If a machine's spec sheet or a forum reply says it has an "E61 group", that single fact tells you a lot — including its portafilter size. Here's what the E61 is, why it's shorthand for 58mm, and how we treat it on this site.
The E61 is a brew group design — the chromed, angular head the portafilter locks into — introduced by Faema on its E61 machine in 1961 (named for the solar eclipse that year). Its defining trick is a thermosiphon: water continuously circulates from the boiler through the heavy brass group, keeping it at brewing temperature so the metal itself stabilises the shot. It also gives a mechanical pre-infusion as you lift the lever. Because the design was never patented into exclusivity, dozens of manufacturers build machines around an E61 group to this day.
The E61 group is built to the 58mm commercial portafilter standard. So when a maker states a machine has an E61 group — even if the spec sheet never prints a portafilter diameter — the 58mm size follows from the standard. That inference is exactly how we handle these entries: the manufacturer confirms the E61 group firsthand, and 58mm comes from the E61 standard rather than the maker's own page.
Because that last step leans on the standard, not a manufacturer's printed number, we mark these entries "measured" rather than manufacturer-verified — an honest, documented inference rather than a firsthand spec. See how we verify for what the confidence tiers mean.
The tells: a tall, chromed, angular group head bolted to the front; a shot that starts on its own as the machine reaches temperature (the pre-infusion); and a maker's spec that names "E61". If your machine has an E61 group, any 58mm basket, tamper, shower screen or group gasket made to the E61 standard is your starting point — though group parts (screens, gaskets) fit by the group itself, so check groupheads explained.
All are 58mm — see the 58mm standard for every basket and part in that format.