If the cap properly captures the paper between in and the end of the tube, there can be no leaks as there is no way for the coffee to get between the paper and the end of the tube. When you tighten the cap, what is it tightening against. Try tightening it with only a filter paper and then seeing it you can move the paper with a stick, if so, something is wrong. Also, look for molding flash that's causing the threads to jam before they are fully engaged. Conceptually if you made a circle of wax paper or the cover of a slick magazine the right size and used that in place of a filter, you should not be able to get any water to pass through or around that blockage.

OK, it does look like the tube end does press against the paper (and the cap) when the cap is tightened very well. Maybe I wasn't tightening it enough sometimes because the inverted method I use tends to make me a bit more careful with the plunger barely inserted under the hot water. I thought tightening helps but not eliminate the side leak under pressure, but I will try some more.

Still not sure of the purpose of the side holes.

Nunas wrote:I don't understand any of the foregoing responses. I have an original Aeropress, which has done hundreds of cups. It has the side holes in question. I fail to see how they serve as a safeguard against over forceful pressing an excessively fine grinds. I've pressed down on mine very hard, many times, while experimenting with different grinds and coffees. Never has any coffee or grinds come out of those side holes. The seal, with only the paper filter as a gasket, is total. Likewise with my metal filter. That said, I too have no idea why Aeropress put those side holes there.

I'm just paraphrasing what Alan Adler posted to Coffeegeek years ago. However, I am going from memory as it isn't trivial to search there anymore.

From the Aeropress website FAQ:

Why are there holes in the sides of the AeroPress filter cap? Doesn't the coffee coming out of them miss being paper-filtered?

The bottom circular rim of the chamber is firmly clamped down on the paper micro-filter when you screw the filter cap onto the bottom of the chamber. Therefore all the coffee that you press down must go through the micro-filter paper. There is a tiny amount of coffee that instead of going straight through the filter and into your mug goes sideways through the micro-filter paper and emerges outside of the chamber in the filter cap. The side holes in the filter cap are there to enable this small amount of coffee to drip down into your mug. If those side holes were not there, some of this coffee would be pushed up and over the rim of the filter cap and then drip outside your mug.

While it varies depending on recipe and brewing technique I find that Aeropress coffee is cloudy in general. A decade ago, while discussing the VST refractometer, Vince Fedele posted on the now dead coffeegeek forum about Aeropress brews:

Vince Fedele wrote:The AP is not the brewing apparatus of choice to make this comparison, because the beverage is not well filtered. The forces are too high, and the coffee is frequently too fine (for the filter). The AP filter is just standard coffee filter paper, die cut to size. Those filters are designed for gravity-drip and coarser grades of grind. While hot water causes the coffee paper filter fibers to swell, making them a better filter, the filter is also weakened when wet. The cellulose fibers of the paper filter separate, deform and distort under higher than gravity-percolation forces. This is one of the reasons AP coffees are often cloudy, not clarified with sediment easily detected on the palate as well as by eye when compared side-by-side to other gravity-drip methods using similar filters, normal drip-grind, and brew formulas.

You can make much clearer Aeropress brews simply by pressing very lightly. It's simple to try. Brew two cups with the same recipe, pushing hard to plunge one cup and barely pressing at all for the other. You will see a distinct difference. The problem is if you have a recipe that demands a hard press. If that's the case then you have to accept some undissolved solids in the brew.

If you're getting larger pieces of sediment in your coffee then you have a leak. Something is wrong with your Aeropress or the filters aren't seating correctly.

K7 wrote:Also, most folks probably won't be able to tell it's leaking sideways because (I think) people typically use a mug and they can't see how it flows. Besides, I think substantial amount (30%?) of coffee is always flowing through the side holes even if you don't press too hard because that's the path of the least resistance.

You're really seeing one third of the liquid coming out the sides? I've never really paid close attention but I'm pretty sure that's not normal. I'll have to do an Aeropress brew this weekend and see how much comes out the side holes. I'd be surprised if it's more than a tiny amount. Sometimes I'm surprised. We'll see.

K7 wrote:I always get some oil that I don't see with v60. I am not sure I buy their claim that AP filters out the coffee oil (that can increase your cholesterol) as much as they claim to do.

If I remember correctly Alan Adler said that Aeropress coffee was tested and that the diterpenes cafestol and kahweol were essentially absent from the brew. But I don't know exactly how it was tested (grind size? water temperature? force on the plunger?) and I never saw any actual test data. So it's anecdotal. Given that some amount of undissolved solids obviously make it into the cup it seems unlikely that the diterpene concentration is undetectable. That said, it's probably way, way less than espresso, press pot, moka, and similar non-paper filtered brews.

Let me clarify. Based on additional input from others and my closer look at the side holes (see photo above), I no longer think it leaks sideways significantly under normal circumstances. When the cap is tightened well, the tube end presses against the paper and there shouldn't be a leak there ...unless pressure is abnormally high (as baldheadracing has alluded to).

Now, as for why I am experiencing the leak, I think it's because of the higher than normal pressure I often end up with. High pressure not because I seek it but because I often want to make two cups (24g) in a single session AND I happen to really like a recipe of fine grind + fast extraction (<40 sec from pour to last drop) for cleanness & sweetness. For single cup, pressure is not that high at all, and it never leaks. But for two cups, more often with darker roasts, pressure is high and it leaks.

Yes, I could grind coarser for larger dose to work around this issue, but then I would lose one huge advantage of Aeropress (or French Press for that matter): easy scaling of recipe. Need to brew two cups? Just 2x water. I find that to be true ...as long as the press time is relatively constant. This is more or less achievable with lighter roasts, but it's tough with darker roasts that generate more fines and often require lower temperature which in turn lead to higher pressure, interestingly.

Maybe I should give up on the find grind, fast pull approach, but I became a huge fan of it in recent months to let it go!

K7 wrote:Maybe I should give up on the find grind, fast pull approach, but I became a huge fan of it in recent months to let it go! :)

I wouldn't change the recipe. I press hard too. I just accept that, like espresso or press pot, the coffee isn't squeaky clean. I like it that way!