Which flush bolts can go on a pair of fire doors?

It depends on how the pair is used. Under NFPA 80 , where both leaves of a fire rated pair are needed for exit, both get labeled fire exit hardware. Where a pair is needed for moving equipment and its inactive leaf is not needed for exit, that leaf may take labeled self-latching or automatic top and bottom flush bolts, and labeled manual bolts only where the authority having jurisdiction approves them. The door's listing, the edition your jurisdiction adopted and the authority having jurisdiction decide what the opening needs.

Can manual flush bolts go on an exit door?

Only where the adopted code allows them. A door in a means of egress has to open without a key, a tool or special knowledge. The IBC allows manual bolts on the inactive leaf of a pair only in specific occupancies and conditions, and then only where that leaf has no knob, panic hardware or similar operating hardware. NFPA 101 generally requires each leaf needed for exit to release on its own, or approved automatic flush bolts arranged as it specifies. See NFPA 101 and IBC Chapter 10 .

Flush Bolts and Strikes

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On a pair of doors, one leaf usually stays shut while the other is used, and the active leaf latches into it. Flush bolts hold that inactive leaf in place. They are set into the edge of the door, flush with it: a top bolt projects into the head of the frame and a bottom bolt into a strike in the floor or threshold.

Manual flush bolts are thrown by hand. Automatic flush bolts extend on their own when the active leaf closes against the inactive one and retract when it opens, so the pair also needs a coordinator, which makes sure the inactive leaf closes first. Flush bolts are made for wood doors or for hollow metal doors, and the door has to be prepared for the one you choose. A dust proof strike in the floor has a spring loaded plunger that rises to fill the strike hole whenever the bolt is up, so dirt cannot collect in it.