I looked at a used 12a local to me the other day and wanted to hear some other thoughts. Engine has no car but the transmission and starter were hooked up so I was able to perform a compression test. Since there was no carb/fuel we couldn't start the engine, so these are cold compression numbers. My understanding is you want to do the test when warm since they lose compression when warm, so maybe subtract 10-15 psi from each reading for a more correct result. I am close to sea level.

Rotor 1: 108, 104, 102 PSI (altitude & RPM corrected values), 106, 102, 100 @ 247rpm was the raw numbers.
Rotor 2: 93, 88, 63 PSI (altitude & RPM corrected values), 87, 82, 59 @ 231rpm raw.

I didn't have a chance to pull the exhaust header (intake manifold was also in place) which I really regret not getting a chance to see the rotor housings. I think those numbers are okay for rotor 1, but if rotor 2 looks to need a rebuild. If the rotor housing is bad then I think it makes more sense to buy a 13b to begin with seeing as how a good used 12a housing is probably $500, and a new one is $1200. Asking is $500 and it comes with the transmission (which I do think I will need eventually, my 2nd gear syncro's have seen better days).

 

One chamber being low could be a bad side seal.

I think $500 isn't too bad. You could probably recoup that selling the irons, rotors, and 1 good housing if the housing #2 is unusable.

Where are you finding new 12A housings? they're NLA as far as I know.

Don't forget that with the 13B you're also going to need a different induction system, though parts availability is better.

 

if you're getting a low compression 12A AND a tranny for $500, then get it. period! as j_tso said, you're probably looking at a stuck/jacked up side seal. you can't lose.
is it your intent to use the 12A to build a 13B? i'm not sure why you're saying this.

 

I shouldn't say new rotor housings, but new 13b housings that are milled down to 12a width. Mazdatrix has them for 1,175 and Atkins for 1,300.

Yes, a 13b would require a new intake, but I think you would have a better engine and part situation in the long run. I'd also like to move away from carbs one day so I am not against the idea. In the meantime though, I would use an adapter plate with a hogged out nikki if I were to go the 13b route.

Thinking about it more you guys sound correct about it being a side seal. If the apex seal was the issue it would effect the two compression chambers bordering it, not one.