Originally Posted by
peugeot.ash
I may get derided for this, but can someone explain the relationship between the rear cassette spacing and the brifters that I use? I already have 8spd 6400 but I have read that these will be compatible with an older 7 speed cassette? Why have some people suggested I need a whole new rear wheel? could I not just change the freehub/cassette if it doesent work?
Shimano 8-speed spacing is 4.8mm cog center to center. 7 speed is 5.0mm. Which means that your shifters pull slightly too little cable per 'click'. But the difference is close enough so that normally 8-speed shifters shift fine over a 7-speed cassette or freewheel. I have always been able to get it to work. If you really want the cog spacing to be exact, then pull apart the cassette and replace the 7-speed spacers with 8-speed ones. Or install 7 cogs from a stock 8-speed cassette. Any Co-op will have piles of old cassettes that can be scavenged.
When you run 7 cogs with the 8-speed shifters, make sure to lock out the derailleur's ability to go into the '8th gear'. Namely the spokes. Use the derailleur cable stops for this.
And all older shops have an drawer of abandoned downtube cable stops that came standard with STI shifters. Maybe half of the ones that came out of the box were ever installed. Our local Co-op sells the best of all of them, the Dura-Ace version (new) for $5 per pair. If you run out of options, and you local shop 'orders' them for you, insist that they come in unopened Shimano bagging with all documentation.