If you spend enough time looking at espresso machines, sooner or later you'll come across the term E61.

And if you're shopping for a portafilter, you'll probably come across phrases such as:

“Fits E61 machines”
“Universal E61 portafilter”
“Standard 58mm E61 fitting”

Which makes everything sound reassuringly simple.

Unfortunately, it isn't.

One of the most common assumptions we encounter is that if two espresso machines both have an E61 brew group, then a portafilter designed for one must automatically fit the other.

That isn't necessarily true.

And to understand why, we first need to explain what E61 actually means.

E61 is a brew group, not a portafilter standard

The E61 brew group was introduced by Italian espresso-machine manufacturer Faema in 1961 — hence the name E61.

It became enormously influential and variations of the design are still used on many traditional and prosumer espresso machines today.

One of its defining characteristics is the way hot water circulates between the boiler and the group through a thermosyphon system. This helps keep the large metal group head hot and provides good thermal stability.

There is considerably more engineering going on inside an E61 group than that, but for somebody trying to buy a portafilter, the important bit is much simpler:

E61 describes the design of the brew group. It does not prescribe one exact portafilter geometry that every manufacturer has to use.

And that is where the trouble starts.

But aren't E61 portafilters all 58mm?

Usually, yes.

Most traditional E61 groups use a nominal 58mm commercial-size filter basket and portafilter.

But 58mm only tells you part of the story.

The diameter of the basket is not what actually locks the portafilter into the espresso machine.

That job is performed by the two protruding tabs on the portafilter head. These are commonly called the wings, ears or lugs.

You insert the portafilter into the group and rotate it. Those wings travel underneath corresponding surfaces inside the brew group and pull the portafilter upwards against the group gasket.

And those little pieces of metal are precisely why a supposedly “standard” E61 portafilter isn't always standard at all.

The wings can be different

Different espresso-machine manufacturers using E61-style groups can specify slightly different portafilter dimensions.

The differences can include:

• the thickness of the wings
• the width of the wings
• their position around the portafilter body
• the angle or shape of the locking surfaces
• the overall height of the portafilter head
• the relationship between the portafilter and the group gasket

Some of these differences are tiny.

But espresso-machine fittings don't have to be dramatically different to cause a problem.

A difference of only a millimetre or two can mean that a portafilter locks far too early, rotates too far, becomes excessively tight or simply refuses to lock into the machine at all.

So why do people call them “E61 portafilters”?

Because there is a broadly recognisable E61-style fitting.

If you put a traditional E61 portafilter next to a completely different fitting — for example one designed for certain domestic Sage, De'Longhi or Gaggia machines — the distinction is obvious.

E61 portafilters normally share broadly similar proportions and geometry.

The mistake is assuming that similar means identical.

Think of E61 as a family of fittings rather than one internationally enforced portafilter specification.

Two manufacturers can both legitimately produce machines based around an E61 brew group while making slightly different decisions about how their portafilters lock into it.

“Universal E61” should therefore be treated with caution

We aren't particularly fond of the phrase “universal E61 portafilter.”

It suggests a degree of compatibility that simply doesn't exist.

A portafilter might fit a very large number of E61 machines. It might fit machines from several manufacturers perfectly.

But that doesn't mean you can conclude:

My machine has an E61 group, therefore any E61 portafilter will fit it.

You can't.

It is particularly important when buying bottomless portafilters, because these are frequently purchased as aftermarket accessories rather than supplied by the espresso-machine manufacturer.

The original portafilter supplied with your machine was designed specifically for that machine. An aftermarket manufacturer is trying to make a product compatible with a range of machines whose fittings may be similar — but not necessarily identical.

Even the locking position doesn't tell the whole story

Another source of confusion is where the portafilter handle ends up once it has been fitted.

People sometimes expect the handle to point perfectly forwards at the 6 o'clock position.

But that isn't necessarily a measure of whether the portafilter is compatible.

The final position is affected by several things, including the thickness and condition of the group gasket. A new gasket may cause a portafilter to lock earlier, while an older compressed gasket may allow it to rotate further.

What matters is whether the portafilter engages correctly, seals properly and can be locked into position without excessive force.

E61 does not mean identical

This is really the important part.

E61 is an espresso brew-group design. It isn't a promise that every component attached to every E61 machine will have identical dimensions.

Most E61 machines have a great deal in common.

But individual manufacturers are still free to alter dimensions and specifications, including the geometry of the portafilter connection.

So if you're buying a replacement or bottomless portafilter, don't simply search for “58mm E61” and assume you've found the right one.

Check the actual make and model of your espresso machine against the compatibility information supplied with the portafilter.

Because when it comes to espresso machines:

58mm isn't a compatibility specification.

And neither, unfortunately, is the word E61.