A set screw applies pressure. It does not locate anything.
Almost every guide opens with “tighten the screw under the cap”, and for one of the three faults below that is exactly right. The trouble is that all three feel the same in the hand, and two of them are not fixed by tightening at all. The reason sits in what the screw actually does: it presses the handle sideways against a flat machined on the spindle. The screw supplies force. The flat supplies position — and the two are separate jobs, which is why a screw that is perfectly tight can still let the handle rotate.
So the hinge is the fit between two shaped surfaces, not the screw. In plan view the spindle is a circle with one side cut off — a D. The screw tip lands either on that flat or on the arc, and the difference is everything. On the flat it is area on area: lots of contact, high friction, and it stays where you put it. On the arc it is a point on a curve: the screw touches along a line, the handle cams over it under load, the screw works back out, and a week later it is loose again. “I tightened it and it still comes loose” is not a bad screw. It is a screw sitting on the round.
Two joints, and one finger tells you which
Hold the lever and rock it, while a finger of the other hand rests on the ring of trim at the base — the collar where the handle meets the body, right above the retaining nut.
- The trim stays still and only the lever moves. That is the handle-to-cartridge joint: the set screw, or the flats where the two parts grip. The cartridge is not moving, the water still switches normally, and none of this needs the water off. This is joint A.
- The trim moves too, and the whole stack rocks as one. That is the cartridge-to-body joint: the retaining nut has backed off, so the entire cartridge is floating in its seat. The lever feels firm relative to the cartridge — people describe this as “the whole tap is loose”, and it is a completely different repair: water off, handle off, a spanner on the nut. This is joint B.
- The lever turns but the water does not change, or it turns freely past its stops. The grip has gone: the flats have rounded, or the splines have worn to nothing. Tightening here does damage rather than good.
That one test is worth more than every product on the shelf, because it splits a free five-minute job from a job that starts with shutting the water off — and because the wrong choice is not merely wasted effort. If you attack a worn grip by tightening the screw, the screw tip is pressed into a thin-walled hub with nothing behind it. It dents the inside of the handle, the dent becomes a ramp, and the handle will never hold again. A ten-dollar handle becomes a handle you must replace.
Three faults, and the exact point where they diverge
The screw has backed out. It is the most common by a wide margin, and it is free. Every time you use the tap you apply a little lateral load to a small screw, and after a few thousand operations it has turned itself out half a turn. That is not a defective fitting; it is what a friction joint does. The fix is thirty seconds plus one thing people skip: turn the handle to the closed position before you tighten, and press it fully home so the screw lands on the flat. Tighten at the open position and the handle ends up clocked round, the red and blue index no longer match reality, and the screw may be sitting on the arc.
The retaining nut has backed off. Same mechanism, different part, and this one is holding the cartridge down against a seal. So it leaks as well as moves — damp at the base of the trim, or a slow drip that appears only when the tap is on. The nut wants to be snug, not tight: over-torque it and you crush the seals on the cartridge body, the handle goes stiff and slow to return, and wear accelerates. That is a real and common self-inflicted fault, because tightening that nut is the intuitive response to a weep. Before the handle goes back on, turn the bare stem with your fingers — stiff means back the nut off a sixteenth of a turn and try again. That is the only moment you can judge the torque without the handle in the way.
The grip has worn round. The screw is already down as far as it goes and the handle still moves, or it spins without operating anything. Two surfaces are involved and only one is usually worn: the hub's internal profile, or the flat on the spindle. A sound spindle with a rounded hub means a new handle, ten to thirty dollars. A spindle that has also been chewed round means a new cartridge. Replacing the wrong half repeats the whole job. And “universal handle” is a marketing phrase: spline count, spline angle, shaft diameter and flat width all vary, so count the teeth or measure the shaft, or photograph the old part and take it with you.
Two details that decide whether it stays fixed
Use the metric key, and start small. Most set screws are 2.5 mm; 3 mm covers most of the rest. The imperial sizes people reach for are 3/32 in — about 2.4 mm — and 1/8 in, about 3.2 mm, and both almost fit. An almost-fit key is the number one cause of a rounded hex socket, and a rounded socket turns a two-minute job into cutting a slot with a saw or replacing the screw. Try 2.5 first, press it in, and check it does not rock before you apply any force. And plug the drain first: a 3 mm screw that gets away from you has an uncanny talent for finding the trap, and then you are not fixing a handle any more. Second detail: a threadlocker, not glue. If the same screw keeps backing out, one drop of removable (blue) threadlocker stops it. Never a permanent grade, and never adhesive on the mating faces — the next person to need that handle off will have to destroy it.