Building a 12a for a friend. Rotor weights?
03-08-14, 11:06 AM
#1
Building a 12a for a friend. Rotor weights?
After the success of my first build Ive decided to help a friend out. His 12a puked an apex seal smashing the front rotor tip and causing a 3 inch groove near the exhaust port on the housing. Everything else is in good shape and will be reused. These were the tiniest apex seals Ive ever seen! They measure around 4mm in height. No wonder one popped out.
Anyway, the engine is an 85, and appears to be 100% factory sealed before dis-assembly. The rear rotor weight is "A" and the bad front rotor weight is "E". Is this normal? I always thought that you could only be one letter apart? Or is that 13b talk? My other 83 engine has a "B" and a "C" rotor.
Should I just find a complete rotating assembly? Can I find any rotor 81-85 year "D" or "E" to replace the bad one?
Anyway, the engine is an 85, and appears to be 100% factory sealed before dis-assembly. The rear rotor weight is "A" and the bad front rotor weight is "E". Is this normal? I always thought that you could only be one letter apart? Or is that 13b talk? My other 83 engine has a "B" and a "C" rotor.
Should I just find a complete rotating assembly? Can I find any rotor 81-85 year "D" or "E" to replace the bad one?
03-08-14, 11:11 AM
#2
If you need any 12A rotors, let me know. I have a couple pairs. As far as the rotor letters, that is not common. they are supposed to be within 2 letters of each other. Just remember that anything goes with a C, but I have a habit of letter matching whenever I can.
03-08-14, 11:32 AM
#4
Get a cheap digital scale that measures up to 7000 grams. I used to always try to keep the letters the same or 2 apart but that only works if the rotors were manufactured in the same lot.
You can find two C rotors from different engines that are over 50 grams apart. Not good.
The best way is to weigh them.
Then you can ignore the letters if you're mixing and matching, as you must do now for your friend. Try to get around 10 grams apart but I've built one up to 28 grams apart and it seems to run without vibration except at that one rpm right above idle where the oil sloshes, but they all do that. Otherwise smoothness.
Make sure all oil and carbon, oh and springs too, are out of the rotors before you weigh.
This one might work for you, but look around on their site for a bit. The My Weigh KD 7000 Multi-Purpose Digital Scale - Black
You can find two C rotors from different engines that are over 50 grams apart. Not good.
The best way is to weigh them.
Then you can ignore the letters if you're mixing and matching, as you must do now for your friend. Try to get around 10 grams apart but I've built one up to 28 grams apart and it seems to run without vibration except at that one rpm right above idle where the oil sloshes, but they all do that. Otherwise smoothness.Make sure all oil and carbon, oh and springs too, are out of the rotors before you weigh.
This one might work for you, but look around on their site for a bit. The My Weigh KD 7000 Multi-Purpose Digital Scale - Black
03-08-14, 07:17 PM
#5
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Anyway, the engine is an 85, and appears to be 100% factory sealed before dis-assembly. The rear rotor weight is "A" and the bad front rotor weight is "E". Is this normal? I always thought that you could only be one letter apart? Or is that 13b talk? My other 83 engine has a "B" and a "C" rotor.
the 12A rotors are different front and rear, and the 83+ rotors are FN or RN for front and rear. the 76-82 rotors are just F or R, and the 12AT is TF and TR
03-09-14, 04:34 PM
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yessir! i need to double check this, but from the start of rotary production to the end of the FD (the Rx8 uses a different process), the rotors were cast in batches, and graded in batches, by weight, and rated A-E. when the rotors are picked, they follow the 2 letter guide, and then they balance the whole thing with the flywheel and counterweight. the tolerance they use is apparently pretty loose, although again i've never seen the spec in print, except that the FD spec is tighter.
every replacement rotor is magically a C weight. this is so when you rebuild a factory engine, the replacement is still in the 2 letter guideline.
the problem is that Mazda makes spare parts, so that you can have a rotor that fits an 83-85 12A that was made in 1982, and one that was made in 2007. both will have the same letter stamp, but really what are the chances that they weigh the same? does the letter stamp apply?
the other part of the problem is that this isn't really documented very well. i've read the bit in the shop manual, and i have a magazine with a tour of the factory, and it has a section about the balancing.
if you want to read the shop manual, this is the newest one i can find with the rotor info, and its from 1974.... http://foxed.ca/rx7manual/manuals/RX...4,%20Small.pdf page 1:12
every replacement rotor is magically a C weight. this is so when you rebuild a factory engine, the replacement is still in the 2 letter guideline.
the problem is that Mazda makes spare parts, so that you can have a rotor that fits an 83-85 12A that was made in 1982, and one that was made in 2007. both will have the same letter stamp, but really what are the chances that they weigh the same? does the letter stamp apply?
the other part of the problem is that this isn't really documented very well. i've read the bit in the shop manual, and i have a magazine with a tour of the factory, and it has a section about the balancing.
if you want to read the shop manual, this is the newest one i can find with the rotor info, and its from 1974.... http://foxed.ca/rx7manual/manuals/RX...4,%20Small.pdf page 1:12
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