I used the Racetech springs.
With regards to tuning; the emulators came with instructions and I used the recommended settings. Never had to mess with them after.
Everything came from PDQ and I was quite happy with the service.
https://pdq1.com/
I guess you could use any springs you like but you'd have to mess with the settings.
I sent the following information direct to Racetech:
Make: Yamaha
Year: 2006
Model: XT660R
Rider Weight: 80kg
Bike weight: 190kg
Spring rate required: 0.8kg/mm
FK Tube OD: 43mm ID: 38.5
Spring: OD: 37mm Length: 633mm (Stock)
Spr Spacer Length: 0mm (Stock)
Stk Set Length: 628mm
Fork Travel: 225mm
Cupped or Flat top: I'm not sure; please see photos here:
https://www.dropbox.com/sh/cc0o6fqot...C27ze8Kla?dl=0
Damping rod top OD: 36mm
Damping Rod Cup ID: 18.25mm
Damping Rod OD/ID (if flat top): 36mm/18.25mm
The stock spring is dual rate:
0.38kg/mm for 0-120mm stroke &
0.6kg/mm for 120-225mm.
They sent their reply:
Here are the item numbers you would need.
FRSP 375580 $114.99
FEGV S4301 $169.99
FPEV AD4301 P $19.99
I just ordered them from PDQ.
I think I used 10w oil (needs changing again actually!) but you can check further up in my old post.
In conjunction with a new Wilburs rear shock (with the right spring for the weight) it transformed the bike.
I still have the bike and use it regularly.