Does a 10 speed crankset work with a 8 speed drivetrain? Also if it does then what speed should the chain be?<br>
Is a 9 speed derailer compatible with a 8 speed cassette? Do all the components have to be the same speed and is it possible for me to mix and match?

"In General" all Shimano road components up to 10 speed play together (there are some exceptions). As far as chain either "should" work, but you'd have to check on the front if there was any clearance issues of the wider 8s chain between the chainrings. 9s derailleur will be fine on 8s (I run a 9s RD on a 6s FW system) Again, if the wider chain doesn't have clearance issues in the cage. Cassette speed and shifter speed should match or have unused "click" if you want to run 9s shifter on 8s.
Just my opinion and experience YMMV

Shimano Dura-Ace Compatibility

Probably, maybe. The wider 8-speed chain may not shift well on the narrower 10-speed rings. When on the small ring, the chain may catch on the back of the bigger ring and jam into the chainstay (aka chain suck).
8-speed chain with an 8-speed cassette. However, you may get away with using a narrower 9-speed chain, which would probably work better with the 10-speed crank than an 8-speed chain.
Yes. All of Shimano's 8 and 9-speed stuff uses the same SIS standard. The shifter will control the speeds, i.e. an 8-speed shifter will index 8 speeds with a 9-speed rear derailleur.
All of Shimano's 7, 8, 9 (Road and MTB) and 10-speed (Road only) shifters and rear derailleurs are SIS. The shifter controls the indexed speeds, and works with any of these rear derailleurs.
It's best to match the chain to the cassette, but you can often get away with a chain that's "1 speed off".

I've been thinking about this recently as I'm doing some basic mods to my old Voyageur. I don't see many 6 speed chains out there, and I've been buying only KMC chains so far, so happened to notice that the description on many chains have a wide range of speeds. Like the [h=1]KMC X8.93 Bicycle Chain 7.3mm 6, 7, 8 Speed[/h]

by KMC Amazon.com : KMC X8.93 Bicycle Chain 7.3mm 6, 7, 8 Speed : Bike Chains : Sports & Outdoors

so are 6-8 generally all compatible, or do you really need to stick to "1 speed off". I mention specifically, because I have 6 speed freewheel, and looking at this chain (on KMC's site it is just listed as 8 speed but is very common for sellers to label it as 6, 7, 8 speed compatible).

Your research is correct; all 6-8 speed chains are all cross-compatible, so manufacturers just make one chain for those drivetrains. The KMC X8.93 is a fine chain; I use it for 6-8 speed bikes I build.

Some good info in this article: Urban Velo - Bicycle Culture on the Skids

Some folks refuse to use 9 speeds or more, because they are concerned about the decreased life of those narrower chains and sprockets. Others call those folks "retro-grouches", but they have a valid point. As chains and sprockets get thinner, they wear more quickly (and are more expensive). I can't prove this with empirical data, of course.

Personally, I stay a generation or two behind the current market out of thrift. I'm just now adapting a couple older bikes to 9 and 10 speeds, as those parts become cheaper.

All **except** Tiagra 4700.

I'm no expert in these matters but I'm pretty sure my 6-speed 105 and 600 groups were stamped 'SIS' as well, but I had no idea all SIS meant they had the same pull ratios and mechanical advantage (I knew some did, but not sure how many). Are you saying my old 1988 6-speed derailleur would work ok on a newer 9-speed cassette? Not that I am going to try that, just curious. I am wanting to upgrade a couple of my bikes to 10x1 gearing, got most all parts ready just havent had time yet, but I'll also keep a couple with thier proper vintage 6 and 7 speed groups.

Thanks, I keep forgetting about that red-headed stepchild.

If you used your 6-speed RD with a 9-speed SIS shifter, it would perfectly index the cogs of a 9-speed cassette.

However, the old RD may not have enough travel to reach all 9 cogs; a 9-speed cassette (36.5 mm) is 7 mm wider than a 6-speed freewheel (29.5 mm).

I'm not familiar enough with Shimano's 6-speed offerings to know if they were all SIS.

Another exception to the rule: Dura-Ace 6, 7, and 8 speed says it's SIS, but it's not really. It's not compatible with other SIS gear (unless you use an alternate cable attachment point).

Shimano Dura-Ace Compatibility

Sorry, I was reminded today as I was looking at reviews of the brifters on some website, and they were all 1-star reviews of people complaining they didn't work with their 10-speed derailleurs (presumably non-4700 Tiagra).

All of the information you require has been writtenup by Sheldon Brown. His webpages are an amazing resource. https://sheldonbrown.com/k7.html]Shim...tes & Freehubs

With changes rom 5 to 6 to 7 and then 8 sprockets on the rear cassette they just widened the whole thing but kept the spacing of the sprockets on the cassette the same. A 5 speed a rear axle was 120mm for 6 speed the axle was increased to 126mm which also worked for 7 speeds. The axle was increased again to 130mm for 8 speeds with the same spacing of the cogs. When they introduced 9 speed cassettes the cog spacing started to get closer together and has gotten tighter on 10 and 11 speed groups. You should try to match the chain to the rear cassette. If you use too wide of a chain it will not engage the cogs fully and if it's too wide it may just slip on top of the gears. Front chainrings are not as critical.

Just about any Shimano rear derailleur will work with any cassette (except for pre 2000 Dura Ace, 4700 Tiagra and Shadow) you just need to match the shifters to the cassette. You can mismatch shifters with cassettes if the cog spacing is the same and you can live with an unused click on your shifter or an unused cog.
Front derailleurs are a lot more tricky. I would try to keep the same model front derailleur and shifter. it just works better especially with the groups that have come out since 105 5700 was introduced.

Take a look at Sheldon's pages. All your questions will be answered.

Always Ask Sheldon, He Has The Answer (AASHHTA)