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@Jonny69
Years ago I had this mad idea that you could probably stuff a few extra gears into an old three-speed plunger mech without it looking out of place if you used gear plates out of a modern 10, 11 or 12 speed cassette. You'd need a narrower chain and switch the derailleur jockey wheels to modern narrow ones. Someone asked a question about setting up a Benelux Mk7 on Retrobike which prompted me to fish it out and have a measure up.
So the original idea was to use an older narrower 5/6/7-speed Shimano Hyperglide cassette hub. These are spaced at 126mm and lot of them have enough spacers that you you can reduce the width from 126mm to 120mm so it'll slot straight into an older 120mm frame. I know you can spread frames but I've lost count of the number of times I've pinched my fingers doing this over the years. Why not just take the spacer out and avoid the issue. Then, see how many gears you can squeeze in the space that the 3-speed freewheel takes up.
The answer is... not many. Probably not enough to make it worth it, anyway. Working in mm, the cog pitch on the 3-speed is 7mm so the total width is only 14mm. The throw on the Mk7 is about 16mm and you could probably increase this to around 18mm if you machined the back off the stop on the inside, and then the springs would be bottoming out. The pitch of Shimano/SRAM 10-speed is 3.95mm and 11-speed 3.7mm so I reckon you could just about squeeze 5 gears in there if you were lucky enough to be able to work between the limits at both ends.
So yeah, you could just about do it, but at the expense of having to run a foreign-looking 36-spoke rear wheel and a modern-looking hub and chain when you get up close.
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