i have an espresso machine which has the the label on the back, "espresso by Arden". i've been to the Arden coffee site and asked about it there but got no response. The machine is damaged and in need of repair and i'd like to possible get some parts, if it's even repairable.. the machines appears to have been dropped

That is a true horror of an expansion valve. You could put a more standard one in, but you would have to change the plumbing a little. A longer but of silicone tube. Plus routing the high pressure tube to the threaded and of the expansion valve. Using this or a similar expansion valve. {link}

With a Speedster skip every 64/83mm grinder as it'll be subpar for the Speedster. Buy a better grinder like most have mentioned here to get the most out of this beautifull espresso machine

You're group gasket is probably getting hard. Try putting a new one as that's the problem in 90% of the cases. Maybe the lip of the single basket is just a tad lower and thus not tightning far enough. I did always love the Cafelat silicone group gaskets

It's indeed not bad to drink distilled water with a good diet but it's bad to make coffee with distilled water as it tastes worthless. A proper extraction needs minerals to make good coffee and that doesn't work with distilled water. Better to descale a bit more often and do maintenance regularly than drinking subpar coffee because of bad coffee

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For the Duetto IV it's this one {link} - the part number is fe1223 and it's described as expansion valve D26 1/4 I think if you confirm that the fitting on the bottom of your existing valve is 1/4 99% chance (but hey it's a free forum here so no guarantees!) it should work work on a Duetto