A friend of mine wants to upgrade his bike, which is an older bike with 8-speed Ultegra 600. He wants to replace the shifters and I came up with an idea, I think it would work, but maybe someone here knows better. Can he install 8-speed Sora brifters, to operate the Ultegra 600 derailleur? I don't see why that wouldn't work, but again I haven't tried it yet.

Cable pull ratios for all 8-speed Shimano product match, except for 7400 series DA. So Sora brifters and 6400 series ultegra are a go.

If you want it all matchy-matchy, 6400 series STI levers turn up from time to time on eBay,

Hope this helps.

He'd be better off going 9 speed. Get some Tiagra STI's and a new 9 speed cassette. The Ultegra 600 derailleur should work just fine. The 8 speed crankset might prevent him from shifting while standing ( Might ?)

Is he "upgrading" just to upgrade? If the 600 brifters are still working well, going to Sora won't be an improvement, although there's really nothing intrinsically wrong with the group. I like the 600 drivetrains I've got, and wouldn't consider trying to "upgrade" them.

Well, the shifters one his bike, although they work well, are all scratched up and he wants to replace them. The Tiagra 9-speed idea doesn't sound bad, I'll bring it up.

Why won't Dura Ace 7400 work with other 8-speed drivetrains? Is the 8-speed Dura Ace cassette narrower then other 8-speeds? Would a 8-speed 7400 Dura Ace rear derailleur be a good replacement for a Sora rear derailleur? What are the compatability issues between DA 7400 and other 8-speed drivetrains? And what about DA 7700?

O.K., take this with a grain of salt, as I've never actually used 8sp Dura Ace, but all the advice and relevant Shimano literature I've encountered says not to mix D/A with the "lower" groups--the sets of 8sp 600/Ultegra level barcons I have carry a sticker on the right-hand one that specifically says it's not to be used with D/A, and at least one set of 600 brifters I've had was similarly emblazoned. I don't know the actual reason, but I guess the rear D/A 8sp derailleur had a slightly different actuation ratio, meaning that, no, it wouldn't be a good replacement for Sora.

I guess the best then would be to go to 8 speed Ultegra, which is considerably better then Sora. Thanks!

O.K., take this with a grain of salt, as I've never actually used 8sp Dura Ace, but all the advice and relevant Shimano literature I've encountered says not to mix D/A with the "lower" groups--the sets of 8sp 600/Ultegra level barcons I have carry a sticker on the right-hand one that specifically says it's not to be used with D/A, and at least one set of 600 brifters I've had was similarly emblazoned. I don't know the actual reason, but I guess the rear D/A 8sp derailleur had a slightly different actuation ratio, meaning that, no, it wouldn't be a good replacement for Sora.

Walrus is spot on. The 7400 series Dura Ace rear derailleur has a different geometry than other 8 speed shimano derailleurs. If you use non Dura Ace shifters, the derailleur doesn't move the same amount per "click" as other 8 speed Shimano derailleurs. In order to use a 7400 series rear derailleur, you need Dura Ace 7400 shifters.