I know this has been discussed before, but I just wanted to chime in with my questions/experiences in mixing 8 and 10 speed components.
I have an older bike with full 105-1056 components -- 53-49 double crank, 8 speed (14-26) cassette, STI shifters, (all from the late 90's), and 8-speed chain (not vintage -- just in case anyone was wondering how long a chain could last, but that's a discussion for a different thread ). Wanting to make it a little easier to get up the hills in my area, I swapped out the crank for a Shimano FC-R700 compact (that is spec'd to be 9 and 10 speed compatible) with 6700 bottom bracket, lowered the FD about a quarter inch, and left all other components unchanged.
I was preparing for some difficulties in the front shifting due to the wider chain, but was pleasantly surprised to discover that, after a painless FD adjustment, everything shifted as well as it had previously. So far, the components seem to be fully compatible.
A number of threads here have mentioned that you need to switch to a 9-speed for such a setup to work. My question is: if there's no problems with the shifting, and the chain isn't rubbing against chainrings anywhere, is there any need to put on a narrower 9-speed chain? Are there any other potential problems I should be looking for? (The chain probably has > 200 miles on it, so I'd just assume not change it if I don't need to)
Crank/BB only swap and you should be fine. stick with the chain that fits your cassette.
If its an 8 stay with the 8 spec.. 9 is internally different. to pack in more cogs in the same space.
only thing that stays the same is the 1/2" distance between the pins & rollers.
measure the chain, 12" of chain should be no more than 1/16th out of spec,