Spiral Balance Wood Window

A handy person from Pflugerville says:
Hi. I am restoring all of our original 1950s wood windows that are 6 over 6 double hung windows. They were of course all nailed/painted inplace.
They have spiral balances and I'm needing some help to determine what product you sell that would be a good replacement for spiral balances they currently have. The top window is 29" tall the bottom window is 30inch tall with a 5/8 inch tube for the balance. It seems some have 2 slots for balances (one spiral balance tune on each side) and the other has only one side routed out for the balance. Please feel free to call me if this is not enough info to select the right replacement parts.
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Bob from SWISCO responded:

This reminds me of our Series 500 5/8" Spiral Balance, often used with older wooden non-tilt windows. Look it over and see if you agree. You could use a bracket like our 17-012 or 17-007 to connect it to the balance to the bottom of the sash. 

A handy person from Pflugerville says:
Yes it looks right to me. Should there be 2 on each sash? Some of them only have one cut out on one side. See photo. Were these once modified from and pocket weight balances?
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Bob from SWISCO responded:

Interesting; there should definitely be balances on both sides of the sash! Most likely these windows used some old counterweight system. At some point some of these counterweights broke, but they were no longer made. A channel would have been routed to use spiral balances for replacements. Thus, I think you'll have to do the same. You'll have to route a pocket or channel to fit the spiral balances in. Using a 5/8" router should work to form this pocket for the balance. 

A handy person from Pflugerville says:
Just to make sure i get the right spiral balance do i want the metal tube to match the height of the sash? If the height measurement is 29 do i want the tube to be 29?

Also, is there a different mechanism i could install instead of the spiral balances?
Can i reuse the attaching bracket i already have to instal a new spiral?
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Bob from SWISCO responded:

To determine what Series 500 length you need, get the height of the visible glass in the window. For a top sash, add 1". For a bottom sash, add 3". For example, if the visible glass height on your bottom sash is 20", you want an S500-23. For strength, simply weigh your sash on a scale and refer to the chart we show on every Series 500 store page. You'll see it once you select your length.

Finally, let's cover installation instructions. It's pretty easy. Screw the Series 500 to the top of the track. Then, clip your bracket (either 17-007 or 17-012) to the end of the spiral before pushing the spiral all the way up into the tube until it can't go up anymore. If your old brackets are undamaged I suppose you could re-use them, but otherwise consider replacing them. Next, use a charging tool like our 17-005 to give it 3-4 turns; this will charge the spring and give it the tension it needs to support your sash weight. Think of it like a wind-up toy. Once the balance is charged, screw the bracket to the bottom of the sash. The two brackets on either side will cradle it, resting the full weight of the window on the balances. If you find that the sash is a little droopy, unscrew the brackets and give the spiral 1-2 more turns. You don't want to turn it any more than that or you risk breaking the spring.

Assuming you have 5/8" balances, I think the Series 500 is your best bet here, and trying to use a different type of balance would take a considerable amount of work, would have no guarantee of working, and be more trouble than it's worth. I would advise against it. 

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