im putting this question in the old school section because i think i may have a pre-rx-7 12A housing and i need some info. im starting another build due to recently re-finding a plethora of parts that i have accumulated over the years and forgot about.

the problem is this... i have 2 12A housings that have excellant chrome with almost imeasurable flaking. these 2 housings i have have some obvious, and not so obvious differences that may make them non-compatible with eachother.

here is a comparison pic.

obvious difference
1. raised vs contoured area around where "12A mazda" is cast
2. smooth vs ribbed outer "shell" of the housing
3. raised boss vs smoth area around sparkplug hole
4. square block vs. "H" shaped casting on the top of the housing


they both have the same spark plug location and same stock port timing on the exhaust, but the only difference im concerned about is the depth of the sparkplug holes. i saw what could have been an anomoly today, when i was spec-ing them out, around the spark plug area so i measured the depth of the spark plug seat to the inside surface or the housing and they were almost .2" different. so then to 100% confirm this, i ran a pair of NGK BUR8 sparkplugs into them and for sure, 1 is just shy of the combustion chamber surface while the other is abt 1/4" sunk in.

does anybody know which housing if from what generation 12A?
...and can i use these housings together in the same motor, as long as i use the sparkplug for whichever generation each housing is?

 

The "H" casted housing was in the later 83-85 12a that had the tall center port iron which appears to be the left one in pic#1.the right housing looks like early style 70's.

 

can they be used together? i belive both housings came from an obvious rebuild that was in my Aug '78 production '79 Rx-7, but that engine ran like butt for unknown reasons, which is why i pulled it years ago and tore it down. im just concerned about the sparkplug hole difference. what sparkplug did the early 12A's use?

as the trochoid shape and the important parts of the castings appear the same, i hope that as long as i make sure the sparkplugs reach equal-ish depth to the combustion chamber that they can be happy partners in my future 12A BP build

 

I might be wrong but I believe that neither of these are from the 70's seeing as how pre 1974 non of the housings said mazda on them, and 74-78? they said Mazda in that cool slant bold font. Maby 79 is the earliest?If anyone knows when Mazda started making housings with that font it would be nice to know.

 

Rx2fan I believe is correct...It looks like you have 79-80 housing and a 83-85 housing...in theory they should "play nice" together.

 

Here is the link to a thread for some 1980 housings...good pics for comparison

https://www.rx7club.com/1st-generation-rx-7-1979-1985-parts-101/1980-12a-housings-good-shape-obo-972281/

 

I believe74-78 will have the big Mazda lettering like the ones in this pic.

 

so it does appear that i have an S1 and an S2+ housing. i just checked NGK site and the plugs for all rx-7 12A's have a 19.5mm reach, the only difference being that 79-80 12A's used 3 electrodes and had a 1.05mm gap vs. the later 4 electrode/1.25mm gap. did mazda just quietly change how they machined out the plug holes? changine the relation of the plug elctrodes to the combustion chamber?

i remember reading through a Paul Yaw article on sparkplug experimenting and he was finding power & fuel economy increases by un-shrouding spark and bringing the electrodes/ground strap closer to the combuston chamber. would you guys think that the difference between the plug reach will equate to any real or noticeable difference in power/idle/drivability?

sorry in advance for all the questions, i just know that the older the generation of rotor head, the more backyard engineering/experimentation has been done

 

The housing will be fine together. The last Bridgeport 12A I built for myself I had one SA housing and one FB housing. I ran second gen leading plugs with a second gen leading coil (Direct Fire Mod) and FB 4 prong plugs for trailing. No problem

http://www.youtube.com/watch?v=09_ukjxh-zM