10 speed chain/cassette w/ 7/8 speed derrailluer
 

I'm going to be doing a makeover on my Paramount. It has the original 105SC group and I want to keep it as original as possible. I have a spare 10 speed cassette and my hub can fit it. If I were to buy a set of Dura Ace 10 speed DT shifters would this setup work? From what I understand the inside width of the different chains is the same, they're just narrower on the outside. I have read from people who said a 10 speed chain worked on their old chairings but I need to know if the RD will work.

The derailleur should be OK. The crank is going to be the unknown. I have a set of 10 speed DuraAce shifters sitting here beside me that just arrived yesterday. I will be trying this on 7 speed tri colour 600 setup in the next couple of days.

I've read the chainrings are okay, I just wasn't sure about the RD. I have an old 10 speed chain, I guess I could put my shifters in friction mode and check it out.

I can say that for the Ultegra and Dura-Ace rear 7s derailers, that trying to reach for an eighth cog always causes a serious indexing issue, as the derailer doesn't quite move far enough on those last two shifts. Note that this isn't a travel limit problem, but an indexing-accuracy problem.

This surprised me at first, but has been confirmed on many occasions.

I noticed that the 8s 105 derailers also arrived with the specific marking on the cage "for integrated 8 SIS", though the cage itself was not the specific source of the problem.

So expect to have to upgrade the rear derailer if you want proper indexing and quiet operation, and also consider the 9s versions as fully 8-speed compatible.

The small 9-10sp chainring has the teeth offset a little to get the closer spacing for the chain. The Shimano spider is the same thickness through the years. If you have trouble with spacing on the front just change to 9-10 speed rings.

I'd have to say it works fine. I have been out around the neighborhood and shifting seems bulletproof. Quiet and accurate. Even the front shifting has zero issues. However, I'm going to put a compact crankset on just for more range.
I've been running 10 speed with a 8 speed 105 rear derailleur for a couple of years and happy with it.

I was a little surprised to see that on the DuraAce 10 speed shifter, the front lever sits up at a 30° angle - just seems wrong, hard to get used to.