Gil

The practical conclusions, in order of how much difference they make:

Buy whole beans and grind before brewing. This is by far the largest single improvement available to anyone buying supermarket coffee, and it costs the price of a hand grinder.

Look for a roast date rather than a best-before date. Roasters who care print the roast date. A best-before date eighteen months out tells you the producer is not thinking about freshness, and that is itself information.

Buy smaller quantities more often. A large bag that lasts two months is stale for the second month.

Store it sealed, cool, dark and dry — and not in the fridge, where it picks up moisture and odours. The freezer works for long-term storage of unopened bags, and repeatedly taking a bag in and out is worse than a cupboard.

If cleaning does not fix it, the next candidates:

Calibration has shifted. Most grinders of this type have an adjustable zero point — a way to reposition the burr carrier relative to the scale. If the mechanism has moved, the whole range shifts, which matches your description of running out of adjustment at one end. Recalibrating is a documented procedure for most models and takes ten minutes.

Burr wear. Real, and slower than people think — burrs last a long time in home use, typically measured in hundreds of kilograms of coffee. Worn burrs produce a broader spread of particle sizes rather than a clean shift in one direction, so the symptom is usually muddy extraction rather than a sudden change. Given that yours changed over a few days, and that the machine is second-hand, worn burrs are possible but not the first suspect.

A loose adjustment collar. Some designs can drift under vibration if a retaining screw has loosened.

Start with the simple things, in this order, because the expensive explanation is the least likely.

Buildup between the burrs. Coffee oils and fines accumulate, and a compacted layer on the burr surfaces effectively increases the gap. This is the most common cause of a gradual or sudden coarsening, and it is free to fix: take the top burr out and clean both burrs thoroughly with a stiff brush. Not a damp cloth — dry brushing, then blow the chamber out.

A burr not seated correctly. If the grinder has been apart, the removable burr must be fully seated in its carrier. A burr sitting on a fragment of coffee or not clicked home sits proud, which coarsens everything.

Something in the chamber. A stone or a foreign object from the beans, jammed between burr and housing.

On these grinders the numbered scale is a relative reference rather than an absolute measurement, so "the same number" does not guarantee the same gap after any disturbance.

Two pieces of received wisdom worth correcting while we are here.

Dark roast does not have meaningfully less caffeine. Caffeine is quite stable at roasting temperatures. There is a small effect because beans lose mass as they roast, so darker beans have marginally more caffeine per gram and fewer beans per scoop — which means measuring by volume versus by weight changes the answer more than the roast level does. Weigh, and the question largely disappears.

Species matters more than anything about preparation. Robusta contains roughly twice the caffeine of arabica. A blend with a high robusta content — common in supermarket espresso blends and in a lot of instant coffee — is a genuinely different drink in this respect, and it is the one variable that can double your figure without changing anything you do.

Two maintenance points that matter more than replacement scheduling.

Do not put the pot in a dishwasher. Detergents attack aluminium, dull the finish, and accelerate corrosion of both the body and the plate. Rinse with hot water, and use a brush rather than anything abrasive.

Check the safety valve. This is the part of a stovetop pot that people genuinely neglect, and unlike the filter plate it matters. It should move freely. If it is crusted with mineral deposit and cannot release, pressure has nowhere to go — which is the one failure mode on these pots with real consequences.

A periodic check that the valve pin moves, and a descale if the water is hard, is worth far more attention than the filter plate ever needs.

If it still is not enjoyable after all that, the honest options are to use it differently rather than to keep fighting it.

Very dark roasts work far better with milk — the fats and sugars round off exactly the harshness you are describing, which is why this roast level is popular for milk drinks. They are also good for cold brew, where the long low-temperature extraction leaves most of the bitter compounds behind, and a large bag is exactly the right quantity for that.

A kilogram is a lot of coffee to dislike. Cold brew would get through it pleasantly.

One thing worth checking while it is apart: whether the mesh is still flat and the spiral plate still springy.

A press filter that has been forced or scrubbed hard often ends up slightly domed, and then it never seals against the wall of the beaker again. That produces grounds in the cup regardless of how clean it is, and people blame the grind.

Replacement filter assemblies are cheap and are sold for most models. If yours is distorted, that is a better use of an afternoon than cleaning it.