Test of the group body prototype

At the end of last week I received the prototype of the group body from the foundry. There was a little confusion with the shipping and the package got left on the doorstep of my office overnight! Very fortunately, despite spending the night on a busy street, it was still there the next morning. I have taken quite a few measurements and, although there are a couple of small problems, I am very pleased with the result. I put the new body into my stripped-down machine with the rest of the original group parts to limit the scope of the testing. The first results were a little worrying: even after a good warm-up, there was a constant flow of water past the lower piston seal. I took a closer look at the piston and decided to replace both seals, which, though they were working fine in the old group, are a couple of years old. Success - no more water past the seal. My suspicion, though I haven't tried to measure this yet, is that there is a certain amount of error in the alignment of the cylinder bore with respect to the top of the group. If the axis of the bore isn't perpendicular to the surface that the rest of the group parts are bolted to it will mean that the axis of the piston wont be parallel with the axis of cylinder. It also makes sense that a pair of cylinder and piston parts that have lived together for decades would generate a unique wear pattern on the seals. I may also add a PTFE (Teflon) guide ring to the piston which will greatly increase the concentricity of the piston with respect to the cylinder.

Finding a good fit for the portafilter seal was another small adventure. The groove for the seal has a couple of out-of-tolerance dimensions which will have to be addressed in the next production run. This means that there is less space (both axially and radially) than is needed for the generic 73mm x 57mm x 9mm Viton seal. To solve the problem for now I found an 8mm silicon seal for another machine (rancilio silvia i think) which is considerably softer and fits quite nicely. 

To make a a long story short: I made coffee on Friday. With old beans and without re-calibrating the grinder the results were actually pretty good. Now that the body has been tested, I can start on the rest of the group parts.