I have just taken delivery of a custom made Tamper from a seller on evilbay and am really pleased with quality of it. He has just over 900 feedbacks with 100% positive, so I thought it would be worth the risk.
But I would like to ask the question of how tight should tampers fit?
I have a Breville 800ES. After reading a few threads on this model I replaced the two pressurized baskets that came with the machine for the non-pressurized ESP4/51.
When I used the new tamper in the ESP4/51 basket I thought I had been sent the wrong size so got the man of the house to check the measurements with a caliper(hope that is right) and it measured spot on.
It seems a better fit in the original baskets (which I dont like using after having used the non-pressurized one).
So does this probably 1mm make a lot of difference when tampering?
Unless you are getting a custom product like a Pullman fitted to your actual baskets a small clearance between the wall of the basket and Tamper is normal. Basket size of say 51, 53, 58 etc is a nominal size and not an exact size so there is variation with in them. My Pavoni Baskets for example are fractionally different between the single and double from the same manufacturer.
As to being a problem up to a mm should be fine providing you are not getting to much coffee up the side walls of the tamper as you use it. If this happens an extra tap on the bench to knock the llose grinds down and a final polish with the tamper will help.
I have actually used one 3mm undersized, to make that work initial tamping was done in a N S E W (compass points) pattern to compact for a start then a final tamp to finish off. This was a major pain so enter another couple of Pullman customs.