Three neat projects
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Joined: Nov 2001
Posts: 2,284
Thankyou for posting the pictures!
X looks just like Y castings with 17 tension bolts, less iron, probably pretty light compared to R5 and 3B castings.
Cosmo plates have primaries that appear to have similar cross sectional area to NO ports but are much more slanty. I'm sure Mazda did this to update the port timing and provide better torque at low RPM. Or maybe they work better with the turbo? Less overlap etc. The secondaries look like FD or T2 ports. You say they're similar in size to 74?
One of these days when I rebuild my 20B I'll lay my '74 template over the secondary to see what differences exist.
X looks just like Y castings with 17 tension bolts, less iron, probably pretty light compared to R5 and 3B castings.
Cosmo plates have primaries that appear to have similar cross sectional area to NO ports but are much more slanty. I'm sure Mazda did this to update the port timing and provide better torque at low RPM. Or maybe they work better with the turbo? Less overlap etc. The secondaries look like FD or T2 ports. You say they're similar in size to 74?
One of these days when I rebuild my 20B I'll lay my '74 template over the secondary to see what differences exist.
Thread Starter
Senior Member
Joined: Nov 2001
Posts: 2,284
X might be the predecessor to Y. It makes sense alphabetically. Ever seen a Z casting? 
Not I.
Interesting how Mazda stayed with the heavier R5 castings into the '80s, that were nitrided, for the J-spec 13Bs. They had the same port timing as US-spec Cosmo and RX-4 from '76 to '78 and all 12As from '76 to '85. The castings were updated but still heavy. They had electronic ignition with the eyebrow semi-circle spark plug shroud of aluminum around the spark plug holes requiring the use of the longer electronic ignition spark plugs like BR7EQ (one step hotter than 1st gen RX-7 BR8EQ-14), high volume 17.5mm oil pumps, no chain tensioners, shorty style oil filter pedestals with the 10mm nuts beneath the iron. No oil pressure threaded hole, which can be anoying to work around. Our 1st gens didn't get the high volume oil pump until '83 when they redid the 12A with lighter rotors etc.
So X in not nitrided. Are the intake ports the same timing and shape as 3B '74-'75? It looks like they were cast for peripheral ports or filled in. Maybe X stands for experimental.
My camera ran out of juice too. I'll try to get a picture of FD vs 3B and FD vs S5 T2. Also FD vs Pineapple Racing in an R5 plate which looks to be an extended streetport.
Interesting how Mazda stayed with the heavier R5 castings into the '80s, that were nitrided, for the J-spec 13Bs. They had the same port timing as US-spec Cosmo and RX-4 from '76 to '78 and all 12As from '76 to '85. The castings were updated but still heavy. They had electronic ignition with the eyebrow semi-circle spark plug shroud of aluminum around the spark plug holes requiring the use of the longer electronic ignition spark plugs like BR7EQ (one step hotter than 1st gen RX-7 BR8EQ-14), high volume 17.5mm oil pumps, no chain tensioners, shorty style oil filter pedestals with the 10mm nuts beneath the iron. No oil pressure threaded hole, which can be anoying to work around. Our 1st gens didn't get the high volume oil pump until '83 when they redid the 12A with lighter rotors etc.
So X in not nitrided. Are the intake ports the same timing and shape as 3B '74-'75? It looks like they were cast for peripheral ports or filled in. Maybe X stands for experimental.
My camera ran out of juice too. I'll try to get a picture of FD vs 3B and FD vs S5 T2. Also FD vs Pineapple Racing in an R5 plate which looks to be an extended streetport.
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