I have a question about chains. I have several bikes that need new chains. They range from 6 , 7, 8 and 10 speed derailleurs. As I understand it, you can't use a chain made for a 6 speed on a 8 speed, or a 8 speed on a 10 speed, (but the reverse is possible)? A 10 speed will fit anything less than a 10 speed. All I want to do is buy several chains that will fit all applications, so I don't have to keep track of boxes of specific chains. Looking for a one size fits all.
Thanks...

I've used 9 speed chains on bikes with 7 speed configurations.

I don't know about 10 speed chains, but they'd likely work too.

If I was you, I'd probably select two chain sizes that cover your range of bikes to stock.

Get 8 & 10 speed chain.
The 8 will cover 8 & less.
Using 10 speed chain on everything is a waste of money and "may" have problems slipping between chain rings on the lower cog count bikes.

I have no experience using 10-speed (or even 9-speed) chains on 6 or 7-speed freewheels/cassettes but I have used 10-speed chains on 8-speed cassettes with no problems at all. Certainly an 8-speed chain will work with 6 and 7-speed.

Where you may have problems with a 10-speed chain is with your crank. 8-speed and below cranks had wider chainring spacing than 9-speed and above. The narrower 10-speed chain can slip between the chainrings instead of shifting cleanly. The 8-speed cassette/10-speed chain combination I mentioned above was used with a 10-speed crank.

I would tend to agree with the others that a 10sp probably won't work with the fewer speed setups but can't say for sure in all situations. You could buy one 10 speed chain and try it out on the other bikes just to be sure,

It's about width .. at 9 'speeds' the chains became thinner, as the gap between cogs narrowed to pack in more sprockets
without adding a significant overall cassette width..

Parts on hand? try fitting the chain on the cassette, or, chainring to see if it slips over the teeth smoothly..

I had a 9-speed chain on an 8-speed drivetrain for a while and the front derailleur would never shift right - no matter how much I fussed with it. It would throw the chain off the inside or similar issue.

One size fits all works about as well for chains as it does hats.

Get KMC X8.93 chains for the 8 and less speed, get KMC X10.93 for the 10 speed.

I don't have a 10 speed, but 5,6,7,8 and 9, and purchase KMC X8.93 in 6 packs, and X9.93 in 5 packs.

Who carries the six pack of the KMC 8.93. I can only find boxes of 25 on ebay.
Thanks, Pete

I have only had problems with the above recommendations when executing a lazy shift, like a ride with the granddaughter on a Sunday. Did not do it for economy but when I would run a 9 speed chain for the RD with a 8 speed crankset. The chain will slip into the no mans land between rings and need to be bumped out, so I no longer do it.

sorry 5 pack https://www.amazon.com/KMC-X8-93-Chains-Missing-Silver/dp/B004JKEOLI

https://www.amazon.com/KMC-X9-93-Chains-Missing-9-Speed/dp/B004JKEONG