For almost all E61 machines, the correct gasket size is 8.5 mm. On our web site we offer an 8 mm as well. I suggest you use the thinner 8 mm gasket if you have an original La Marzocco portafilter. The reason is the La Marzocco portafilter has slightly thicker ears, which prevents it from locking in as secure as it should. The slightly thinner gasket allows the portafilter to move farther to the right and results in a more secure fit.