Changzhou Colewell Machinery Co., Ltd.

Essential Tools for Measuring Gas Struts

Before beginning the measurement process, gathering the right tools ensures accuracy and efficiency. A measuring tape or ruler that displays both millimeters and inches is essential, as gas strut specifications may be listed in either unit system. Most manufacturers use metric measurements, so millimeter markings are particularly important.

A digital caliper provides precise measurements for shaft diameter, cylinder diameter, and end fitting dimensions. Digital calipers typically offer accuracy to 0.01mm, which is sufficient for gas strut applications. These tools are available at hardware stores and are invaluable for measuring cylindrical components accurately.

Additional helpful items include a notepad or smartphone for recording measurements, a permanent marker for marking reference points, and a camera for documenting the strut's appearance and any markings on the cylinder body. Safety glasses should be worn when working with gas struts, as they contain pressurized nitrogen gas. Work gloves protect hands from sharp edges and accumulated grime on older struts.

Step 1: Measuring Extended Length

Removing the Gas Strut

The first step in measuring gas struts involves safe removal from the mounting position. Before removing any strut, secure the object it supports with an alternative support mechanism such as a prop rod or have an assistant hold it. Never rely on a worn strut for support during removal, as it may fail unexpectedly. Following proper lockout/tagout safety procedures is essential when working with equipment that could move unexpectedly.

Most gas struts attach via clip-on ball socket fittings or bolted eyelet connections. Ball socket fittings typically have a C-shaped retaining clip that can be pried away with a flat-head screwdriver, allowing the socket to slide off the ball stud. Eyelet fittings require removing a bolt or pin. Once detached at both ends, carefully set the strut on a clean work surface.

Taking the Center-to-Center Measurement

With the strut removed, fully extend it by pulling the piston rod out until it reaches maximum extension. The critical measurement is the center-to-center distance between the two mounting points. This means measuring from the exact center of the mounting hole on one end fitting to the exact center of the mounting hole on the opposite end fitting.

For ball socket fittings, the center point is the center of the ball socket opening. For eyelet fittings, it's the center of the circular hole. This center-to-center measurement is the true functional length of the strut when installed. Never measure from the outer edges of fittings or from the cylinder body ends, as this produces incorrect specifications. According to NIST measurement standards, proper reference point selection is fundamental to accurate dimensional measurement.

Record this measurement in millimeters. Common extended lengths range from 200mm for small cabinet applications to 800mm for large vehicle tailgates. Automotive hood struts typically measure 400-600mm, while office chair gas springs measure 180-250mm when extended.

Measurement Tip: If the strut won't fully extend due to lost pressure, apply firm pulling force to extend it completely before measuring. A strut that resists full extension indicates it has already failed and confirms replacement is necessary.

Step 2: Measuring Compressed Length

After recording the extended length, the next measurement is compressed length. Push the piston rod fully into the cylinder body until it reaches maximum compression. Depending on the force rating, considerable pressure may be required—higher force struts (above 400N) may need body weight or mechanical advantage to compress fully.

Once fully compressed, measure again from center-to-center of the mounting holes using the same reference points as the extended measurement. The compressed length represents the minimum space required when the application is closed. This measurement is crucial for ensuring the strut will fit in the available space without interference.

Typical compressed lengths range from 50% to 60% of the extended length. For example, a strut with 500mm extended length usually compresses to approximately 300mm. The difference between extended and compressed length is the stroke length—in this example, 200mm of stroke. Record the compressed length in millimeters for accurate specification.

Step 3: Measuring Shaft Diameter

Shaft diameter (piston rod diameter) is the third critical measurement. Using a digital caliper, measure across the diameter of the exposed piston rod. Take this measurement on a smooth, clean section of the rod away from any threaded areas or the seal area near the cylinder body.

Open the caliper jaws and place them perpendicular to the rod, ensuring they contact the rod at its widest point. Read the measurement from the digital display. Common shaft diameters include 6mm, 8mm, 10mm, and 14mm. Automotive applications typically use 8mm or 10mm shafts, while heavy-duty applications may use 14mm shafts.

If a caliper isn't available, a ruler can provide an approximate measurement, though less precise. Place the ruler against the rod and read where the diameter crosses the ruler markings. For verification, measure at two or three points along the shaft to ensure consistency. Record the shaft diameter in millimeters—this specification must match exactly when ordering replacements.

Step 4: Measuring Cylinder Diameter

The outer diameter of the cylinder body (tube diameter) is measured next. This dimension helps identify the correct replacement model and ensures compatibility with any mounting brackets or clips. Use the digital caliper to measure across the widest part of the cylinder body, ensuring the measurement is perpendicular to the cylinder's axis.

Measure the bare metal tube, not including paint or coatings. If there's a crimp, weld, or fitting connection, move to a smooth cylindrical section for accurate measurement. Common tube diameters range from 15mm to 28mm. Small cabinet applications use 15-18mm tubes, automotive applications typically use 19-23mm tubes, and heavy industrial applications may use 23-28mm tubes.

The relationship between tube diameter and shaft diameter affects the strut's capacity and durability. Typical ratios range from 2:1 to 2.5:1. For example, a 10mm shaft usually pairs with a 22-23mm tube. Record the cylinder diameter in millimeters for complete specification documentation.

Step 5: Identifying Force Rating

Locating the Force Marking

Force rating indicates the lifting power the gas strut provides and is measured in Newtons (N) or pounds-force (lbs). Unlike the previous measurements, force rating cannot be measured with rulers or calipers—it should be stamped, printed, or labeled directly on the cylinder body.

Examine the cylinder body carefully, particularly near the end caps and along the middle section. Look for markings like "400N," "200N," "500N," or "100 lbs." These markings may be stamped into the metal, printed on a label, or laser-etched. Clean any dirt or grime from the cylinder surface to reveal worn markings. Some manufacturers use color coding systems in addition to numerical markings.

Understanding Force Rating Values

Force rating represents the extension force—how hard the strut pushes when extending. A 400N strut exerts approximately 40 kilograms (90 pounds) of pushing force. This force remains relatively constant throughout the stroke, though it may decrease slightly (5-10%) from full extension to full compression.

If markings are completely illegible and the strut is one of a pair, check the matching strut for readable markings and use that specification for both replacements. If neither strut has legible markings, consult the equipment manufacturer's documentation, parts catalog, or measure the weight and geometry of the application to calculate required force.

Understanding the conversion between units prevents ordering errors: 1 Newton equals approximately 0.225 pounds-force, or conversely, 1 pound-force equals approximately 4.448 Newtons. A 400N strut equals about 90 lbs, not 400 lbs. Always verify which unit system the specification uses and record both values if possible.

Step 6: Identifying and Measuring End Fittings

Common End Fitting Types

End fittings connect the gas strut to mounting points and come in several standard types. Ball socket fittings are the most common, featuring a plastic or metal socket that snaps onto a ball stud. Eyelet fittings have a circular hole for a bolt or pin to pass through. Clevis fittings feature a U-shaped fork secured with a clevis pin. Threaded fittings have male or female threads that screw directly into mounting points.

Many gas struts have different fitting types on each end—for example, a ball socket on one end and an eyelet on the other. Always identify and measure both ends separately. The fitting configuration must match the original to ensure proper installation.

Measuring Ball Socket Fittings

For ball socket fittings, use a caliper to measure the inner diameter of the socket opening. This measurement must be precise—common sizes are 8mm and 10mm, and even 1mm difference affects fit and safety. An 8mm socket fits M8 ball studs, while a 10mm socket fits M10 ball studs.

Also note whether the socket uses a clip-on retainer (C-clip or circular clip), a threaded collar, or a push-on design. Document the socket material—plastic sockets are common for lighter applications while metal sockets handle heavier loads. Record the socket diameter and retention method for accurate replacement ordering.

Measuring Eyelet Fittings

For eyelet fittings, measure two dimensions: the inner diameter of the hole and the overall width of the eyelet housing. The hole diameter must match the mounting bolt or pin size—common sizes are 8mm, 10mm, and 12mm. Use the caliper to measure the hole's inner diameter accurately.

The eyelet housing width (the distance between the mounting surfaces) must fit between the mounting brackets with appropriate clearance. Measure this width at the widest point of the eyelet. Also note the housing thickness around the hole, as this indicates the fitting's load capacity. Record all three measurements: hole diameter, housing width, and material thickness.

Fitting Orientation

End fitting orientation significantly affects installation. Some gas struts have both fittings aligned in the same rotational plane (0 degrees), while others have fittings oriented at 90 degrees to each other. Looking at the strut from the end, note whether turning one fitting would align with or remain perpendicular to the other.

Take a photograph showing the fitting orientation from the end view. This visual reference prevents ordering struts with incorrect orientation that won't fit properly despite having correct dimensions. Most suppliers can provide struts with specific fitting orientations if specified during ordering.

Safety Note: Gas struts contain compressed nitrogen at pressures typically between 100-150 bar (1450-2175 psi). Never attempt to disassemble, puncture, or heat a gas strut. If a strut shows oil leakage or cylinder damage, replace it immediately. Dispose of old gas struts following local regulations for pressurized containers.

Recording Your Measurements

After completing all measurements, organize the information systematically for easy reference when ordering. Create a measurement record that includes all five critical dimensions plus end fitting specifications. A complete record should contain:

• Extended length (center-to-center): ___ mm

• Compressed length (center-to-center): ___ mm

• Stroke length (extended minus compressed): ___ mm

• Shaft diameter: ___ mm

• Cylinder diameter: ___ mm

• Force rating: ___ N (___ lbs)

• End fitting 1: Type and size

• End fitting 2: Type and size

• Fitting orientation: Aligned or 90 degrees

• Application/location: Specific description

If measuring multiple gas struts for different applications, label each record clearly. Include details like "2019 Ford Explorer - rear tailgate - passenger side" or "Kitchen upper cabinet - left door." This specific labeling prevents confusion when ordering multiple different struts.

Supplement written measurements with photographs showing the strut laid out with a measuring tape, close-ups of end fittings, and any markings on the cylinder body. These photos provide visual confirmation if questions arise during ordering. Store measurement records in multiple locations—written notes, digital files, and backed-up photos ensure information isn't lost.

Common Measurement Mistakes to Avoid

The most frequent measurement error is failing to measure from center-to-center of mounting holes. Many people measure from fitting edges or cylinder body ends, resulting in measurements that are 20-30mm incorrect. This seemingly small error causes replacements that won't fit properly. Always identify the exact center of each mounting point and measure between these centers.

Another common mistake involves measuring a partially compressed strut that hasn't been fully extended. Gas struts lose pressure over time, causing them to compress slightly. Always manually pull the rod to full extension before measuring. If significant force is required to extend it fully, this confirms the strut has lost pressure and needs replacement.

Confusing Newtons and pounds-force creates serious ordering errors. A 400N strut provides about 90 pounds of force, not 400 pounds. Using the wrong unit system results in ordering struts four times too strong or one-fourth the required strength. Always verify the unit system and convert if necessary—multiply pounds by 4.448 to get Newtons, or divide Newtons by 4.448 to get pounds.

Measuring only one strut when a pair is installed can cause problems if the struts are actually different specifications due to previous incorrect replacement. Always check both struts and verify they match. If they differ significantly, one is likely incorrect and checking manufacturer specifications helps determine the proper specification.

Neglecting to verify fitting orientation leads to ordering struts that have correct dimensions but won't install properly. A strut with 90-degree fitting orientation cannot be substituted with one having aligned fittings, even if all other measurements match. Always document fitting orientation with photos or notes.

Verifying Measurements Before Ordering

Before placing a replacement order, invest five minutes in verification to prevent costly errors. Review the recorded measurements and physically check them against the strut one more time. This double-check catches transcription errors, misread measurements, or overlooked details before they result in ordering incorrect parts.

Verify that measurement relationships make logical sense. The compressed length should be 50-60% of extended length. The stroke length (extended minus compressed) should reasonably match the travel distance of the application. Force rating should be appropriate for the object's weight—a small cabinet wouldn't need 800N, nor would a heavy truck tailgate work with only 100N.

When communicating with suppliers, provide the complete specification list rather than partial information. Experienced suppliers can spot inconsistencies or errors based on their knowledge of typical applications. Ask them to confirm the selection makes sense for the stated application. Reputable suppliers prefer clarifying specifications before purchase rather than processing returns afterward.

If original equipment specifications are available from service manuals or parts catalogs, compare the measurements against published data. Differences of more than 5-10mm in length or more than 10% in force rating warrant remeasurement to determine which specification is correct. This verification step prevents installing replacements that don't match original equipment specifications. The SAE International standards organization provides technical specifications for many automotive components that can serve as reference points for verification.