Retrofitting historic wooden sashes with spiral balances

A contractor from Newburyport, MA says:
I need to replace several window frames but preserve the wooden sashes because they are in one of several condos in a historic building. The upper sashes will be permanently fixed in position and I intend to fit the lower sashes with spiral balances.The windows are grooved already, so I can route them out further to accept the balance instead of grooving the new frames. The sashes are 27' and weigh 18lbs, can you tell me what balances and additional hardware I'll need. I understand that once purchased I can't return them but there is already one window fitted with a spiral balance in this unit so I am reasonably sure this scheme will work.

Thanks, Pat
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Paul from SWISCO responded:
Good Morning,

Thank you for contacting us. This sounds like an interesting project, and we are glad to help. The sash weight is a good start, this will help us determine what balance tip color you'll need. Moving on, do you have any existing balances available? We will need balance diameter (either 3/8" or 5/8"), and the length of the balance tube, as shown here:
balance measurement photo
Once we have this information we can begin making recommendations as to what spiral balances you may be able to use as replacements.

As for returns, you can return window balances. Please see our Return/Exchange Policy.
A contractor from Newburyport, MA says:
Thanks for replying. The sashes I'm working with do not currently have spiral balances. At some point they were retrofitted into an ill fitting PVC Casing with a coil balance, all of which have failed and must be replaced. My intent is to remove the entire existing frame, permanently secure the upper sash, and retrofit the bottom sash with a spiral sash balance.

I can modify the groove to take either3/8' or 5/8'.

They weigh 18-20 lbs, and I would obviously prefer to over compensate if possible.

The Sashes are just shy 0f 28" X 34 5/8" and the over all finish height of the frame is 54", so the distance from shut to open is 26".

My main point of confusion is what clips and stops I need.
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Paul from SWISCO responded:
Sorry for the delay.

I would take a look at the Swisco S500-28 Spiral Balance.
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