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Yup, the Sachs 6 speed used 1/8th sprockets. As you say there are some available on eBay and some places may have NOS, but that’s not really an option for a business like ours. I don’t have time or tools to make spacers up- I’ve tried, and it’s a faff. Tbh most Brompton wheels of that age will have worn rims anyway and probably due for replacement, so a new wheel makes sense to avoid future sprocket anxiety issues.
jj72
@M_V
For clarity, I don't know what width cogs the derailleur bikes use now or have used in the past, I'd have presumed 3/32" so that you could use a 3/32" geared chain but jj72 post seems to suggest it used to be 1/8" cogs.
The current 2/6 speed cog kits have what appears to be a plain toothed smaller cog and a larger cog with ramps and pins.
If there were 3 splined cogs with ramps and pins fitted to older 2/6 speed bikes then yes, I can see that those are no longer available. Or maybe the cogs were a special thickness in order to work with the drivers which (I think) were primarily designed to work with single cogs and these are not available anymore.
However, it seems like you can make it work with plain toothed cogs which are available, in 3/32" and 1/8" and down to 13t so @hugo7 father's could be sorted out without the new rear wheel.
Maybe the shop that's doing the work can't source the cogs from their suppliers or they need a bit of modding which the shop can't/won't do which is fair enough but there are definitely ways and means and Sturmey/3 spline cogs are definitely still available.