What the seal actually does

A moka pot works on pressure. Water in the lower boiler is heated until steam pressure forces it up through the coffee bed and out into the upper chamber. The rubber seal sits between the two halves and holds that pressure in. When it hardens, cracks or takes a permanent set, pressure leaks out at the seam, the pot brews more slowly and the coffee comes out thin and unevenly extracted. A pot that has started hissing at the joint, dribbling at the seam or needing to be screwed together unusually tightly is telling you the seal is finished.

The filter plate is the perforated disc that sits directly above the seal and keeps grounds out of the finished coffee. It rarely fails outright, but it does clog with coffee oils and scale over years of use, so Bialetti supplies a fresh one alongside the seals. The set contains three seals precisely because they are consumables: one goes in now, two go in the drawer.

Choosing the right size

Take the old seal out of the pot and measure it across the outside edge. Match that figure to the chart below. If the old seal has already crumbled or been thrown out, measure the recess in the upper chamber where the filter plate sits instead.

The 3 cup and 4 cup pots use an identical seal, which is why they share a single variant here.

Which pots it fits

This set is made for the classic octagonal aluminium range, including the Bialetti Moka Express, plus the Dama, Break, Happy, Fiammetta, Moka Melody, Alpina, Moka Timer and Rainbow. It does not fit the stainless steel Venus or Musa, the Brikka, or the induction-compatible models, which use their own seal profiles. If the funnel basket in your pot is also warped or corroded, the Bialetti replacement filter funnel is sold separately in the same size range.

Fitting it

Unscrew the two halves, lift out the old filter plate and work the old seal free with your fingers or a blunt tool, taking care not to score the aluminium. Rinse the groove with warm water and no soap. Seat the new filter plate first, bumpy side facing up, then stretch the new seal around it so it sits flat all the way round. A seal that sits proud on one side will leak, so check it before screwing the pot back together. This walkthrough covers the awkward part, getting the old one out without damaging anything:

A note on grind

A fresh seal solves leaks, but it will not fix a channelling brew. Moka pots want a grind slightly coarser than espresso and finer than filter, roughly the texture of table salt, filled level in the funnel and never tamped. Pre-ground supermarket coffee is usually too coarse and too stale to hold together as a bed. A hand grinder like the 1Zpresso X-Ultra covers the moka range comfortably and makes far more difference to the cup than the pot itself does.

What’s in the box

• 3 × Bialetti rubber seal
• 1 × Bialetti filter plate