Clearance for spring

A professional from Texas says:
I work at a theatre in Texas and we are building a functional sash window for our set. How much clearance on either side is needed for the spring to be able to slide in and have enough tension to hold the window up? We might be looking at using 2, one on each side. The window pane is going to be 20" x 30". Thanks in advanced.
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Dave Sr. from SWISCO responded:
Believe it or not, no one has ever asked us this question before. I would say 3/8 to 1/2" play for each side. I would suggest fabricating the sash on the tight side and "plane" it down if you have to.

I have used these in the past for repairing wood sashes with a chain weight system. They did work quite well at holding up the wood sash. I would also recommend using at least two per sash and possibly installing two more on the bottom if you need extra drag to hold the sash up.
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