Stiff link after 20 miles

[Leaway2]

I bought a 1980's steel Ammoco. I replaced a few items. bar tape, pedals. When I took the rear skewer out, the spindle was snapped , so I replaced it and the balls. I also fitted a KMC 8 speed chain, which claims to be 6 speed compatible.
The freewheel is 14 - 24 6 speed but obv's not hyperglide (42 52 on the front).
Dérailleur is a Shimano Touney friction
I went out for a ride yesterday which was not the first ride, but the longest at 23 miles.
During the last 7 miles the chain started to jump. When I got home and looked at the chain, it had a stiff link.
Now I have done a couple of 10 - 12 miles on it before and the chain was OK, so what causes a chain to develop a stiff link?
Is the chain compatible with this older freewheel/chainset?
*Edit* miscounted. it is 14 -24

 

[I like Skol]

How was the chain joined, missing link or the old fashioned way by pressing a partially removed pin back in? I had an issue with a new missing link recently that would get stiff after riding a few miles. Fixed that by removing it and found a burr in the eye of one of the chain ends that must have been there when I reduced the chain length. If it is joined 'the other way' then my money would be on the stiff link being where you joined it and it just needs backing off a smidge.

 

[Leaway2]

I never use the missing link after one broke and ripped my deraillier off. I use a splitter.

 

[Ajax Bay]

If you've pressed the pin back in correctly (ie to rejoin the chain) that should work. But I guess since it's stiff you need to adopt an @Fnaar approach till the stiffness is not apparent. Have to assert that using a quick link correctly is low risk (think of the hundreds of thousands (millions?) of chains linked like this around the world), even given your one-off experience. (Yes - that chain is fine for 6,7,8 speed.)

 

[Leaway2]

Thanks, The fact that the link was not "stiff" when I set out, is what I am having a problem with. Anyway "stiffness" mid ride is something I’ll have to get used to.
My problem with quick links is one has broken, but also I can never get the buggers off anyway as I don't have the correct tool (ooh missus).

 

[mjr]

My problem with quick links is one has broken, but also I can never get the buggers off anyway as I don't have the correct tool (ooh missus).

Any set of parallel-face pliers or grips will open them easily (if you imagine the quick link as a rectangle when viewed from above when closed, you push opposing corners to open it) or they can be pushed open if your fingers are strong enough (mine aren't).

Reportedly, they fail sometimes, but it seems relatively rare compared to how often people seem to screw up rejoining chains with pins and chain tools.

 

[User6179]

Anyway "stiffness" mid ride is something I’ll have to get used to

I bought a chain from Ribble (8 speed) and unless it is swimming in oil a few of the links stiffening up , I started using it in January and I wonder if it got corroded from salt on the road on its first outing .