I was out riding today when I went to grab a gear on my 8-speed XT thumb shifter, when I got a "pop" and the thumb lever just went all the way forward with no resistance, other than the feeling of the ratchet skimming across teeth. The finger shifter has no problems metering cable out and moving the derailleur to smaller cogs, but the thumb shifter just won't do anything.

I'm thinking maybe a spring broke, but I'm not sure what to look for. Any ideas?

I had the exact same problem and after disassembling the integrated shifter I realized that I was indeed in over my head with the complexity of the assembly. I spoke to the mechanics at my bike shop and they recommended replacing the entire assembly. They noted that the probability of getting it to work reliably after the repair was low and I didn't want to deal with an on trail failure. I found this article XT Shifter Repair which will give you an idea of what you're up against. I managed to find some new old stock Shimano XT 8 speed shifter set on the Internet for around $75 so I now have a backup should my rear shifter fail. Good luck and if you find you need some internal part let me know, I'm sure I saved the old assembly. Be Self Propelled

try tri-flow first. I used that when they wouldn't catch. But they were dry from sitting.

Have you checked the shifter cable? This is your first point of call. I recently flushed my (granted, Ultegra, but it's the same concept) right shifter housing out and replaced the inner cable after I had the same symptoms and it's working like a charm now. Agree with the Triflow suggestion as well.

I flushed it with liquid wrench teflon spray lube (basically triflow) to no avail. I'll check the cable next. I need to find a 3.5mm allen to disassemble the shifter though.

Thank you very much for both of those.

Ok, I got the shifter halfway apart (and stripped the little nut in the process) and it looks like there's a spring on that little pawl that moves the teeth around that's broken. If I press the pawl with my finger, it shifts fine.

I have a feeling I'm not going to be able to fix/replace that spring, and there aren't many 8-speed XT shifters for sale out there (Mine are SL-M740s). I'm wondering if SRAM makes anything that's compatible.

i have had good luck with liquid wrench penetrating oil. that teflon spray doesn't leave any oil behind to soak into the tight spots.

I've seen SRAM shifters compatible with Shimano 8-speed, just don't get the ones that are named X-something, because those aren't. Why not simply buy 8-speed shifters from another Shimano model line?

Well because I wasn't really sure if they still made 8-speed anything anymore, and if they did whether the new stuff will be compatible with my old stuff. Also, after doing a Google search just now, I see a lot of things like "Alivio", "Altus", and "Acera" and I'm not sure if any of those are any good. I replaced the shifters on my Trek 9700 last year with SLX stuff, and the old shifters were comparatively junk.

I have no problem using something else, but I have XT/XTR on the bike and I want something with decent, positive lever action, and not flappy-floppy action like my old 9700 shifters.

Alivo, Acera and Altus are all good-quality groupsets, they're low-end, so they aren't quite as light as XT, but they won't break on you any more than XT will. All the Shimano 8-speed stuff should still be compatible, but if you do want high-end parts, you may have to look on eBay. You might get lucky and find some that have just been sitting in the back room of a bike shop somewhere.

Weight's not an issue since I don't race (hence my 27 lb bike), but I do like a good feel to the shifter. I like it to be crisp and have a short "throw". My old 9700 shifters made you shove the thumb lever a mile to get the first click, and the XT only move a few mm to get the same action.

I'll check ebay and see what I can find, otherwise I'll figure out which of the Shimano A* groups is the best of the lot.

I swung by Performance tonight and shifted some of their 8-sp bikes with the Shimano A* groups. The shifter action was pretty sloppy. But I noticed that they had 10-sp XT stuff on sale for pretty cheap. This gave me the idea to do a 1x10 with my existing crank, and the XT stuff on the back. I'm not sure if my XTR rear der would work with 10-sp stuff, but I could ebay it to offset the costs if it didn't.

Or I could just keep my eye out for another XT shifter on ebay and stick with what I've got.