I am planning a flow profiling mod to my machine. I spoke with a few folks behind the scenes about what-all I need to do and I think I have most of it figured out but I am stuck on sourcing/choosing a needle valve appropriate for this application.

I could probably just choose a random one and get by, but I thought I'd ask the forum for some quality options.
I guess I'm targeting a 60-600 mL/min adjustment range @9bar

I tried to source a Slayer needle valve but they don't wanna give me one. I also talked to these guys:

but I get the impression he wants to sell me the total solution, which I don't mind but will take time since they've only designed one for the linea at this point.

I use a Swagelok Metering Valve. SS-2MA2 as it happens, but any of the M series here https://www.swagelok.com/downloads/webc ... 01-142.pdf should be fine, or possibly the S series even. You'll need a solenoid valve too of course - beware that many of commonly available 2 ways are designed for boiler refill duties and not capable of resisting 10 bar.

Awesome! that's exactly what I was looking for. I'm planning on picking a 2-way solenoid from the GS3 parts catalog.

If you choose a 2 way from GS3 catalog it will be an NO valve. That means when auto fill is activated, boiler filling will suffer as the flow valve will always be inline to the machine (at least until the TDR activates the valve). Upon further thought - you may be able to connect a relay to the autofill solenoid to activate the 2 way whenever the autofill operates.

I ended up using a 3 way universal to allow regular (line pressure) fill but to close when the 3 way gets engaged (until a TDR releases it and starts the pump). Note the universal designation vs the NO or NC (with valves even 3 way have NO and NC configuration of the seals - a universal means a switchover between to hydraulics lines).

I used a plastic John Guest valve which works pretty well. All the flow control is done in Plastic.

We discussed the mod and part choices rather extensively at Mod La Marzocco GS3 to flow profile?. Note all of the hosing is done on the inlet side of the pump (so it doesn't really see that much pressure....).

In hind sight - probably the most important mod for a plumbed in machine is a pressure regulator. Good luck with the mod.