Sash windows need a 3mm gap in between sash and body to operate without having friction protecting against clean vacation. Over a window that's more than painted, twisted, and warped the tolerance could be a lot more like 5mm in spots to allow passage in the sash at “sticky points”. https://installingsecondaryglazin79001.blogs100.com/36310646/the-2-minute-rule-for-sash-windows