There was a thread on another subforum yesterday that referenced Sheldon Brown's 1/8" rule for chain/sprocket replacement. Of course, I ran out to the garage and measured all my chains, and sure enough, on both of my two recent craigslist purchases, the chains had exceeded the 1/8" stretch threshold that, according to the article, would necessitate replacing not only the chain, but the sprockets/cassette as well.

The two bikes in question are: (1) An early to mid 1970s Raleigh Record, and (2) an early 1990s Bianchi Advantage. Neither has any trouble with shifting or skipping or anything like that, and I don't see any obvious signs of strange wear patterns.

I understand that replacing the chains without replacing the sprockets will probably result in skipping, so I'm reluctant to do that. I'm also reluctant to "fix" something that doesn't act broken.

My questions:

1) What if I don't do anything? I guess, ultimately, the chain breaks, and then I have to deal with it. Does any other bad thing happen?

2) If I were to decide to replace both the chains and the sprockets on one or both bikes, it seems that doing this for the Bianchi is probably straightforward, and I could probably use one of any number of replacement cassettes on the market. (Is that true?)

But what about the Record? Do they make new replacements for the freewheel sprockets for those? Or would it make more sense to think about replacing the whole hub or wheel, while I'm at it? And if I did, would it be possible to fit a new derailleur that would allow me to put a granny sprocket on there?

Any guidance would be really appreciated. Thanks!

The longer you wait, the more likely that you will also need to replace the chainrings as well. As for the Record, new freewheels are still available, so you can keep the vintage bike vintage.

Your answers depend on how much you expect to ride these bikes.

If you don't replace anything, you might be able to ride until the chain has stretched to between 1/4" & 3/8" over 12" (2-3%) before it begins to skip. With decent care that's many thousands of miles, but at that point you may need to also replace chainrings. Back in the day, riding a chain and freewheel together until it the chain slipped was standard practice, and often chainrings were not needed until this was repeated a few times.

I suspect that factors that determined how the chainrings survived included the chainring material, and how evenly the various freewheel sprockets were used. Riders who tended to do 90% of their riding would have the chain start slipping sooner, than those who mixed things up better, and so would probably do better on the chainring front. These days I rarely hear of chainring slip except on smaller mtn bike chainrings, si I suspect that larger rings tend to fare better.

I don't think it's possible to run a chain to the point where strength is compromised, so the slippage will protect you from the dangers of a broken chain.

There are freewheels and cassettes available for either bike, so if you want to replace the chain there shouldn't be a barrier other than cost.

i don't replace chains, sprockets or chainrings until they begin slipping. then i replace the chain and sprocket and maybe the chainring if i haven't reversed it once already.

Thanks for the replies. I examined the rear cogs on both bikes, and the Record looks nearly mint - I guess they were making them out of pretty strong stuff back then. So maybe just a chain will suffice. On the Bianchi, most of the cogs look good. One of them has one sort of funky looking tooth, but that may have just been the light.

I'm realizing that buying old bikes can turn out to be pretty expensive, if you want to try to make them perfect.

If the gear teeth look fine, you might get away with just replacing the chain.

I think one of the standard procedures is-
Replace chain.
IF it skips, put old chain back on and ride until it starts to skip and then replace both.

That approach would be totally in keeping with the way I usually approach things. I love it!