How To: Rebuild a Caldwell Tape Balance

Occasionally, I get the joy of working with my brother Daniel Sidler of Barron Restorations in Austin, TX. This week he was generous enough to share the technique he used to rebuild a tape balance since finding parts for some of these is almost impossible.

He’s a got far more patience than I do to figure this out, but now the rest of us can benefit from his diligence to figure this out. There is finally a step by step tutorials to restore a caldwell tape balance on the internet! Rather than simply buying a new tape balance you can use this process to get an old balance working again. Enjoy!

I stumbled into this repair on a sweet 1932 dormer window that seemed like a layup—right up to the moment the tape balance snapped and the spring recoiled like a mousetrap. Replacement info was nonexistent, so I tore into the broken unit, mapped the guts, and rebuilt it. The result: a smooth, reliable tape balance and a repeatable process you can use on your own restoration projects.

Tape Balance Anatomy

Before you touch a screw, get familiar with what’s inside. A typical Caldwell-style tape balance is a compact little machine with a few critical pieces that all have to play nicely together:

• Body & Faceplate: The metal housing that mounts into the jamb. Note the geometry: the top is more narrow and the bottom flares out. That asymmetry matters—get it flipped and the tape will bind.
• Clock Spring Cartridge (Drum): The heart of the unit—thin, tightly wound metal spring that provides lifting force.
• Brass Tape: The “belt” that runs out to the sash. It attaches directly to the clock spring drum.
• Square Retaining Nuts & Screws: Two small square nuts live in the body and accept the faceplate screws. They’re your third hand during reassembly.
• Orientation Clues: Pay attention to how the tape sits when fully wound and which way the spring needs to turn to add tension (clockwise in the rebuild shown here).

I cleaned everything, then added a thin layer of high-performance grease (the same kind I use on trailer axles) to all moving parts to keep things smooth.

Safety note: clock springs are under real tension. Keep fingers clear, control the wind, and don’t let the assembly spin free. Oh, and wear your safety glasses!

I benchmarked the working unit with a luggage scale. Pulling the tape down registered ~10 lbs; the return force was ~5 lbs. That became the target for the rebuild. In practice, the magic number ended up being 8 full revolutions of the clock spring during wind-up to match those forces.

Step 1: Disassemble, Clean, and Prep

Lay out the parts in order as you tear down the unit. Clean everything and apply a thin coat of high-performance grease to moving parts. Keep track of how the tape was oriented in a working balance—it’s easy to get turned around because the assembly reads “backwards and upside down” when it’s open on the bench.

Pro tip: Thread the two faceplate screws a few turns into the square retaining nuts ahead of time. You’ll thank yourself later when you’re holding tension with one hand and aligning parts with the other.

Step 2: Repair the Brass Tape End

If the tape broke at the drum (common), trim away the fatigued section. Using needle-nose pliers, re-create the factory bend at the end of the tape to fit the drum slot and attach the brass hook onto the new end of your tape. If you lose a little length (about 2″ in my case), plan to move the sash mounting point higher on the stile during reinstallation. A chisel makes quick work widening the rope groove to accept the tape end.

Step 3: Map the Orientation

Dry-fit the clock spring cartridge on the faceplate so you know what “top” and “bottom” look like assembled. Hint: top is narrower, bottom flares. Get this wrong and the tape will bind.

If you have another functioning balance, peek at it to confirm the wind direction and how the tape nests on the drum when fully wound.

Step 4: Pre-Rig the Tape for Pull-Through

Zip-tie the free end of the tape. That little tail gives you something to grab and pull through the body opening under tension later without fumbling.

Step 5: Wind the Clock Spring (Clockwise)

I braced the cartridge against a ½″ lip on my workbench for leverage. Keep your thumb on the spring as you wind so the coil stays seated and doesn’t try to escape.

• Start with 5 turns (that’s roughly how many rotations it takes to fully unwind the tape).
• Then add tension: 8 full revolutions total hit the target forces (~10 lbs down / ~5 lbs up) in my test setup.

Step 6: Attach the Tape Under Tension

Still controlling the spring, attach the tape end to the drum in the correct orientation (this is where a reference photo is handy). Once it’s locked, let the spring wind the tape onto the drum, but leave 3″–6″ of tape hanging out. Do not let go of the assembly yet—the drum still wants to unwind your afternoon.

Step 7: Marry the Cartridge to the Body

While maintaining control, pull the zip-tied tape through the body opening. Flip the cartridge upside down into the body and use those pre-set square nuts to capture it with the faceplate screws.

Adjustment tip: I found I needed to tighten fully to align, then back off slightly to prevent binding so the mechanism runs smoothly. All the geometry has to sit just right.

Step 8: Reinstall in the Jamb & Set the New Sash Attachment

On site, set the lower sash in the jamb and pull the tape down to see where it naturally wants to land on the sash stile. If you shortened the tape, raise the attachment point accordingly. A little wood removal on the stile makes room for the new location. Fasten the tape end, operate the sash, and confirm you’re getting that clean, controlled travel.

Results & Notes

• With 8 spring revolutions, the rebuilt unit matched the working balance forces and operated smoothly.
• If the tape was shortened during the repair, moving the mounting point up on the sash stile is a simple, effective fix.
• If the mechanism binds after assembly, slightly loosen the faceplate screws until motion frees up. Over-tightening can pinch the works.

This isn’t exotic factory magic; it’s careful setup, smart orientation, and controlled spring tension. Take your time, respect the spring, and you can bring these tape balances back to life without hunting unicorn replacement parts.