I bought a set of eight S900-26's. They don't hold up the sash.
I put a charge in them as I saw on your video. They don't stay up. When I put the old balances back in, they work but they make the same noise that made me want new balances.
I did notice that the spiral on the old balance twists the opposite way, compared to my swisco balances.
I don't think you have the Series 900 balance. This has an outer tube diameter of 9/16", but looking at your balance the inside spring is larger than 3/8". This means the outer tube, which appears to have broken off, is probably 5/8". I think our Series 600 is a better option for your window.
Just looking at the raw spring on your old balance, my suspicion is that there was originally an outer plastic layer on the balance, but it would likely not have been a separate part; it's usually just the outer skin of the balance that covers the wound tube you can see. Our balances have an outer metal layer.
Sometimes, though, there actually is a separate balance cover. We can't supply anything like that, since usually the manufacturer would be the one to choose which cover to use. It's not needed for the function of the balance, though. I hope that helps.
