I have read previous posts and the various theories around flat v's convex tamper bases. Interested in the theory of matching the shape of the baser to that of your showerscreen.

I have an e61 HX and the showerscreen is slightly convex. I gather the theory is to keep uniform distance between the screen and puck across the width of the puck - but why would this make a difference? I would have thought the pressure in the group would be uniform throughout?

Hi Kade, and welcome to H-B. This hypothesis has been mentioned numerous times, but I've never seen any data to support it. In general, I find a convex tamper to be slightly more forgiving than a flat tamper. A convex piston may help to prevent "donut" pours by pushing grinds towards the edge of the basket, rather than compressing them in the center.

Again, these are minor effects. IMHO grind, dose, and distribution have a far more profound impact on the pour than the tamper piston shape.

I'm in complete agreement with John on all counts.

That said, Reg Barber now offers a piston called a C-flat, which is flat in the center, and curved at the edges -- could be a nice compromise if you can't decide between convex and flat.

If you like to updose, a profile that approximates the shower screen give just a scoch more room before the screen tears up the puck on lock in. If you dose a l'Italiano, any shape will do.

I have an R-B C-Flat, an R-B American curve and a generic flat and until I get very close to hitting the shower screen on lock in, it's swings and roundabouts for the various profiles.

A piston that is about .25mm smaller than the basket is optimal from a cleanliness standpoint. A looser piston collects coffee around the rim and a tighter fit is prone to binding.

RapidCoffee wrote: This hypothesis has been mentioned numerous times, but I've never seen any data to support it.

Well, having never seen it before I mentioned it in the 'Tamper Roadshow', It may have been me that started that rumour. It was a SWAG based on the fact that I was having far better results with the flat tamps than with any of the curved bases, and my screen is flat. I won't tell you where I pulled that from, but suffice it to say that everyone has one...
OTOH, I ended up reversing that hypothesis, and suggesting later that maybe it's the shape of the bottom of the basket, and not the dispersion screen, that dictates the optimal shape of the tamp. My LM styled 14 g baskets that I use on Mis Silvia are rounded on the bottom, and the convex seems to do just fine. Since I've finally gotten a naked for her as well, I may do some more research (HA!) that is far more reality based. Don't expect any double-blind tests, or any Excel spreadsheets, though... ; >