I discovered on my 1982 Holdsworth Avanti that it takes 5mm derailleur cables - SIS was introduced a couple of years later with the funky new Shimano cables.
So I used brake cale with no ferrules.
My latest build (1953 Holdsworth Whirlwind) definitely doesn't need 4mm derailleur cables.
Went with 4.5mm stainless coiled outer + ferrule
vintage cable outer 4.5mm OD -
https://www.veloduo.co.uk/products/v...3e363576&_ss=r
ferrules -
https://www.veloduo.co.uk/products/f...railleur-cable
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plain: 4.5 ID + 5.0 OD to fit vintage Campagnolo Rear Mechs
stepped: 4.5 ID + 5.5 OD to fit brazed on rear derailleur cable stop
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The 4.5mm OD worked for me to fit a cable stop similar to the one pictured.
