV engines were cast iron blocks and heads with non cross flow iron heads, they were only made in a 1390cc version.
Isuzu Japan were primarily manufacturers of commercial and heavy equipment until they ventured into passenger vehicles buying the rights to produce a Japanese version of the 1953 Hillman Minx the Mark 6 known as the PH10. The Mark 7 badged as the Isuzu Hillman Minx PH11 in 1954 then in 1955 they released a new model PH12 (Hillman Mark which used the new Hillman 1390cc OHV engine.
In 1956 Hillman released a completely new body design known as the Audax Hillman which was also carried evr to the Isuzu range as the PH100 model.
In January 1958 Isuzu introduced a new model, the "Super Deluxe", at a higher price than the Minx. Then in March 1958 they introduced a more basic "Standard" model (designated PH50), at a lower price. The Standard model replaced the ‘mid-range’ Minx, leaving Isuzu with Standard and Super Deluxe Models. Then in August 1958 the range was face-lifted to resemble the Series 2 Hillman Minx (this model was designated the PH200. At the same time, power was increased from 46–50 bhp (34–37 kW; 47–51 PS) in the Minx and to 55 bhp (41 kW; 56 PS) in the Super Deluxe. The car was also now rated to carry six passengers instead of five.
In October 1959 the Isuzu Minx was face-lifted again to resemble the UK Series 3 Hillman Minx (model designation PH300). At this same time it gained the enlarged (1494 cc) motor. There were now two versions of the engine; the Standard with 60 bhp (45 kW; 61 PS) (7.5:1 compression ratio) and the Super Deluxe (62 bhp (46 kW; 63 PS), 8.5:1 CR). They were face-lifted again in October 1960 (model PH 400) to resemble the Minx Series 3A - except that the Japanese models retained the smaller windshield from the earlier Minx. There were further minor revisions in October 1961 and October 1962. In April 1963 another minor revision was made in which power was lifted to 68 bhp (51 kW; 69 PS) (Standard) and 70 bhp (52 kW; 71 PS) (Super DeLuxe). Revisions after October 1960 were not given new model designations and continued to be called PH400. Isuzu phased out the Hillman Minx in June 1964 at which time 57,729 Isuzu Hillmans had been made.
There was very little change to the appearance of the cars from October 1960, the last model still resembling the Series 3A Minx. The Isuzu Minx retained the 1494 cc motor until the end, never receiving the 1592 cc motor that was released in the Series 3C Hillman Minx in 1962. They were fitted with 4 speed, column-shift manual transmissions and drum brakes right until the end. Nor did Isuzu ever build the revised body shape of the Series 5 Hillman (introduced by Rootes late in 1963). But while Isuzu did not change the appearance of the Minx, they did make changes to the specifications, particularly on the Super Deluxe models. These became progressively better and better equipped than their UK Minx counterparts. Initially this meant full instrumentation (including ammeter, oil pressure gauge, clock) and a radio. A lockable glove box was added and such features as a handbrake warning light and reversing lights. Self-dipping headlights were also offered as was a boot-mounted air-conditioning system
The Isuzu Bellel - was introduced on 11 April 1961. The Bellel was a conventional 4-door sedan which certainly owed little to the Minx in terms of styling (it was similar to contemporary Farina designs like the Fiat 2300). Larger and heavier (1,190 kg (2,624 lb)), and equipped with a 2-litre (1991 cc) engine, the new Isuzu was a size above the Minx and did not replace it.
The true Minx replacement was the Isuzu Bellett. Introduced in June 1963, the Bellett was actually slightly smaller than the Minx, and the styling was much more low-slung. The Bellett was available in "Standard", "DeLuxe" and "Sport" variants, with both 2 and 4 door bodies and came with a 1471 cc petrol motor or an 1800 cc diesel. At the same time Isuzu released the Bellett Express estate with 1300cc and 1500cc engines. The Bellett Express, while looking completely different from the Hillman Express, had similar features like two doors, side hinged tailgate and sliding rear passenger windows. Isuzu did not drop the Minx as soon as the Bellett was introduced; they continued to produce it until mid-1964.
These Isuzu engines were known as the G series petrol engines and C series diesel engines, the basis for the engines that went on to power the whole range of Isuzu built passenger and light commercial vehicles. The Isuzu G engines were redesigned with both alloy twin cam crossflow OHV engines as well as alloy non crossflow OHV models then they were again redesigned with crossflow OHC alloy heads. These G series and C series engines types carry common design features. Pistons 1 & 2 and pistons 3 & 4 are paired with a wider gap between 2 and 3.
Later 4Z petrol and 4F diesel engines were evenly spaced pistons but all carried over common G and C series features such as crank journal sizes and engine bore sizes.
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