Doors and Windows - Capping replacement windows?

We are thinking about updating our original 100+ year old windows with vinyl replacement windows. I feel pretty confident on installing them myself from what I have seen and been told. However, if installed I would have to remove the storm windows and frames. I was told that if I remove the storm windows I will have to 'cap' the windows with some type of aluminum flashing. I have no idea what they mean, what it looks like or where it goes. I have been told you cannot buy it premade and will need my own brake to make them or hire it out. Could someone please explain what I need here? Every show I've watched on popping in replacement windows never mentions 'caps'.

A lot will depend on how the storm windows are currently mounted. If they are mounted inside the frames of your present 100 year old windows, no such "cap" would be needed. And with the right vinyl windows, you'll probably not have much use for the storm windows, either, If you can retrofit fit the vinyls into the original wood frames and these storm windows didn't disturb that frame when they were installed, you should have no problems. If what you have is any different, post the details and we can help you.

Capping is simply a way of covering, with aluminum, the exterior wood framing around the window so as to eliminate painting. It is not a necessary part of a window installation.