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[Datsun 1200 encyclopedia]

A12GX Engine

From Datsun 1200 Club

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Category: Engine Mechanical

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.


Contents

Overview

Also see: Hitachi Twin Carb

A12 GX Engine
album

A12 GX engine - South Africa version
post

GX Engine - South Africa modified
album

Performance

A12 69 horsepowerSingle carb Datsun B110
A12T 80 horsepower Twin carbNissan Cherry E10
A12GX 83 horsepower Twin carbDatsun B110
A14 80 horsepowerTwin carbDatsun B210
A14 92 horsepowerEGIDatsun B310
reference: Performance

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).

1002.jpg 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.
  • A12T (twin carb) cylinder head from Nissan Cherry has a different part number but seems identical

Identification

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

  1. Extra-large oval ports 30x35 mm (larger than A14/A15 28x34 mm)
    18268.jpg
  2. 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.
    4441.jpg
  3. Heart shaped combustion chambers
    3795.jpg GX: Type B
  4. Extra hole drilled for rocker oiling
    18271.jpg
  5. Valves: 35mm inlet, 29mm exhaust (smaller than A14/A15)
  6. Combustion chamber: 29.1 cc (as per FIA papers)

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
    18258.jpg Dual-step valve spring seats

Valves

A12GX valves are the same size as standard A12, and smaller than A14 valves. But they differ from standard A12 in material.

  • Intake
  • Exhaust
  • Dual Spring (inner coil, outer coil)
    • Cylinder head is machined for dual-step spring seats
  • Different valve stem oils seals (Option on standard A12)
  • Valve retainers are same as standard A12

Engine Block

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

  • Pistons: Flat top pistons for 10:1 compression ratio on hi-gas models
    • These are same as Datsun 1000 coupe (A10 engine) and Nissan Cherry A10 pistons (same part number)
    • NOTE: some GX engines have standard A12 9.5:1 dished pistons
  • Camshaft: 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 (PIPE-suction, water pump) has an additional hose fitting for the manifold coolant hose.
    20648.jpg 11267.jpg 14429.jpg
  • PIPE-connector (breather pipe at rear of engine) is standard. Distinguish from Cherry block which has a right-angle pipe. Datsun 1200 pipe is straight out.

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

Block Ventilation

See main article: Block Ventilation

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:
    24761.jpg
  • 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.

Intake Manifold

A12GX vs A12T - Manifolds are nearly identical, but A12T has two extra fittings

A12GX
26609.jpgAlbum click to view

Center fitting (yellow arrow) is for IMS speed screw assembly.
24759.jpgAlbum click to view
Far left fitting (no arrow) is for power brake booster. NOTE: Early Nissan Cherry A12T may be missing this fitting

A12T/A12GX versus A14T port size
* A12T: 30x35 mm
* A14T: 28x34 mm
18268.jpgAlbum click to view

A14T identification
24760.jpgAlbum click to view

A12GX/A12T variations
5758.jpgAlbum click to view 14760.jpgAlbum click to view

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.
14558.jpg

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:
12996.jpg

Air Cleaner

See main article: GX Air Cleaner

18250.jpg

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

12371_4bb2a0bf36ee6.jpg

11086.jpg 568.gif 10728.jpg

12778.jpg 2585.jpg

NOTE: Nissan Cherry A12T has a similar exhaust manifold, but it is single out. The Datsun 1000 Coupe exhaust manifold is similar.

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 recommends the GX manifold for Rally competition use instead of extractors.

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

Distributor

GX has special distributor, but it is not dual-points. The GX distributor differs from the standard 1200 model in only two parts. These two parts are the spring for the mechanical advance weights & the cam [for the points]. Both of these parts are standard A10 [non Coupe] components.

It uses only one spring for the mechanical advance mechanism instead of two in the standard distributor.

The engine from the KB10 [Datsun 1000 Coupe] used a different spec distributor to all other A10 powered models.

Spring 22110- 18001 [One only req]
Cam 22132- 18005

NOTE: Part numbers in the xxxxx- 18xxx range are A10 engine or B10 car part numbers.

The distributor in the photo below is the factory competition [race engine] distributor.
18240.jpg

Rocker Cover

The Rocker cover is unique to the A12GX engine:
14415.jpg

  • 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)


17453.jpg
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

NOTE: A12T in Nissan Cherry has a unique rocker cover that is different from A12GX or standard A12. Anyone have a picture?

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
  • intake ports 30mm x 39mm oval 1.5 mm tolerance

reference: FIA Amendment March 1970

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 6 mm elongated section  ?
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

B210 GX car model came either with single-carb engine, or twin-carb engine.

Japan: Highest trim level
* Some GX models had the twin-carb engine (A12T or A14T)
* Other GX models had the single-carb engine (A12S or A14S)
USA:
* GX models had a standard A14S (single-carb 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.

Late B210 in Japan had a twin-carb A14T engine option and came with special GX-T badging.

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 and was not factory offering, but a racing/competition offering.

Late B210 (1976-1977) for the Japan market changed from the A12T to A14T (Twin Carb) engine. This had a regular oval port cylinder head and did not use the large oval ports of the A12GX Engine.

Part Numbers

See Main Article: A12GX Part Numbers

8995.jpg 14539.jpg 14541.jpg 14542.jpg

Photo Index

See main article: A12GX Engine Photo Index

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