Measure a torsion spring by its wire, inside diameter, length and wind

Four numbers describe any torsion spring: the thickness of its wire, the width inside its coils, its length when unwound, and the direction it was wound. Most of them can be read off a spring that has already broken, and having them in hand turns a photo into a firm quote.

We use those numbers to load the right springs for garage door spring replacement, then confirm each one at the door before anything is wound. Measuring is for the photo and the text message. It is never a reason to put a hand, a tool or a tape on a spring that still carries its wind.

The four readings that identify a torsion spring

DASMA’s measuring tip names four readings, wire size, inside diameter, overall length and winding direction, and suppliers also ask about the ends, which are standard on most springs.

Wire size, inside diameter and length together set a spring’s lift and its cycle life, so a near miss on any one changes how the door behaves. Winding direction decides which side of the center bracket the spring mounts on. DASMA’s glossary counts only the working coils as active, not the dead coils held on the cones.

How to read wire size from a 10-coil and a 20-coil count

Wire size comes from the length of a stack of closed coils: measure across 10 coils, then 20, and find the reading on a wire chart, since no tape reads a single wire accurately.

Hook the tape on the end of a coil, or slip its tip between two coils, and count ten coils along the broken spring. Read where the tenth coil meets the eleventh, to the nearest sixteenth of an inch. Then count twenty and read again; the second figure should be twice the first.

Pull the tape away from its end hook as you read, because pushing it in adds about a sixteenth. Every coil in the stack has to sit tight against the next, since a gap makes the stack read long and points at a heavier wire.

The 20-coil reading is what settles close sizes. Wire of .2343 inch and wire of .2375 inch both read 2 3/8 inches across ten coils, and only across twenty do they separate, at 4 11/16 against 4 3/4.

Suppliers prefer this coil stack to calipers: a caliper on a curved coil reads large, rust first thickens wire and later thins it, and many newer springs wear a thin coat of paint. Coil sides rub together in use, so they corrode less and stay free of paint.

The chart below converts coil stacks to wire sizes across the residential range, from one spring supplier’s published table, with lengths rounded to the nearest sixteenth of an inch.

Because the chart rounds to sixteenths, dividing a reading by 20 lands near the wire size rather than on it, so match the reading to the nearest row. The same supplier says .200 and .237 wire are no longer used, so a reading on either row deserves a second look.

The paint stripe on a spring hints at its wire size too, and our guide to spring color codes lists every color. DASMA calls color a partial identification, so when the paint and the tape disagree, the tape wins.

Finding the inside diameter on the cone

Read the inside diameter from the number cast into the winding cone or the stationary cone, because a tape cannot reach inside a spring that sits on its shaft.

• 1 3/4 inch cones are marked 175 or 134.
• 2 inch cones are marked 2.0, 200 or 2000, sometimes with a letter in front, such as P200.
• 2 1/4 inch cones on Raynor doors read 1264 to 1267.
• 2 5/8 inch cones are marked 258 or 263.

A flashlight helps, since the casting is small and often grimy. One supplier notes that a residential spring rarely runs larger than 2 5/8 inches. Do not bend a tape around the coil to estimate it; a curved tape gives a wrong number.

Measuring the length of a spring that broke in two

Length is the unwound length from the first coil to the last, cones excluded, and on a broken spring you find it by measuring both pieces and adding them together.

The broken spring has already let go of its stored energy, so its two pieces can be measured where they hang, with the tape and nothing else. One parts retailer says a total within about 3/4 inch of the true length is close enough to order from, and we confirm the rest at the door.

Do not take a length from the spring that survived on a two-spring door. A wound torsion spring grows by about one coil for each turn it carries, so it reads an inch or two long, and measuring it would put you beside a loaded spring.

A painted or chalked line is a useful clue. Clopay’s instructions have installers chalk a straight line along a spring before winding, and that line wraps once around the coil for every turn of tension, so a stripe that spirals around a spring means it is still wound.

Telling a left-wound spring from a right-wound one

Look at the end of the coil: the wire’s tail points the way the spring was wound, and from inside the garage the right-wound spring normally sits left of the center bracket.

Left and right are always read from inside the garage, facing the door, as the door makers’ manuals describe them. Cone color is the trade’s shorthand: DASMA’s terminology sheet assigns red to a left side, right wound spring and black to a right side, left wound spring, and Clopay colors its cones and drums to match.

Suppliers describe a hand check as well. Point your right thumb along the center of a broken piece; if your fingers curl the way the wire runs, it is right wound. Do it in the air beside the piece, and never wrap a hand around a spring that is still wound.

Why the two springs on one door can measure differently

Two springs on one shaft are not always twins, because doors leave the factory, and come back from earlier repairs, with unmatched pairs that still balance the door.

One spring supplier estimates that about a third of installed doors carry unmatched springs, put there by makers to save cost and by service companies to avoid stocking another size. The pair still lifts the door, but each spring wears through its cycles at its own pace.

So send readings from the broken spring and a photo of the survivor, noting which side each sits on. DASMA’s sheet on cycle life lists a spring applied to the wrong door among the faults that shorten spring life, so copying an old spring exactly can copy an old mistake. When readings and door size disagree, the door’s weight decides.

What to photograph and send for a spring quote

A good spring quote comes from a handful of photos and a few facts, all taken from the floor or a stepladder while the door stays shut and the opener has no power.

1. The whole shaft. Both springs, the center bracket and both drums in one frame show one spring or two and where the break sits.
2. The break up close. Include the gap and any lift cable hanging slack beside the track.
3. The cone casting. Shine a flashlight on the number cast into the broken spring’s cone.
4. A tape along 20 coils. Lay it on the broken spring only, with the reading in focus.
5. The wire’s tail and any paint. Show the coil end where it meets a cone, plus any stripe or cone color.
6. The door itself. Note its width, its height, whether it is insulated, and any maker’s label on its inside face.

That is enough for us to load the likely springs. Final readings happen on site, with the door clamped in its track, before a winding bar goes near a cone.

Where measuring stops

Measuring ends at the tape: the door stays down, the opener stays unplugged, and every bolt and set screw on the spring side stays exactly as you found it.

Do not lift a door with a broken spring, by hand or with the opener. It now carries nearly its full weight with nothing to counter it, so nobody should stand or walk under it, and pulling the release cord can let it move without warning.

The set screws on each winding cone, the bolts through the center bearing plate, and the bottom brackets at the lower corners, some with red heads, all hold a wound spring’s force. Loosening one while any spring on the shaft is wound can release that force in an instant.

Winding and unwinding belong to a technician with solid winding bars and a clamp on the track, whatever the spring system. Our guide to the dangers of replacing a spring yourself explains why, and the readings you send still save a trip.