How to Measure a Gas Strut: The 8-Step Checklist I Use Before Every Order

If you've ever had to order a replacement gas strut, you already know it's not as simple as "give me the same one." There's extended length, compressed length, stroke, tube diameter, rod diameter, force rating, mounting style... miss any of them, and a $25 part becomes a $25 paperweight.

The good news: measuring a strut takes ten minutes. A tape measure, a caliper, and eight steps—then you're done. I've been ordering parts for a living as an office administrator at a 40-person automotive service company, managing roughly $200K in annual parts and maintenance spending across 8 vendors. I report to both operations and finance, which means I get questions when a return eats the budget. Gas struts come up more often than you'd think: hatches, toolboxes, utility lids, even a customer's 350Z. Here's the checklist I use every time—refined by my share of wrong orders and returns.

Step 1: Identify the mounting type

Before you measure anything, look at the ends of the strut. Three mounting types cover almost everything:

• Ball socket — a ball-shaped stud that clips into a socket. Most common on hoods and hatches.
• Eyelet — a hole through a bushing, bolted directly to a bracket.
• Flat mount or fork — a tab or U-shaped bracket that bolts flat against a surface.

Your replacement strut needs the same mount type on the same end. A strut with eyelets won't fit a ball-socket bracket even if all the dimensions match. I've seen a technician order a double-eyelet strut because the numbers were right, only to realize the bracket has balls. Check the mount first—it saves you from the rest of the process.

Step 2: Measure the extended length (center-to-center)

Pull the strut out to full extension. If it's too weak to extend on its own, give it a gentle pull—they get lazy with age. Then measure from the center of the top mount to the center of the bottom mount.

Center-to-center. Not outer edge to outer edge. This is the #1 mistake in gas strut ordering. Measuring edge-to-edge adds the length of both end fittings, and suddenly your "correct" replacement is an inch too long. The most frustrating part: the wrong strut looks perfectly fine until you bolt it on.

Step 3: Measure the compressed length

Push the strut fully closed and measure center-to-center again. The compressed length has to fit the space available when your hatch or lid is closed.

This is the measurement that gets skipped most often. A lot of people—myself included, in year one—order based on extended length alone. Then the lid won't click shut, and you get to order a second strut with express shipping and explain to finance why a $25 part actually cost $65.

Step 4: Calculate the stroke

Extended length minus compressed length equals stroke. Every manufacturer spec sheet lists it, so having this number on hand makes cross-referencing part numbers a lot easier.

Step 5: Measure the tube and rod diameters

Use a caliper. A tape measure won't give you millimeter precision, and these dimensions are all in millimeters.

The tube diameter is the width of the main cylinder body. Common automotive sizes: 15mm, 18mm, 19mm, 22mm, 28mm. In my experience, roughly 75% of the struts we order are 18mm—but that's an estimate from memory, not a dataset, so measure yours.

The rod diameter is the width of the chrome piston rod. Typical values: 6mm, 8mm, or 10mm. Pair that with the tube diameter, and you'll know which mounting hardware variants you need.

Step 6: Find the force rating

Every gas strut has a force rating in Newtons (or pounds), telling you how much push it delivers at full extension. The rating is usually printed on the tube—look for something like "350N" or "450N," or a part number that encodes it.

If the printing is worn off, look up the original part number online. Koyo is one of the brands we regularly order—struts, bearings, radiator products, steering racks—and they do a good job of labeling force ratings on the tube. For OEM applications, the factory spec is your safest reference. A vehicle like the 350Z has well-documented fitment data for both the Koyo radiator and steering rack, and the same logic applies to gas struts: use the data, don't guess.

Don't be tempted to uprate or downrate the force. A strut rated too high slams a hood open and stresses the hinges. A strut rated too low lets the lid sag. The force rating is a specification, not a suggestion. When I first started, I assumed "stronger = better," and that's how I learned that a 600N strut on a light aluminum hatch is a recipe for a bent hinge.

Step 7: Inspect the end hardware

With the strut off the vehicle, check the ball studs and sockets. Ball studs commonly measure 9mm, 11mm, or 14mm. If the stud is worn or the socket clip is loose, order new mounting hardware at the same time. These clips cost a couple of dollars, and replacing them while you're already mounting a new strut is a no-brainer. Otherwise, you'll do the whole job twice.

(Should mention: also check the brackets on the vehicle. If the bracket hole has stretched or the mounting surface is bent, no new strut is going to fix that. I've learned to check this before ordering, not after.)

Step 8: Write it all down, then order

At this point you should have eight data points:

1. Mounting type on each end
2. Extended length (center-to-center)
3. Compressed length (center-to-center)
4. Stroke
5. Tube diameter
6. Rod diameter
7. Force rating
8. End hardware size

Compare these against the replacement strut's spec sheet. Don't rely on the part number alone—similar-looking part numbers are a classic trap. And when the new strut arrives, measure it before installing. A 30-second check beats another round of returns.

Even after I'd double-checked everything on a recent order, I had that moment of doubt. What if I misread the caliper? What if the supplier's listing was outdated? The two days until delivery felt long. Then I opened the box and the numbers matched perfectly—the technician had the old strut swapped in ten minutes. The relief, honestly, was disproportionate to the stakes. But that's how it goes.

Common mistakes that cause reorder loops

• Measuring edge-to-edge instead of center-to-center. Adds the length of both end fittings to every dimension. This is the most common error we see.
• Skipping compressed length. The #1 cause of "the lid won't close."
• Guessing the force rating. If it's printed on the tube, use that number. If not, find the OEM spec. Guessing means doing it twice.
• Ignoring worn mounting hardware. Loose clips cause rattles and premature wear on new struts.
• Assuming "same brand = same fit." A Koyo part number has to match the application just like any other brand. We order Koyo parts (including the 350Z Koyo radiator, steering racks, and a fair number of their other components), and the brand's reputation doesn't make an incorrect part number fit better.

Know when to ask a specialist

Here's a realization that took me a few years and more than a few wrong orders: knowing when to stop guessing is a professional skill. A gas strut replacement is a straightforward measured swap—I'm comfortable with that. But if the application is unusual, like adapting struts to a heavy aftermarket spoiler or a custom enclosure, I don't guess the force rating. I call a specialist.

The same goes for other components. When we order a radiator tube, a set of ruthenium spark plugs, or a steering rack, I check the manufacturer's fitment data before placing the order. Suppliers who publish proper spec sheets and part-number references earn my trust because I can verify their claims. Suppliers who go blank when you ask for a part number? That's a red flag. Per FTC guidance, product claims should be substantiated—and in practice, a supplier who can't substantiate fitment is asking you to gamble.

I'd rather work with a specialist who knows their limits than a generalist who overpromises. The vendor who told us "that application isn't our strength—here's who does it better" earned our business for every other part we need. That kind of honesty is rare, and it's worth more than a lower price.

Bottom line

Measuring a gas strut is a ten-minute job. You need a tape measure, a caliper, and eight data points:

Mounting type, extended length, compressed length, stroke, tube diameter, rod diameter, force rating, and end hardware size.

Write them down. Verify the replacement against your numbers. And if anything feels off, ask a specialist before you order. It costs nothing to ask—returns cost time, budget, and patience.

Kavita Iyer

Kavita Iyer is an automotive filtration analyst specializing in engine air filters, oil filters, fuel filters, cabin air filters, and replacement filter elements. She uses ISO 5011, ISO 4548-12, and ISO 19438 methods to evaluate fractional efficiency, contaminant capacity, pressure drop, bypass behavior, seal integrity, and flow restriction. Her work helps distributors, fleet operators, and service networks compare filtration performance, establish replacement intervals, and avoid choices based only on dimensions or marketing claims.