Can a BL-7800 10-speed downtube shifter be used on an 8 speed hub/cassette in SIS/Index mode? IF so, is there a way to "lock-out" the first two or last two index settings so I don't shift too far and end up shifting my chain right off my cog?

Cable pull per click is wrong.

No, but u can use it in friction like a real man.

Use it in friction mode. Will not work/will work very poorly in indexed mode.

The limits of derailleur movement are set at the derailleur using the end point adjustment screws, not the shifter. If your derailleur shifts into the spokes or ontot he axle that is a derailleur adjustment problem, not a shifter adjustment problem. In fact, I was taught to, and still do to this day, adjust end-point screws with no cable attached.

Hm, thanks for the answers. Couple more questions...

1. Why the "taboo" of using indexed shifting? Is it generally just known to be worse than friction? I always thought index would be better because you could mentally known how many gears you've moved by the clicking sound or clicking "feeling" in your fingers.

2. What if I use 7402 downtube shifters, will indexing shifters meant for 8-speed SIS significantly improve the performance or otherwise work better?

You're missing the point. A 10S cassette fits in the same space as an 8S cassette. Therefore, to compensate, the 10S shifter pulls less cable per shift. If you tried using a 10S shifter with an 8S cassette more than half of your shifts would probably be off, or work very poorly.

Using an 8S index shifter with your 8S cassette should work perfectly. There's no "taboo" about using indexed shifting. That's why virtually all bikes today use it.

Ah, but I forgot. What rear derailer do you have? If you have an old Dura Ace RD, you MUST use Dura Ace shifters with the number of speeds to match your cassette. The old Dura Ace stuff used a different cable pull than everything else. If you don't have a Dura Ace rear derailer, then you CANNOT use Dura Ace 8 speed shifters. You need a lesser model like Ultegra, 105, RX-100, RSX, etc with the number of speeds to match your cassette.

Or use friction shifters.

You can also do alternative cable routing on SOME combinations of DA/Non-DA stuff. If you already have the stuff, you might as well do friction. Not hard to shift friction on ramped 8/9/10 speed cassettes.

Hm, will 7-speed 7401 go another "notch" and be a jury-rigged 8-speed downtube shifter (indexed) ? Or is the cable pull still different and will mess up the shifting while indexed? IE friction only.

Cable pull is slightly different, but I've used a 7 speed downtube shifter with an 8 speed cassette. It worked fine for me. The last shift isn't a "click" but rather it goes into the friction portion of the lever travel. That doesn't matter though since it's only 1 shift and is limited by the limit screw. It was actually the smoothest shift in the whole system.

But to make any of this workable you must specify: what rear derailer? Dura Ace or something else?

Dura-Ace, same series 7400. or 7401/7402 I can't tell the difference.

I have no experience with the Dura Ace stuff, but a 7 speed Dura Ace shifter might index it ok.

A 9 speed non-Dura Ace might also work, but no promises. The old DA used less cable pull than everything else, so the way I figure it a non-DA 9 speed should pull about the same cable as a DA 8 speed.

It's all about the cable pull!

Oh, I forgot to ask, does the 8-speed "600 series" downtube shifter work? I may use that until I can find DA ones.

No but a 9-speed might. 8-speed Dura Ace is it's own animal but I think I read somewhere that it's spacing was the same as 9-speed everything else. I've never tried it, though, so if you spend the money and it doesn't work, don't blame me.

Your question about index shifting brought back memories. I'm a retro grouch. When index shifting came out I thought "What could possibly be the big deal?" When I bought my first index shifting bike I made sure that I could convert it to index shifting on-the-fly when it drifted out of tune. It only took 5 miles into my first ride before I was sold. "What have I been missing?" Now I'm a retro grouch the other way. I don't mix and match 7-speed and 8-speed stuff because the cog spacing isn't exactly the same and the indexing doesn't match up closely enough to suit me.

Oh - and what I said about index shifting - I had the exact same reaction when I got my first STI equipped bike.

Only in friction. 600, Ultegra, 105, RSX, RX-100, are all interchangeable. Dura Ace is different.

Since you have the shifters, you could always put on a non-DA derailer and everything would be perfect. Therefore avoiding all the hassle and compatibility issues caused by the old DA system.

Cassette spacing? I believe all Shimano 8 speed cassettes have the same spacing, DA or otherwise. It's the cable pull that's different between DA and non-DA.

But yes, I believe a "9-speed everything else" shifter would work with an old DA rear derailer to shift 8 speeds.

Why don't you just install a 10 speed cassette with the shifters?

I don't think a 7400 derailleur would take 10 speed cassettes? if it does...10 speed DA with mostly 7400 parts. I even got a stem, but JUST missed out of a 7400 headset.

Ok.

If I use a 9-speed shifterI can index 8 of them and either the highest or lowest gear becomes friction as long as the limit screws keep it from going too far?

Does the 7402 derailleur accept 10 speed cassettes?
If it can use 10 speed cassettes does that mean I can use 10 speed shifters, or will the derailleur not work?

Correct. To run a 10 speed cassette you'd need the 10 speed shifters, 10S chain, and a rear derailer that will be compatible (Ultegra, 105, 9-speed or newer DA, etc.) You current derailer would not work.

You'll actually just have an extra click somewhere. If you set it up like you would any other shifter, the extra click will be locked out by the lower limit screw, and you will never notice it when riding.

I guess I'm confused on what you want to do now but here is some info on Sheldon Browns site

I'm trying to put together a Dura-Ace 8 speed 7402 (mostly) together; I just missed out on a NOS headset. I'm cursing myself for not paying attention to auctions.

Yeah I've gone over his site, but it was like a year ago. I've totally forgotten exists (blasphemy!). Thanks for the heads-up.

P.S. I just noticed the sticky.