Just Picked up a 78 12a
09-19-11, 08:42 PM
#1
Just Picked up a 78 12a
Hey I just bought a 12a motor that used to live in a 78RX3. I makes 6 strong chuffs when rotated by hand and looks to be in very useable condition. It has a single distributor, not sure if it is converted from twin dizzy or not. Also has a stock carb and manifold. Looks like a Hitachi style carb. Def not a Nikki. I know most of the 12a motors interchange pretty well 79-85, but this is the earliest motor I have had. Anything special I need to know??
09-19-11, 10:58 PM
#3
I know 74-85 were single dizzy. The 78 engine will not have nitriding so it's not hardened. The oil seals will be worn. The oil pump will be small. The thrust assembly could be a bit worn (due to low oil volume = higher wear). The eccentric shaft will have a blind plug - check it for oil leakage. rotarydave2006 is correct about the intermediate plates having various port sizes, however a US-spec 12A will have tall runners. All four ports will be the typical 76 to 85 12A size. There are no nice 74 spec ports in there. The exhaust ports will be of the opening early and closing early variety, similar to the US-spec RX-7 12As. The housings should have thermal reactor holes like US-spec 79-80 12As. The points dizzy should be similar as well.
What I'd do with it is tear it down, inpect it, swap the side palte for nitrided Y or R5 plates, change the oil seals and throw in some new o-rings. Also update the corner springs with the NF01 (so called FD corner springs) but only, and I mean ONLY do this upgrade if swapping to nitrided side plates. I recently saw the aftermath of what these NF01 corner springs will do to a non nitrided side plate. Deep wear! An alternative are the RX-8 corner springs. They can be found in the RX-8 section of Mazdatrix' website. Or you can find them at Atkins being sold as FD corner springs for the cheaper RX-8 price. You've been warned.
As for apex seals, use them if they're still good enough. At least get new apex springs. Also get new side springs; they're very cheap.
To use that engine in a 7, you'll need a 7 oil pan. To use it in your GLC, I've gotten away with an R5 13B pan the same shape as any REPU or Cosmo or RX-4. I assume the RX-3's pan shape would be compatible as the engine bays are similar to the GLC.
Hmm, what else? Oh the flywheel is going to be 30 pounds and the pressure plate will be 215mm. I doubt a 225mm disc can be fitted to it,. However, if you're tearing the motor down, you can swap to an 81-82 front counterweight and the nicer 23 pound 81-82 flywheel. I suggest swapping the front CW here because the 81-82 cars had a different one compared to all 12As before, and I don't nwo whether you'll run into vibes if you don't. Also be sure to check end play and adjust as necessary to get it to around .002" to .003". Also throw in an 83-85 "high volume" oil pump.
What I'd do with it is tear it down, inpect it, swap the side palte for nitrided Y or R5 plates, change the oil seals and throw in some new o-rings. Also update the corner springs with the NF01 (so called FD corner springs) but only, and I mean ONLY do this upgrade if swapping to nitrided side plates. I recently saw the aftermath of what these NF01 corner springs will do to a non nitrided side plate. Deep wear! An alternative are the RX-8 corner springs. They can be found in the RX-8 section of Mazdatrix' website. Or you can find them at Atkins being sold as FD corner springs for the cheaper RX-8 price. You've been warned.
As for apex seals, use them if they're still good enough. At least get new apex springs. Also get new side springs; they're very cheap.To use that engine in a 7, you'll need a 7 oil pan. To use it in your GLC, I've gotten away with an R5 13B pan the same shape as any REPU or Cosmo or RX-4. I assume the RX-3's pan shape would be compatible as the engine bays are similar to the GLC.
Hmm, what else? Oh the flywheel is going to be 30 pounds and the pressure plate will be 215mm. I doubt a 225mm disc can be fitted to it,. However, if you're tearing the motor down, you can swap to an 81-82 front counterweight and the nicer 23 pound 81-82 flywheel. I suggest swapping the front CW here because the 81-82 cars had a different one compared to all 12As before, and I don't nwo whether you'll run into vibes if you don't. Also be sure to check end play and adjust as necessary to get it to around .002" to .003". Also throw in an 83-85 "high volume" oil pump.
09-20-11, 08:36 AM
#4
Thanks for all the info! I really don't know what I will use this motor for. To tell the truth...I was just geeked to find an older 12a and was hoping they were loaded with goodness like the older 13b 's seem to be. I see now this is not the case. O well. I will prob take it apart as suggested and build it to more reliable specs. Thanks again for the tips.
09-20-11, 01:16 PM
#5
Oh by the way if you do swap to an '81-82 flywheel, a 225mm disc will fit, just barely. 
and with RB's long pressure plate bolt set, you can fit a 225mm pressure plate. Lots of grip, light stock flywheel! You can do the same on the older 30 pounders but not worth it if you can't fit a 225mm disc. You'll need to test fit stuff if interested in pursuing this.
and with RB's long pressure plate bolt set, you can fit a 225mm pressure plate. Lots of grip, light stock flywheel! You can do the same on the older 30 pounders but not worth it if you can't fit a 225mm disc. You'll need to test fit stuff if interested in pursuing this.
09-20-11, 04:05 PM
#6
Thanks Jeff...all of this flywheel stuff made me go and check my parts bin. I have a VERY heavy flywheel with no dowels and then I have a much lighter flywheel with dowels. I am sure I have to front CW to go with each, but it might be tough to tell them apart. It would be REALLY nice to score a light weight steel unit from racing beat, but I am having a hard time parting with the 400+ dollars when there are so many other things I could using that money for.