This is pretty insightful. I've always thought the 1st/2nd position spacing was wider because 7 and 8 speed has the added sprocket washer (per Shimano diagram). I never realized it continued to the 2nd/3rd... only for the 8 speed? It also explains why some people are able to run 8 speed shifters with 7 speed cassettes fairly well.
Since .1mm = .004", going from 3.00 to 3.20 is only .008" and surely within the Centeron pulley movement. But that can still pose an issue.
I've gone way outside the box on fitting an 8 speed cassette into my 126mm dropout Cannondale. Intentionally using the wrong 9 speed freehub body on a FH-7700 to make the cassette closer to the spokes but having to add .8mm spacer behind the cassette and still keeping the OLD between 127mm and 128mm.
Everything worked; keep in mind I'm running a 14-16-18-20-23-26-30-36 cassette and with the .8mm behind it, there is a challenge to get everything on the freehub body. But the 2nd position, downtube shifter, did not have a positive firm click and sometimes needed some additional nudging, or it was a bit too noisy. Dialing in the 2nd position was detrimental to the rest of the cassette. I started to expand the spacing between the 2nd and 3rd position and that helped. But being space limited, I couldn't go too far. The OP's spacing measurements helped me understand how much space I needed.
The result is a pretty whacked out cassette that is runs very smooth:
14t - 11 speed Ultegra 1st position with oversized integrated spacer sanded down to fit.
16t - 10 speed XT with a 3.25mm spacer behind it.
18t - 8 speed Sunrace standard spacing.
20t, 23t, 26t, 30t - 8 speed Shimano standard 3.0mm, except 23t to 26t is 3.08mm
36t - 9 speed Sunrace standard 3.00mm spacer.
John
Last edited by 70sSanO; 08-12-21 at 02:49 PM.