yes my X's are non nitrided. they seem much like the R5, but its got an extra boss on the front iron that interferes with the electric distributors. can't compare ports cause the X's are factory PP irons, and the R5's are super rare here. 17 tension bolts, which would make it like 1980, but non nitrided makes it pre 2/78? weird! we didn't get em here

FC housings are a different post, i'll have to go take pics too, its like alphabet soup. s4 and s5 di have slightly different ports. the center have different timings, between s4/s5 and na/t2. s5 and s5 t2 have slightly different port runners.

i don't know what mazda supplies now though

 

I've got three R5 engines here. I just took a quick look at one front plate. It's an "A". I'll check my two others later today.

 

thanks jeff,, didnt mean to steal this thread
just think getting down all these factory plate and exhaust port differences and combinations is important to have noted and down in pics if possible
- and entirely worthy of its own thread , of which your opening posts would make a great start

they have used the X plate as a PP part because the delete bolt/s will allow the MFR PP housings to be used
this X has delete bolt at the normal 3 oclock position as seen in rear R5 plates
( though in X including the front plate , R5 front not )
but also a bolt at the 12.30 position ( on the rear plate,, not the front )
R5= 18 bolts
X= 17 bolts

good note about the later dizzy pot clash
,, and actually its something that i had noted with some runs of some s4/5 NA and turbo plates,, and not others
( one of those running changes )

the notes you make about the evolution age of the X plate being pre 1978 in some qualities and post 80 in others is also something i had noted

it may be interesting to note that many 1st gen s1rx7 engines in aus
( here the definitions between the two s1/2 rx7 engines is blurred by jap import variations )
will still use the R5 front plate ( and that crap NO middle plate )
and be set up with the later chain and cogs ,, ignoring the mounts for the old tensioner
and this amounts to 18 bolts and nitriding


-- an example where the factory has continued to re-use a previously superseded plate


it seems the oz and jap s1 rx7 12a engine is R5/NO/R5
i think there is another N ( s2 rx7 ? )
and later a Y for s3 rx7
H for 6p 12a ( jap rx7, jap HB cosmo )
and [ maybe M for 12at ] ( jap s3 rx7 and HB cosmo )

however,, the exhaust port on SOME of these R5/NO/R5 rx7 12a's is later open and closed timed and also larger
this exhaust cut from the factory is curious as you can clearly see a smaller shape
( still slightly later open than some of the ( jap import ? ) 12a's )
and its been milled again by another machine to stay open even later
[ yes i think its factory , i have 3 pairs like this ,, and only one pair of the s1/s2 type housings with the little,, low port ]

---- as if there is actually 3 exhaust port profiles across the 12a SD range


i think we need a thread to lay out the foundations of all of these observations and to post up the pics
( sounds like i also need to make that degree wheel and get batteries for the good camera )
it would nice if it could be updated as people pull down factory engines to note the combinations
( much of my stuff is already stripped and most of it goes on distant recall as to what matched to what )

 

rotor housings are different again! they get simpler as time moves on though..

anyways, the PP has an X center plate too, its a short runner, the top of the iron is drilled an tapped for the coolant sensor like an rx2/3 has. i believe its got the air injection port under the center runners, like a catalyst engine though.

i did go thru the 79-80 parts catalog and here's what i learned. the 79 engines have 18 tension bolts and the 80 engines have 18. so the irons and rotor housings all have different part numbers, but it would seem that is really the only difference.

the 1980 california spec engine got an egr valve, and its essentially the same as the s4 fc, it sits on top of the center iron. it uses a CA only center iron and rotor housings which were always hard to get.

it seems like you can spot an SA engine due to the sharp corners of the irons, the later irons get more rounded.

ok so rotor housings. lets see you have to remember that in the US all of our cars had the big pollution control setup on it, so there is not a lot of variation in engine spec. for example if you could order new us spec parts the 76-80 12A is the same. ALL of them.

so rotor housings are simple.

the 71-73 12A got a 2090-23-050 (i think, its a 2090 part) the engine is a REAPS engine, even in 1970, single exhaust port, 80 open and 48 close (maybe a typo?) the later ones get the SIP process and have a TCP cast into the top.

74-75 12A is the big MAZDA font housing. 1881-10-100 for a 12A, 1757-10-100 for the 13B. for the US these are the big port housings, the FSM says 71 and 48.5. these engines also got the later closing intakes. then the fuel crisis hit...

76-78 12A13B, still TR, still black painted irons. these rotor housings are small font, the "mazda automobiles" ones. they get the 75 open and 38 close that the 12A keeps until 85.

79-80 SA engines, it just says mazda.

81-81 N201, looses the exterior ribs, the air injection is changed for the catalyst system.

84-85 N249, slightly stronger steel insert, its got some ribs over the spark plugs, and a little extra boss on the intake side.

84-85 we got the N304 rotor housing in the GSL-SE, which is similar to the cosmo re-egi engine, its the first US engine to get the oil metering nozzles.

i show 83 as an N231, but i don't think that's right.

the only other 12A housing we got was the N210-10-100, which is i think a euro part, its the same as the N201/N249 but has 10 degrees more port timing, so it closes @48. it was the "cheater" housing for the racers.

 

this is great stuff. cheers to you all.

Jeff ~
i really don't see your idle going to hell with what you have. even with my big streetports, i haven't had any idle issues thus far. when the car is cold (on choke) i start getting a subdued "brap-brap", but other than that they idle as smooth as stock - and that's with a carburetor and enlarged runners.

interestingly, i'm working on a Gen II engine right now that i streetported, but while i did modify the secondary runners, i just removed the flash on the primary ones, so when i read what you're doing with yours it really tickled me.

 

Looking great so far. Great thread.

 

12a housings,, i provide a 13bt stock housing as a datum comparison
an rx4 13b housing is also provided to show its the same timing EO and EC as the smaller 12a ports



these ones are curious,, later mazda font and with 12a immediately on top of the mazda
- without the H casting mark found on all s3 12a's
( maybe s2 rx7, N226 ?)

that is a stock 13bt housing to compare with
these housings are the only pair with large exhaust port but later EO,, same EC as the 13bt
( ie this port is large,, and further up than other 12a engines , and factory )

---all other 12a housings have 13bt /rx4 EO timing ----


small port ( same as rx4 ) ,, double cut port,, and a port same as 13bt ,, all 12a OEMs
-added with that large late port shown above
== 4 different timing variations of EO and EC from the factory for 12a


as many individually different OEM 12a housings i could get in a stack
stock 13b rx4 front,, stock 13bt at the rear to compare
---note the small 12a ports are same EO and same EC as the rx4 13b--
---note one of those little ports is clearly a s3 rx7 one
( H on top,, dimples near the plug ribs ,,no oil squirt hole ) - N249



same stack , outside view so you can gauge which font is which


closer view of the double nibble
this one is s3 12at ( has oil squirt )
and yes i cleaned it back to inspect for any porting
this is too straight , with no tooling marks that may have indicated it was played with aftermarket outside of the factory
-- at any rate its identical to some others ( with other fonts ) ,, this is factory

notes --
far end to close end , ignoring the 13bt start
-- 12at housing- double bite cut ,, and same EO and almost the same EC as 13bt
---the other s3 housing has the low,, small 12a port
--- next ,, modern s1 type font with a 10mm gap up to the 12a boss , large port,, same factory double cut
--- another enigma,, this one has the largest port,, 13bt timing on the dot , one clean factory cut
( still the 1 cm gap in the fonts ,, so s1 rx7 " large port" )
--- same as the ( s1 ) one beside it,, but little , low port ( US N201 ? )

so ,, conclusions
-- 4 different exhaust port profiles for 12a
low,, small port, same EO and EC timing as the rx4 13b example ( which sounds like many US ones )
large ,, double cut port,,rx4 13b EO almost 13bt EC
large high ports ,, late EO timing , 13bt EC
large port, 13bt EO and EC timing

note rx4 13b and 13bt have same EO ,, 13bt has a later EC

-- the port differences seem to be repeated across several different font styles so there is no rule of thumb


my guess is the double cut ones are factory modified maybe to fill orders for a non available large port housing

 

1881-10-100a/3625-23-050 74-75 12a rotor housings
8871-10-100/150 76-79 12a
8341-10-100/150 80 california spec 12a
n201-10-100 81-83 usa
n249-10-100 84-85 usa
n210-10-100/150 (81-85?) euro 12a housing
n236-10-100 12a turbo housings



trying to order these chronologically as i see them ( and clarify the jumble above )
note if there is front and rear specific housings
then the rear? housing has the /150 end prefix instead of /100

this is how i piece them together,, there may be some errors and point them out if you see anything

--all measured from a flat bench
( provided in a form for all those without a degree wheel )






1881-10-100
1974-75
original font
smooth around the spark plugs
EGR/ACV port is the original banjo bolt through the sump style
EO=
EC=


---------------------





8871-10-100
76-79 12a
note the new mazda font , but with a 10 mm gap to the 12a cast letters
in between,, a blank casting strip where perhaps " automobiles " has been hurriedly deleted from the casting
late EO port ( high port )
EO= 82 mm ( approx )
EC= 104 mm


------------------------

N201-10-100
early EC port ( small )
note casting font has evolved to delete the delete casting so to speak
- still 1 cm gap
EO= 78mm
EC= 97mm

----------------------

N210-10-100 ??
late EC port ( large )
EO= 78mm
EC= 104mm

------------------

???
double cut port ( medium )
EO= 78 mm
EC= 102mm

--------------------

N249-10-100
final font ,, no gap in the casting to the 12a moniker
dimples and small shrouds around the spark plugs
H casting on top
early EC small port
EO= 78mm
EC= 97 mm

-------------------



N236-10-100
final font
spark plug dimples and shrouds
double cut port
and oil squirt location = 12at
double cut medium port
EO= 78mm
EC= 102mm

----------------------

so far missing from this list


N225 1981 12a 6p
( with the 225mm clutch if manual and with the funny shaped little front counterweights of the late ones )
which is out of bunch chronologically as its actually a s3 type motor
that japan released while everybody else still copped s2 type housings ( rotors and weights )
they have the H housing casting, dimpled area at the plugs, but importantly one of the shrouds is segmented
inside the rotors are marked NF or NR,, a give away for s3 rotors

and N231 1983 rx7
which should be visually similar,, with final font and dimples with H

 

interesting... the 1881 looks right,

the SA i think is right, they usually have the mazda machined smooth like the attached pic

our US spec N201 looks just like the N249, except without the little webs around the trailing plugs and its missing the little nubbins where the oil injectors could go if it was all grown up.

the attached pic of the motor is an N249. an N201 would not have those little bumps where the oil injector sits on the later engines.

when you could get these new, we just ordered whichever was cheaper, the differences aren't visually apparent

the new rotor housing pics are the n236 turbo

i think i can explain why your n201 is different from ours, for the 1981 model year we got the lean burn catalyst engine, that japan got in 1980. if aus/nz stayed with the reactor, that explains it...