Just how bad can $400 13b be?

Seriously, I see it for sale with a 5spd trans too locally.

Guy says it's been kicked around the basement for awhile.... looks like he was building a drag b2200...

I'd roll the dice for 400 bucks if I could turn the motor.

There's many different flavors of 13B.

'74-78, '84-85, '86-88 and '89-91 nonturbo, '86-88 and '89-91 turbo (yes, the year range matters, they are different), FD twin turbo, Cosmo twin turbo, and the four or five different flavors of RX-8 engine...

They all have their specific idiosyncracies.  Anything pre-85 might have bad rotors and they do not tolerate high revs.  Anything '86-95 (well, technically '86-02) has the best internals and rotor housing chrome, but they tend to crack the lands out for the coolant seals, causing them to fail.  RX-8 engines are just goofy because people like to run them out of oil, and the side seals do not live very long because of the thermal stress of going over the exhaust port.  Also Mazda did not give them nearly enough oil injection.

If you can turn it, you can peek in the exhaust ports with a bright light and see if the rotors are scored up, too.

It should "chuff" out of the exhaust evenly for each face of the rotor (three per rotor, six chuffs total). You have to spin it briskly to get it to chuff.
 

You can get a cheap borescope that plugs into your phone and look down the intake/exhaust (or spark plug holes if the manifolds are attached) for chrome flaking.

Depending on how and how long it has been stored, the internal soft seals might have deteriorated.

I can tell y'all it's a NA 6 port.

Okay, that narrows it down to '84-85, or one of the two flavors of '86-91.

'84-85 is easy.  You see the boss with four holes at the bottom of the intermediate housing on the spark plug side?  (Actually it is three holes and a coolant drain plug, but anyway)  An '84-85 engine will have a very similar looking boss on the ports side.  Also, it will not have an EGR valve on the top of the intermediate housing, just blank iron.

'86-88 do not have the boss on the ports side, they mounted the engine on that side with three bolts going through the oil pan.  To tell if the engine is an '86-88 (9.4:1 compression) or a desirable '89-91 (9.7:1 compression, lightweight rotors) , again look for the presence of an EGR valve on top of the engine or a blanking plate if somebody's removed it - '86-88 had EGR valves and '89-91 did not.

There are other differences between the three engines (series 3, series 4, and series 5 RX-7, respectively) mainly with respect to spark plug placement and the location of the air injection ports at the intake manifold face, but the motor mount boss(*) locations and the presence or lack of an EGR provision are the quick identifiers.

* - Series 3 13Bs, aka GSL-SE engines, did not USE those bosses, they mounted the engine via the front cover like all other first generation RX-7s.  The bosses are a carryover from where this engine was originally used in the Cosmo and Luce, and where that intermediate housing was used in the 12A Turbo also in the Cosmo and Luce)

In reply to Knurled. :

86-88.

Thanks for the quick reference.

Okay, good.  That's also probably the most common.

Maybe 50/50 that it has a cracked coolant seal land, unfortunately the only fix for this is to replace the side housing(s) that failed.  (See: the 13B currently in my '81, living the K-Seal life until I yank the engine overwinter)  Rotor housings generally stay in good shape although they do tend to get a weird "spine" wear in the center over 200,000mi.  Easily fixed with some elbow grease and some sandpaper held over a dowel.  Rotors are USUALLY good (apex slots not hammered wide) unless it FODed itself, see Brett Murphy's post above.   If it FODed itself, the rotor and rotor housing on that side are typically garbage.

$400 for a nonrunning core is a decent deal nowadays.  If it runs and doesn't pump combustion gases into the coolant, all the better.  With this realistic outlook, you won't be disappointed.

If it does need a rebuild, there are many cheap ways to go about it.  Make your own O-rings and gaskets, that sort of thing.

How bad?  It can be useless junk.  The housing are wear items, and not cheap to replace, so to get it running well, can be thousands,

and as such  $400 is just a downpayment to get in the game.   

I haven't seen a worn out '86-up rotorhousing yet.  Side housings yes, from insufficient air filtration, but the rotor housings are way better than, say, the 12As that had housing chrome as the primary wear item.