I recently acquired an '89 Super Galaxy tandem. This has Stronglight
cranks on, with a single 86mm BCD for a triple. The bicycle's original
gearing was 48/42/28 chainrings with a 13-28 six speed rear freewheel.

Of course, tandems being pretty quick, we found ourselves spinning out the
48/13 almost immediately. Hence I ordered a 52t 86mm chainring and fitted
it.

However, the chain falls down the gap between the 52t and 42t chainrings,
which is a nuisance. Furthermore, I intended to replace the rear hub with
a 7-speed cassette hub; we plan to do the End to End on this tandem and I
don't expect to be able to find 6s freewheels in the back of beyond, I
have a bunch of existing 7s gear, and an 11t top sprocket would also
alleviate the gear range problems. However, I'm concerned that a narrower
chain will only exacerbate this problem.

I'm not sure what the answer is. "New cranks" is obvious, but expensive.
Returning to 48/42/28 might sort it out, but then the gearing's less
useful. Another idea that springs to mind is drilling into the 52 and 42
just below the teeth on the 42, and bolting them together - both to make
them slightly closer, and to make it impossible for the chain to descend
below that level. Grinding down the flats that engage with the spider
would also decrease the spacing, but seems pretty drastic.

I'd be grateful for any advice people can offer.
--
David Damerell Kill the tomato!