So I recently came by a rather aged Clements 10-speed drop-handlebar bike. It sat in my parents garage for 15-20 years that I know of and possibly longer still, so needs a little TLC.

It's got shifter levers mounted on the stem and is very much a vintage bike in that it doesn't have nice indexing, you just move the lever and listen for the chain moving where you want it to be.

I'm curious how I'll get on with a drop-handlebar bike (I've ridden an MTB for the last couple of years) so I'm keen to fix it up to test it out. Then I can either keep it or sell it to fund something else.

I'm wondering what to do when the cassette wears out because I'm just not seeing 5-speed cassettes for sale. I can find 7-speed, but I'm not sure how well that will work with the existing setup. Am I right in thinking that if the cassette is comparable in thickness, and the tooth counts of the largest and smallest sprockets are comparable, then it should still work? I figure if I'm just shifting a lever until the chain moves then that should theoretically support any number of gears, as long as the rear derailleur is designed to move far enough to cater for them all.

Obviously I don't want to take a bike that might be worth something and turn it into a bike worth nothing if I can help it, so any pointers welcome!

7 speed freewheels are wider and the dropout spacing will be larger than your frame.

You can still get 5 speed freewheels here: https://www.jensonusa.com/store/produ...l.aspx?sc=FRGL

5-speed freewheels do exist, but are getting harder to find since the push is for more and more gears. As long as you replace the chain when it's worn, the existing freewheel should last a while as they're quite robust. (There are no 5-speed cassettes- cassettes came about when the length of unsupported axle became too much for an 8-speed freewheel)

A 7-speed conversion may be do-able, but will require widening the rear triangle to suit the slightly longer axle. A steel frame can be slightly bent in this way without problems. Using the original wheel with the new freewheel will require the wheel to be redished and spacers added on the right side. Alternatively you could try to source a pre-built wheel with a 7-speed freewheel or cassette.

If the bike has friction shifting, the limiting factor is the derailleur. If it can cover the range of a 7-speed freewheel, then the system will work with the old shifter. A modern freewheel should be easier to shift due to the shaping of the teeth.

5 speed freewheels are readily available on line and are cheap. Niagara Cycle Works sells five speed freewheels for $9. SJS Cycle sells one for $8.22. Bike does not have a cassette and a cassette will not fit. Rear wheel is not wide enough to accomodate a wider freewheel. When you replace the freewheel, replace the chain as well. Chains are also cheap, Niagara sells a 5 speed chain for less than $5.

it's a question of semantics and technical differences.

You can't find 5 speed cassettes because they don't exist (and never did) The cassette system, wherein the freewheel mechanism is in the hub, and the sprockets slide on, came out when bikes were already up to 6 and 7 speeds.

The older freewheel system, has the freewheel mechanism in a separate screw on module along with the sprockets comes up to 8s. 5s freewheels are still around and you'll have no problem finding them if you search the right term. buy 5 speed freewheel

Thanks everybody, I didn't realise that it was a freewheel rather than a cassette. That would explain why a search for a 5-speed cassette turned up nothing at all...

It looks like I can get some form of replacement parts if/when they do need it from UK suppliers, which saves the postage a lot of US-based firms charge if they'll even ship abroad.

Looks like there's plenty for me to learn here!

Mostly true, but 5-speed cassettes were available for a short time at the very beginning of the cassette era. I'd bet very, very few were ever imported to the U.S.:
https://www.sheldonbrown.com/shimano1982/pages/24.html