Originally Posted by
jemaleddin
Thanks! On my way home a young man jokingly(?) suggested we swap bikes and I demurred, but his much newer Trek was probably worth 5 times mine.
Strangely: while messing around with freewheels and cassettes, I had my LBS make sure my derailleur hanger was straight. The RD-7402 is meant to be 7 and 6 speed compatible, and it certainly seems to be!
All 7400 series derailleurs have a 1.9 cable pull ratio. All other (old) Shimano derailleurs and shifters, like that 105SC, have a 1.7 cable pull ratio. The upshot of that is that a non-7400 9 speed shifter will make a 7402 derailleur move far enough per click to match 8 speed spacing. Based on that, 7 speed 105 levers should be shifting that 7402 5.52mm per shift, which is the same as 6 speed spacing, rather than 5mm 7 speed spacing.