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The Datsun A12GX Engine is legendary. While it may not be a powerful as an A14 engine, it was a stock package offered in Japanese Domestic Model Datsun 1200 GX Sedans and Coupes and was rated at 20% more horsepower than the Standard A12 engine. The GX engine featured a matched pair of Hitachi HJE38W carburetors, larger cam, giant intake ports, and streamlined exhaust for superior high-RPM power.

Also see: Hitachi Twin Carb engine
A12GX kit photo gallery

A12 GX Engine

A12	69 horsepower
A12GX	83 horsepower
A14	85 horsepower

reference: Performance

1 GX Cylinder Head
- 1.1 Identification
2 Engine Block
- 2.1 Block Ventilation
3 Induction
- 3.1 Carburetors
  - 3.1.1 Choke Linkage
- 3.2 Air Cleaner
4 Exhaust
- 4.1 Compared to Extractors
5 Rocker Cover
6 Specifications
7 B210 A12 'GX' Engine
8 A14 and A15 'GX' Engine
9 Part Numbers
10 Photo Index

GX Cylinder Head

The GX cylinder head has the largest ports of any factory A-series cylinder head. The ports are larger than A14/A15 ovals. This makes it suitable for extra-high RPM usage. However, the valves are standard A12 size, smaller than A14/A15 valves. The latter is a better head for regular-RPM usage (under 4000 RPM).

GX cylinder head and intake

 Main article: A-series_Cylinder_Heads

GX has intake ports shaped differently for flow and swirl, however it is said that a ported and polished standard A12 head with larger valves fitted flows about the same as a standard GX head. Also see head trivia.
The GX head bolts on an A14/A15, but is very similar to the oval port A15 head. However, the 1974-up heads provided oil to the rocker shafts in different locations so you may need to drill the oil passage in the newer block
There is apparently an A14 Oval Port GX head which flows more than the A12GX head.
reference: GX Head or A15
GX head uses standard A12 size valves. No bigger than stock. The Nissan A14 Competition head was used by Nissan for racing is similar but has the larger A14-size valves.
GX fuel pumps have a slightly stronger spring under the diaphram to boost fuel delivery pressure a smidgen.

Identification

To be a genuine GX head, it must have these features:

Extra-large oval ports 30x35 mm (larger than A14/A15 28x34 mm)
Four large-diameter water holes (5-6mm), one below each port (regular engines have three small-diameter, or none at all). These send coolant to the water-heated GX inlet manifold.
Heart shaped combustion chambers
GX: Type B
Extra hole drilled for rocker oiling
Valves: 35mm inlet, 29mm exhaust (smaller than A14/A15)

CAUTION: Some A12T heads also have the same features. How to tell the difference between Cherry Coupe (E10) A12T engine and A12 GX engine? Unknown. See discussion'Virgin Pone'.

Also note:

1200 GX heads usually (always?) have no casting numbers at all
A genuine head may have had the dual-valve springs removed. But it will probably still have the dual-step valve spring seats
Dual-step valve spring seats

Engine Block

The GX engine block is a standard A12 part. However, it has some extra parts attached to it:

Balanced crank
Flat top pistons for 10:1 compression ratio on hi-gas models (some GX engines have standard 9.5:1 dished pistons)
Slightly longer duration cam than stock 1200, for 6800 RPM usage (up 400 RPM over stock cam)
Water pump inlet pipe at front of block has additional hose fitting to recieve manifold coolant outlet hose.

The block crank and rods are the same as the standard A12.

Block Ventilation

The GX engine has a reverse flow block ventilation system, compare with the standard A12 engine. In the GX, the block breather hose goes to the air cleaner, while the rocker cover hose goes to the air cleaner.

Induction

The GX engine uses a twin-carb inlet manifold with extra-large oval ports to match the cylinder head.

See main article: Hitachi Twin Carb

Features include:

a matched pair of left- and right- hand Hitachi HJE38W carbs. These are variable venturi (SU-type), with 38mm bore (~1.5 inches)'
Oval port inlet manifold with water jackets to match up with th water hoes in the cylinder head
Intake manifold has 2 studs at the top of each pair of inlet ports (cylinder head matches this). Standard A12 has only one stud per pair of intake ports.
Manifold has provision for PCV valve:

It is reported that SU carburetors (H4 or HS4 1.5 inch carbs) will fit the GX manifold, with some linkage creativity.
The GX manifolds will fit an A14/A15 engine but port mismatch occurs
Fitting A14T manifold to A12 GX head will likely result in leaks since the GX head ports are higher up than the manifold ports.

Carburetors

The left-hand and right-hand Hitachi 38mm carburetors have a unique center linkage. They are 'dome-top' Hitachis, not the 'flat-top' models used in newer models like the B210.

The L-series twin-Hitachi engine linkage reportedly does not fit.

There is a stamped aluminum heat shield. The carb throttle return springs connect to a stamped hook on this heat shield.

Choke Linkage

18410-H2302 WIRE COMP,-choke control GX

The A12GX used a twin-cable Choke knob. However, you can use the stock single cable with a simple bracket on the carbs to operate both:

Air Cleaner

Exhaust

The GX engine has a dual-outlet cast exhaust manifold that bolts to twin head pipes which blend into into one pipe further down.

See main article: Exhaust Piping

Compared to Extractors

Word has it that the GX exmani works as good as common extractors (headers), though not as good as the best tuned-length extractors. The factory 1200/B110 Competition Tune-up manual recomends the GX manifold for Rally competition use instead of extractors.

NOTE: Many Datsun 1200 extractors may not clear the GX inlet manifold.

Rocker Cover

The Rocker cover is unique to the A12GX engine:

accelerator clip is on front pad, not rear
fuel line clip boss is on rear left side, not front left side
Datsun logo (not Nissan)

Arrows: Accelerator cable clip

GX: center feed accelerator cable
A12: rear feed accelerator cable

Circles: Left side threaded boss for fuel line

GX: rear boss
A12: front and rear bosses

Specifications

Nominal Rev Limit: 6800 RPM (instead of standard A12 6400 RPM)
Actual Rev limit: reported fine at 8,000, float at 8,150 rpm

references:

FIA Amendment March 1970

Datsun 1200 group 1 FIA recognition

Comparison
part	A12GX Engine	A12 Engine
Carb	Hitachi HJE38W side-draft (two carbs)	Carb Hitachi DCG306-1 down-draft
Carb Venturi	Variable	Primary: 26 mm, Secondary: 30mm
compression ration	10.0	9.5:1
combustion chamber	29.1 cm3	36.6 cm3 or 29.1 cm3
valve opens	BTDC 20 degrees +- 7 degrees	?
valve closes	ABDC 56 degrees +- 7 degrees	?
valve opens	BBDC 58 degrees +- 7 degrees	?
valve closes	ATDC 18 degrees +- 7 degrees	?
In. Valve dia.	29 mm 1.14 in	?
valve lift	9 mm 0.35 in	?
exhaust ports	27w x 29h +- 1.5 mm	?
intake ports	30 mm circle with 4.5 mm elongated section (30 x 34.5 oval?)	?
exhaust ports	26w x 28h with 3 mm radius +- 1.5 mm	?
center ex port	69w x 28h	?
intake ports	28/27 mm circle	?
Flywheel weight	?	9.5 kg
Flywheel with all turning parts	?	13.1 kg
hp	83ps @ 6,400 rpm	hp 68ps JIS @ 6,000 rpm
max rpm	6,800 rpm (80 ps)	hp 68ps JIS @ 6,000 rpm
max torque	10.0 kg-m at 4,400 rpm	9.7 kg-m at 3,600 rpm
max speed car	160 km/hour	150 km/hour

B210 A12 'GX' Engine

The B210 A12 twin-carb engine was designated A12T by Nissan. It was basically the same as the B110 twin carb (1200 GX Engine), but based on the newer cylinder block. The head was no longer drilled for the oil gallery that was unique to the A10 & 1200. It also used flat top carbs and the inlet manifold had provision for a few more fittings to allow compliance with newer smog laws. There was also provision for a vacuum takeoff for the brake booster.

See main article: Hitachi Twin Carb

The B210 A12T induction seems to be the most commonly encountered twin-carb A-engine setup in Australia.

The B210 A12T exhaust manifold was the same as the GX one, but the fuel pump was of course the newer type for the new cylinder block.

The air filter housing base plate was revised a little to accomodate more fittings too, but the main cover piece was unchanged, other than for the change in colour from red to blue. It seems that L series SSS air filters also went from red to orange.

A14 and A15 'GX' Engine

Datsun Competition referred to an A14 'GX' cylinder head, however, this appeared to to be a late A12T head fitted with the larger A14/A15 valves. This would be the best A-series cylinder head but was not factory offering, but a racing/competition offering.