By Kent · Lock Depot Staff Writer

Order the wrong non-keyed knob for a door and you find out the hard way, usually when a tenant calls to say they cannot get back into their own bathroom, or a maintenance guy walks in on someone who thought the door was locked. The Arrow MK series covers four separate functions on the same Grade 2 cylindrical body, and two of them get confused constantly because both sound like "lock the door from one side." They do not work the same way, and putting the wrong one on the wrong door is a callback waiting to happen.

MK02 privacy function: locked from inside, opened from outside in an emergency

The Arrow MK02 is built for a door where the person inside is the one who locks it. Push the button on the inside knob and the outside knob goes dead until it is released, exactly like a residential bathroom or bedroom lock. Nobody outside can turn the knob to get in, but every MK02 ships with an emergency release on the outside face so a maintenance tech or supervisor can get in with a small tool if someone is unresponsive or a child locks themselves in. This is the function for a restroom, a private office, or any room where the occupant needs to control access for as long as they are inside it, and where someone else legitimately needs a way in if something goes wrong.

MK04 patio function: locked from outside, always free from inside

The Arrow MK04 flips the logic. The pushbutton lock lives on the outside knob, and the inside knob is always free, with no way to lock someone in and no way for the inside button to override it. This is built for a secondary entry, a patio door, a side gate into a mechanical yard, anywhere a building manager wants to secure the door from the outside on the way out, without ever creating a door that could trap someone inside during an emergency. If your local fire code cares about free egress on a given opening, and it usually does, the MK04 is the function that guarantees it regardless of what happens on the outside knob.

Two more MK functions worth knowing before you order

MK01 passage: no lock at all

The Arrow MK01 retracts the latch from either side with no locking mechanism at all. Standard hardware for corridors, closets, and common areas where a lock has no purpose. It is the cheapest and simplest of the four, and it is also the one people accidentally order when they actually needed a privacy function and did not check the model number closely enough.

MK03 communicating passage: independent locks on both sides

The Arrow MK03 is built for a door between two rooms that sometimes need to be separated and sometimes do not, like a shared wall between two hotel rooms or two offices. Each side can lock independently of the other, which neither the privacy nor the patio function allows.

What all four share, and where they stop being interchangeable

All four MK functions share the same body: Grade 2 cylindrical construction built to BHMA A156.2 Series 4000 and UL listed, a standard 2-1/8" bore prep with a 2-3/4" backset, fitting doors from 1-3/8" to 1-3/4" thick, non-handed so the same part works on a left or right hand door. The TA knob in 626 satin chrome is the most common configuration Lock Depot stocks across the line. What none of the four MK knobs do is meet ADA. If the door needs accessible hardware, Arrow's RL lever series is the ADA-compliant swap, same functions, lever instead of knob. Grade 2 is also the ceiling here; it is the right call for apartment buildings and light commercial work where a Grade 1 mortise or heavy-duty cylindrical lock is more lock than the door needs, but it is not the spec for a high-abuse institutional opening.

Before you order: check the prep, not just the function

All four MK bodies share the same 2-1/8" bore and 2-3/4" backset, so if you are replacing an existing MK-series knob the prep is already cut and any of the four will drop in physically. The mistake that costs a second trip is ordering the wrong function for a prep that is otherwise correct: a locksmith swapping a worn-out MK01 passage set on a door that was supposed to get a privacy lock in the first place will fit the new knob fine and still get the callback. Confirm the function against what the door is actually used for, not against what was already installed. If the existing hole pattern does not match 2-1/8" and 2-3/4", stop and measure before ordering; a non-standard prep on an older building sometimes needs an adapter plate rather than a straight swap.

Common questions

Can the MK04 patio function be locked from the inside at all?

No, and that is the entire point of the function. The inside knob is always free to open the door. Only the outside knob carries a pushbutton lock. If you need a door that locks from either side, the MK04 is the wrong function; look at the MK02 privacy function instead, or the MK03 if both sides need independent locking.

Is the MK02 privacy function secure enough for an exterior door?

No. Privacy function is designed to keep casual entry out while someone is inside a room, not to secure a building envelope. It has a documented emergency release from the outside by design, which is a feature indoors and a liability on an exterior door. Use a keyed entry or deadbolt function on any door facing the outside.

Does the MK series meet ADA accessibility requirements?

No, none of the four knob functions do. Arrow's own documentation is explicit that knob hardware does not meet ADA; for an accessible opening, order the equivalent function in the RL lever series instead.

What is the difference between MK01 passage and MK03 communicating passage?

MK01 has no locking mechanism on either side, ever. MK03 looks similar day to day but each knob can be locked independently, which MK01 cannot do at all. If the door needs to stay lockable from either room on occasion, MK01 will not do that job.

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