decided to replace my chain today only to find out the new one has thicker sideplates.(both are sram 971).on the 12 tooth cassette,it jams between it and the 11 tooth,which prevents proper contact,so it slips on it.the rest work just fine.put the old chain back on and all is well.any ideas?

Something is wrong here. I gather you have a 9s drive train, so any 9s chain should work. Even an 8s chain will work and a chain would have to be visibly thicker to jam the way you describe.

The only thing I can imagine is that you got the wrong chain in s 971 box.

how right you were.it was a sram box,which i had to take a screwdriver to get the container open.reminder to self,wear bifocals so i can see the fineprint on the chain.it was a kmc chain and quick disconect link.how it got there i have no idea,all i have had is sram stuff.good call!

A KMC 9-speed chain will be no wider than a SRAM 9-speed chain so whatever you have is for 8 speed or, more likely, 5/6-speed.

IF it's jamming between the 11 & 12, you might be missing a shim that would go between those 2 cogs.
Sometimes they use a "generic" 11T and toss in a shim to get the proper spacing.
That way, for example, they can use a 9 speed END cog for an 7/8 speed cassette.

That's what I was going to guess as well.

Yes, this can be an issue, but it's not consistent with the facts as reported. The OP days that when he put the original chain back on, all was well. Assuming he didn't work on the cassette at the same time, and make a mistake, then magically correct it without knowing, that makes the chain itself the only variable.

Not having any more info, except that the chain in the box was a different brand than the one on the label, I'll venture that the OP might have been the indirect victim of a retail return scam. Some one buys the chain, then returns it after switching the chain to a used or cheaper model. The vendor accepts the box, credits the buyer and puts it back in stock, without checking the contents.

Of course, that could be fanciful nonsense, and the OP might have had this chain sitting around for eons and forgotten what he put in the box.

BTW- the OP might also have a spacing issue, not bad enough for the right chain, but enough to make a critical difference with the wrong one.

well i have learned a lot about this subject today.the chain was replaced only.i could have sworn i ordered two chains so when the time came i could switch them out and keep rolling.i know that was what i wanted and would have done but its been over a year so i am not going to say that was what happened.the bike is a specialised rockhopper that was bought new with a sram cassette and sram crankset.i still have them on the shelf.i upgraded to better sram parts.so if my memory has failed me it came from the factory with sram parts and a kmc chain.if not i got the old switcharoo.i have got to start writing this stuff down when it happens.i never knew getting old could get this complacated!thanks guys.