Adaptor płatę?

I have an SC3, which I bought about 9 years ago and fitted a 125mm four jaw chuck to it.  Bought an adaptor plate to fit the spindle and machined it in situ to fit the chuck.  Obviously an independent four jaw requires less accuracy than a non independent chuck.

A 100mm chuck should be capable of holding more than 1.75″ diameter, don’t you have the external jaws? If there is something fouling the jaw steps, then, as already mentioned some soft jaws would work. A 125mm chuck would probably require an adaptor plate, and by looking through ebay there are lots of barstock offcuts, some cast iron, but most mild steel that could be machined on your lathe to make the chuck adaptor for a larger chuck. You would need to first make a drawing of the spindle flange, and then attach the blank disc to the spindle, or a faceplate if you have one. Don’t worry about drilling/and or tapping holes in the plate to hold it on, there will be plenty of room to drill mounts in other parts of the plate later. We have chuck backplates with up to a dozen holes in them, plain and tapped and they work just fine. Also mentioned is a four jaw independent which is so much more versatile than a scroll chuck, just takes more time to set up to use.

This is why lathes used to be supplied with a faceplate (often rather than a chuck to keep apparent cost down) and the end caps should have been cast with a chucking spigot.