I looked around a bit for the calipers I once had, but couldn’t find them. So I took a 3 mm allen wrench to test the gaps between the cogs and found that some of them did indeed appear to be just over 3 mm and some just under.
So I looked at some of the eBay listings for new/old SunTour stock, but they were a little pricey. And you never know how much to trust an eBay seller’s description.
So I went to a local shop that seems to have gone over mainly to secondhand bikes and repairs, thinking that if anyone had an old SunTour freewheel sitting on a back shelf, it would be them. Unfortunately, they did not (though the owner did rummage around a little thinking they might).
Since I had the bike with me, he said “let’s just pop this off and see how a Shimano works”. He pulled an “almost unused” MF-TZ21 off a secondhand bike and replaced the freewheel. On the first test the shifting was a little crunchy, but after some adjustment it worked smoothly. Then we put on a new chain I had brought, and it worked even better.
Of course, that was just riding it around a parking lot, so I wasn’t entirely confident about it. But today I did my favorite short local ride, a ten-mile mix of asphalt and gravel, with some ups and downs, used a wide range of gearing, shifted under load, and so on. Everything seems great.
Not sure this would work in all cases, but it worked for me.
It’s also the kind of place where you go back into the workshop with them and chat and pick up pointers, which is a big plus in my book.