Hey guys i need your help. I've been researching different cylinder head option for my a12..
This is pretty much the dilemma..

I have access to a A12T / a12gx head.
And also can get a a15 head.
Based from my research:
The a12gx has bigger ports but same valve dia as the stock a12.
And the a15 has smaller port than the a12gx but have bigger valves..

Honestly i would like to get the best of both worlds..
I was wondering if its possible to install a bigger valves on the a12gx?
Or would it be better togo with the a15 and just port it?

Any suggestions would be awesome!

Go for the larger valves installed on the GX head! You can't beat those ports and anyway porting would cost a fortune compared to the larger valves.

Sounds like its going to be a beastly little a12!

Would an a15 valves work?

H89 head beats the gx hands down but might need 76mm bores to clear the valves unless you can notch the block to suit them. GX ports are way too big for efficient midrange they fall apart there and only come on song above 4000rpm with the right induction and cam as the stock cam does nothing for down low and mid range.

Nissan went big ports even on their race L series heads then went smaller on the last heads due to the loss of midrange in all tunes. Same with the GX head.

Some rallye guys also took all my round port stuff as they have race all kinds of datsun A series setups in the past 20 years and love the torque opened up round ports make with bigger valves and high comp pistons.

Its all a balancing act there are many choices

Beastly A12's love the big ports. You've got to throttle them all over the place but that's the fun of it. Never had so much fun with my old A12 and when it was opened up it was an absolute pleasure, the a-series singing at high revs!

Bigger valves and you're pushing the power band further up the rev range, if this doesn't do it for you just run the stock GX head and upsize the valves later on down the track.

A12 have a nice short stroke, so they suit monster revs. Monster revs take big flow numbers, and cam/carbs to suit. It's well doable to see revs up around 9.5k. 5 o'clock traffic, forget it though.

As far as I know, I don't believe the H89 head requires any modification what so ever to fit the A12 Block?? Unless you go bigger valves. Plenty of A12's with H89 Head getting around with standard bore.

With turbo or boost a different thing (even Sykines turbo mainly previously stock H89 with stock bore A12 proved how good they are from factory for boost) but NA has one big issue when pushing the envelope for a powerful A12. Gordon Dobey (ex-nissan works mech) schooled a friend and myself some years ago now (around 1998 and it wasnt hard to do to 2 relatively young punks!) when we took my A12 and H89 head to him for some work. He enlightened us on the issues of valve shrouding and that the Gx head was better with its smaller valves and as Bart mentioned needs RPM to get them to work. Unlike the GX heads, the H89 on stock 73mm bores does suffer from shrouding which affects NA potential. The gx in this case is better with stock A12 bore providing the rest of the GX spec parts are used eg. comp, cam, induction, exhaust and ignition. Not many know or worry about shrouding but this is where bigger valves make little if less under the curve compared to well setup with less valve shrouding.

GX is the more exxy of the heads and most unfortunately are too machined to be useful down the track..............so make sure you dont just get a very machined one with little life in it left for further rebuilds or surfacing. This is the case with many speedway GX heads and why they are passed on more often than the usually popular high school tart.

On the other hand the h89 is just right from the start in every possible way for NA providing you up the bore, comp, cam it, right valve springs, in and out taken care of with a good tuner should be a good powerful street & weekend zesty donk.

Depends what your present and future goals are really. The good thing is that if you spend 120 for a reground cam, headers, ignition, carbs, match porting, valve springs and tuning you will have a different beast that later if you want more you only need to change to a better head, inlet manifold and tune or boost it with lower comp pistons or deck plate. Beyond that its a rebuild with bigger pistons or go larger block with higher comp etc.

The main things is to start with the necessities then aim higher by taking it step by step as a lot of people including myself have gone for the lot many times over and over again only to find small well thought additions are the most rewarding and less migranes also included.

With boost best take the advice of those who have done it and are still doing it, have a dyno or can help with any part of the build to avoid disaster and short lived results.

Well there you go. Beyond my scope of knowledge...all I was answering was that yes it will fit with no mods and is an improvement on its own however marginal it may be.

Up to you decide your goals and go from there.

Thank you for the response guys.
I am very sorry but I forgot one of the important factors which I should have included on my question..
My purpose with the set-up and what i have planned in my head..

ok so this is the story..

I have a spare a12 engine.
I pretty much want a set-up that I will use on track days and at the fine edge of still can be used on the street.

What i have in mind is:
a12 block with:
a14 crankshafts
a13 Con-Rod if I can find one. if not maybe modify a12
76mm pistons
Tomei Camshafts (Undecided specs)
Nismo Pushrods
Nismo double valve springs (if a15 head is used)
Nismo retainers
Yella Terra Rockers or Tomei Balnit rocker shaft (Opinions?)
Twin side webbers or Single side (depending on which one will give me more power)
Custom exhaust (will do it myself)
and I am currently researching for a 76mm Metal Head Gasket..

Head is my major concern..
with this setup.
should I still go for the a12gx head? or should i go with the a15?

Guys. if you see something wrong Or can suggest better setup with what I have in mind. please let me know.. I am very much hungry for more information with regards to building my a-series engine.. as I am more familiar with Honda B/d-series engines.

Thank you very much for your time.