87 mazda rx7?????????????

Yay, a rotary question! The apex seal is a spring loaded piece of metal that sits in a groove at the apex of each lobe on a rotor of a Wankel rotary engine. The rotor is a triangle shaped engine component equivalent to a reciprocating engines piston. It resides inside the engine in a section called the rotor housing. Therefore the apex seals are at the tips of the rotor, inside the engine. They seal the tips of the rotor like a piston ring. I have to guess you're looking at an RX7 being sold advertised as having stuck apex seals. Stuck apex seals are the product of carbon buildup inside the engine. The seal gets wedged in its groove and no longer seals the periphery of the rotor housing and the rotor, making the engine run poorly, if at all. The remedy is either engine rebuild, or trying to clean the carbon by taking off the exhaust manifold and cleaning the apex seals with a brush. I've also heard that some Automatic Transmission Fluid poured in through the spark plug holes can help in freeing stuck apex seals.
 
on a rotary engine, it does not use pistons. instead the rotor, a triangular mass with rounded sides, rotates in an oval like section of the engine. in this rotor you will find 3 apex seals on each well apex. a two rotor engine like your is, has a total of 6 apex seals. this is also the weak point of the engine really, if you overheat it, the engine will run poorly until those seal are replaced.
 
It seals part of the combustion chamber and you don't want to go anywhere near that engine. Take the engine out and send it to a rotery engine rebuilder or it will never work properly again.
 
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