The A12GX Engine is the one with the largest ports -- bigger than A14 or A15 -- and was rated at 83 hp. Only the A14E engine was rated higher

Here's one listed this month on Yahoo! Auctions for $6,500 USD
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  a GX beauty item.jpg (64.49 KB)


  b GX on a pallet.jpg (62.76 KB)


  c engine left side.jpg (62.40 KB)


  d GX with cast header and collector pipe.jpg (64.29 KB)


  e GX with cut-down starter housing.jpg (62.18 KB)


  f Hitachi Twin Carbs.jpg (64.32 KB)


  g overhead valve engine.jpg (90.28 KB)


  h NISSAN cast head.jpg (64.23 KB)


  i crankshaft and block breather net.jpg (64.65 KB)


  j GX-hi flat-top pistons.jpg (60.42 KB)

Very nicely presented engine - but it looks like it has round ports! Even if the inlet ports are oval, rather than round (which could be the case as none of the photos show the ports straight on) it doesn't look like a GX head, whch has oval inlet ports where the top of the ports seem to be the smae height or even slightly higher than the manifold studs

Well spotted there, heads are just enlarged round ports
My sambar kei truck with supercharger and 660cc makes 82hp
with just pulley change and bigger injectors and cost me
halve that pretend A12GX so for me 6500usd is a bit much.

Here's one that appears to be a created GX engine, rather than a factory one. But it's all period parts, and if it has the high compression pistons, totally equivalent

has a Cherry front-dizzy M08 block, but with competition carburetors. These are flat-tops from 1973 but without the float bowl vents that production A12T carbies had. They were sold by Datsun Competition outside of Japan

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  a twin carb A12.jpg (171.71 KB)


  b M08 block casting.jpg (142.15 KB)


  c Top twin carb.jpg (120.07 KB)


  d air cleaner.jpg (184.42 KB)


  e A12 NISSAN JAPAN.jpg (150.18 KB)


  f right stock.jpg (117.19 KB)


  g right spark delay.jpg (94.38 KB)


  h air cleaner base.jpg (145.60 KB)


  i left Alternator.jpg (165.68 KB)


  j exhaust.jpg (83.06 KB)

Late to the party but just wow on the $6500 price tag.

It didn't sell. Perhaps the price was too higher, or maybe the price was right and the 0 bids were for another reason?

The 2nd one with the flat-tops got 24 bids and sold for 201,500 yen, what is that, $2,015 USD?
https://auctions.yahoo.co.jp/jp/auction/j1165374226

I offered my A12GX bottom end for $100 when I was selling my kb10
I ended up giving it away but little did I know that next to an A12
it weighed almost halve less, so we pulled the sump to find it had
a knife edged crank and some skinny rods with the flat tops.
Later owner found rods where kawasaki rods and camshaft was hollow
which was he said could have been using gun drilling.
It spent its life at 10k rpm plus as it was ex-speedway but how
crazy the extent some of these oval trackers go for to get the last
bit of hp, when a bit of boost on a stocky similar to TopGear a12
with an SU can make double the torque and same race hp at normal RPM.
His used a 10 degree locked dizzy which today could be helped again
with a H2O/Meth control unit for more boost, so there is no need for
big money investment for a GX when a little A12.
Yet hard to even find that these days!

yes the GX short block is identical to regular A12 except for the pistons. It also uses a Suction Pipe with an extra fitting on it for the heated manifold