Coil Spring Compressor Tool

A strut assembly carries the spring under tension even at rest. Pulling the top nut off an uncompressed spring lets the strut shaft and spring separate violently, so the compressor's job is to squeeze the coils down far enough to take that load off the shaft before anything is loosened. The same tool is used in reverse to load the new spring back onto the strut during reassembly.

This step comes up most often when installing lowering springs on the factory strut, replacing a worn strut or shock under an existing spring, or rebuilding a coilover to change the spring rate. See lowering springs installation and cost and coilover installation and cost for where this fits in the overall job.

External compressors use two C-clamp-style hooks that grab the spring from the outside and are tightened down with a wrench on each side. They work well on struts where something runs through the center of the coil and blocks an internal tool.

Internal compressors slide inside the coil and use a jack-screw with two threaded arms that pull toward each other as the shaft is turned, which self-centers the load and is the type most shops reach for on everyday MacPherson struts. A Klann-type compressor adds plates and a telescoping jack-shaft and is called for on some high-tension OEM springs, such as certain Mercedes applications.

Bench-mounted hydraulic or pneumatic strut presses are the shop-grade step up from either hand tool: faster, built for volume, and priced well beyond what a home garage needs.

Reusing the factory strut body with a new spring, whether it is a lowering spring or part of a lowering kit, almost always means compressing a spring at some point. Swapping a strut or shock under a spring that is staying in the car is the same job.

Coilover kits are different: the spring is already compressed onto a threaded lower perch from the manufacturer, and ride height is set by threading that perch up or down with a coilover spanner wrench, not by compressing the spring. See coilovers vs lowering springs for how the two jobs differ. A spring compressor only comes back into play on a coilover if the spring is being removed to change its rate or to service the damper.

A compressed coil spring stores enough energy to cause serious injury if it lets go unevenly or all at once. The core rules, drawn from spring compressor safety guides and manufacturer instructions:

• Confirm the compressor actually fits the spring's coil diameter and free length before starting; if it does not fit, use a different tool.
• Tighten with a hand wrench or ratchet, a few turns at a time on each side, so both arms stay directly opposite each other on the coil at all times.
• Never substitute zip ties, worm clamps, or any improvised restraint for a proper compressor.
• Stay off the spring entirely once it is compressed just enough to free the strut, and never force it beyond the point it is designed to travel.
• Keep a compressed spring under load only as long as the job takes; do not leave it compressed and walk away.
• Wear safety glasses, heavy gloves, and proper work clothing, and keep hands and fingers clear of the coil gaps.

Match the tool to the spring, not just the car. Compressor listings state a usable coil diameter and uncompressed spring length; a budget internal set from Harbor Freight, for example, is rated for springs up to 10 inches long with up to a 6 inch coil diameter and a 5/8 inch rod, and needs a separate 3/4 inch wrench to turn, at the time of writing. Oversized or undersized hooks are a common cause of a compressor slipping off mid-job.

*At the time of writing, based on the sources listed below. Beyond size, look for forged steel arms over stamped sheet metal, dual locking pins or hooks that keep the spring captured if hand pressure is released, and a visible inspection before every use for cracks, rust pitting, or a bent shaft; a damaged compressor is not worth the risk it is meant to remove.