Please note that although the gaskets are strong under compression, you should never use a sharp tool like a screwdriver to install them. This may tear the silicone.
Cafelat's group gaskets are made from food grade FDA silicone. Unlike regular gaskets that are made from rubber these do not have any smell or taste whatsoever, and are easy to change since they will not bake rock hard like rubber does. If you have ever struggled to change an old gasket you cannot help but notice the awful smell and the black soot that comes off the rubber. And that is what you are potentially consuming with each drink.
Cafelat have spent months testing and adjusting the material so that these have the same compression strength as rubber, but are also softer so that it is easier to create a good seal.
Size for E61 8mm gasket: 73mm x 57mm x 8.5mm (Blue colour): SGS food test report
Cafelat have found that the replica E61 groups made these days are pretty much all the same, the different machine manufacturers then use either an 8mm or an 8.5mm group gasket.