Getting Gas Strut Sizing Right

The correct gas strut size comes down to five measurements: extended length (centre-to-centre), compressed length, stroke length, shaft and cylinder diameters, and the Newton force rating. Get all five right and the replacement fits perfectly, holds the lid open at the correct angle, and lasts the full service life. Miss one and you are back to propping the boot open with a piece of pipe.

Gas It Lift Struts supplies replacement and custom gas struts across Gauteng and nationally by courier. Whether your vehicle is a Johannesburg taxi, a Pretoria SUV, or a canopy bakkie out on the East Rand, the sizing process is the same. The sections below walk through each measurement step by step.

Key Takeaways

• ✓ Measure extended length centre-to-centre, never end-to-end
• ✓ Record compressed length to confirm the strut fits in the closed position
• ✓ Stroke length is the visible rod travel distance, not the total body length
• ✓ Force rating in Newtons (N) is printed on the strut body label
• ✓ Shaft and cylinder diameters determine which end fittings are compatible
• ✓ Call Gas It Lift Struts with your measurements for same-day dispatch confirmation

Step 1: Extended Length (Centre-to-Centre)

Pull the gas strut to its fully extended position. Measure from the centre of the mounting point at one end to the centre of the mounting point at the other. This is the single most important measurement and the one most people get wrong.

For eyelet mounts, measure from the centre of the circular hole. For ball socket mounts, measure from the middle of the ball socket opening. Do not measure from the outer edge of the end cap or the tip of the fitting. The difference might only be 10 mm, but that 10 mm is enough to put the lid at the wrong angle or prevent the fitting from seating properly on the bracket pin.

A standard gas spring for a passenger car boot typically extends between 300 mm and 500 mm centre-to-centre. Canopy struts on a bakkie often run longer, from 400 mm to over 600 mm, depending on the lid geometry. Write down this figure before moving on.

Step 2: Compressed Length

Fully collapse the strut and measure the same centre-to-centre distance in the compressed position. This confirms the strut will physically fit inside the compartment when the lid is closed. If the compressed length is too long, the lid will not close fully. If it is too short, the strut will hit the end of its travel before the lid reaches the fully closed position, putting stress on the pivot points.

Gas It Lift Struts checks both extended and compressed lengths against your vehicle before confirming stock. Customers looking for boot gas struts on saloons and hatchbacks sometimes find that aftermarket struts have the correct extended length but a slightly different compressed length compared to the OEM part. That difference matters on tighter boot lid geometries.

Step 3: Stroke Length

Stroke length is the distance the rod travels as the strut moves from fully compressed to fully extended. Measure the exposed length of the metal shaft from where it exits the cylinder body to the base of the end fitting. Do not include the fitting itself in this measurement.

Stroke length is directly related to how far the lid opens. A shorter stroke on a bonnet strut means the bonnet opens to a lower angle. Most vehicle applications have a defined stroke from the manufacturer. When replacing a failed strut, match the stroke exactly. On custom installations, such as a fabricated canopy or a workshop lid, the stroke is calculated from the geometry of the pivot points and the required opening angle.

Step 4: Shaft and Cylinder Diameters

Two diameters matter: the thin metal rod (the shaft) and the main cylinder body. Common shaft diameters for automotive applications are 8 mm and 10 mm. Common cylinder diameters are 18 mm, 22 mm, and 28 mm. These dimensions affect which end fittings will fit the strut and determine the load the strut can physically carry.

Use a vernier calliper for an accurate reading. A tape measure works at a pinch for the cylinder, but the shaft diameter is narrow enough that a tape measure can read 0.5 mm to 1 mm low. For bonnet gas struts and other under-bonnet applications where clearance is tight, getting the cylinder diameter right prevents fouling against surrounding components.

Step 5: Force Rating in Newtons

The force rating tells you how much lifting force the gas strut generates. It is printed on the label attached to the cylinder body and expressed as a number followed by the letter N, for example 300N or 500N. This is the measurement most often ignored and the one most responsible for failed replacements.

A strut with too low a Newton rating will not hold the lid open. A strut with too high a rating will force the lid open faster than expected and may prevent it from closing without extra downward pressure. For most passenger car boots and bonnets, OEM Newton ratings are in the 200N to 600N range per strut. Most vehicles use two struts per panel, so the combined force lifts the full weight of the lid.

Gas It Lift Struts sources replacement gas lifts and gas shocks matched to the original Newton rating for your vehicle. Customers fitting canopy gas struts to a custom or modified canopy may need a custom Newton calculation based on the actual lid weight. Call the Gas It Lift Struts team with the lid weight and dimensions and the correct rating will be worked out before the order is placed.