This tamper has a convex base to put an arc into the tamping surface so as to distribute the water into the coffee puck more uniformly (see inset photo).
When choosing a coffee tamper, lever espresso machines will have portafilter baskets with 53 millimeter diameters or smaller, and standard espresso machines will have portafilter baskets with 57-58 millimeter diameters, so you need to get the right coffee tamper in either case. If you're not sure, measure the diameter of your portafilter basket first.
Many people want a coffee tamper that fits snugly into the portafilter, and some prefer a millimeter (or so) of wiggle room, so that they can more easily manipulate the coffee tamper while tamping. But note that the coffee tamper doesn't get pushed down into the bottom of the filter basket (which can be slightly tapered). So it's the inner diameter at the top of the basket that you'll need to measure to size your coffee tamper correctly.
You should look at our article "How to Use a Coffee Tamper Correctly for Uniform Extractions" if you're not exactly sure how to use a coffee tamper correctly. With some simple technique, this coffee tamper will help you to make high quality extractions.
We recommend that you do not wash these coffee tampers in the dishwasher, because over time, the dishwasher will damage the finish and discolor the stainless steel head of your coffee tamper. To clean, simply wipe the tamper down with a damp cloth in soapy water. Your coffee tamper should not get "dirty" in the first place.