Ever tried to hold open a heavy hood, a truck bed cover, or an RV compartment door, only for it to slowly, or worse, suddenly, come crashing down? It’s not just annoying; it’s a real safety hazard. That sinking feeling often points to a failing gas strut, also known as a gas spring or lift support. Replacing them yourself is a straightforward job that can save you a bundle, but it all hinges on one critical step: getting the right measurements.
You might think grabbing any old strut that “looks about right” will do, but that’s a recipe for frustration, damage, or even injury. The precise length and force rating are crucial for proper function and safety. This guide will walk you through exactly how to measure gas strut length, ensuring you get the perfect replacement every time. We’ll cover everything from the tools you need to what to do if your old strut is completely missing, empowering you to tackle this common fix with confidence.
Why Accurate Measurement Matters: Beyond Just Length
Before we dive into the “how-to,” let’s understand the “why.” A gas strut isn’t just a simple rod; it’s a carefully engineered component designed to hold a specific weight and extend to a precise length. Getting the measurements wrong can lead to a host of problems.
An incorrect length can prevent your hatch from opening fully or, conversely, over-extend it, damaging hinges or the strut itself. An improper force rating means the strut won’t hold the weight, leading to a sagging or collapsing door, or it could be too strong, making it difficult to close. For off-roaders with heavy custom hoods or toolboxes, this precision is even more critical for safety and convenience.
The Risks of Guesswork
- Safety Hazards: A strut that can’t hold its load is a danger, especially when working under a hood or retrieving gear from an overhead compartment.
- Component Damage: Too long, and it can stress mounting points; too short, and it won’t provide adequate support.
- Premature Failure: An ill-fitting strut will work harder, leading to early wear and tear.
- Frustration: Having to reorder parts and redo the job is a waste of your valuable time and money.
Essential Tools and Safety Gear for the Job
You don’t need a full workshop to measure a gas strut, but a few key tools will make the job accurate and safe. Gathering these beforehand will ensure a smooth process.
Safety should always be your top priority. Gas struts are under pressure, and while measuring isn’t as risky as removal, it’s wise to be prepared.
Must-Have Tools
- Tape Measure or Ruler: A standard retractable tape measure or a sturdy ruler will work perfectly for measuring length.
- Marker or Grease Pencil: Useful for marking pivot points if your strut is still attached.
- Notebook and Pen: To record your measurements accurately. Don’t rely on memory!
- Digital Caliper (Optional but Recommended): Excellent for precise measurements of shaft and body diameters, as well as end fittings.
Safety First!
- Safety Glasses: Always protect your eyes from debris or unexpected strut movement.
- Gloves: Protect your hands from grime and sharp edges.
- Prop Rod or Assistant: Crucial for safely holding the hood, hatch, or cover open while you measure or remove the old strut. Never rely on a failing strut to hold itself up.
The Step-by-Step Guide: how to measure gas strut length Correctly
Measuring a gas strut is straightforward once you know which dimensions are important. There are two primary lengths you need: the extended length and the compressed length. You’ll also need to note the diameter of the rod and body, as well as the type of end fittings.
This process works whether you’re measuring a strut for your Jeep’s rear hatch, your RV’s storage bay, or a custom toolbox on your off-road rig.
Step 1: Safely Support the Object
Before you do anything, ensure the hood, hatch, or cover is securely held open. Use a sturdy prop rod, a piece of wood, or have a friend hold it. A failing gas strut cannot be trusted.
This is arguably the most critical safety step. Skipping it can lead to serious injury or damage to your vehicle.
Step 2: Measure the Extended Length (Center-to-Center)
The extended length is the most common measurement for gas struts. It’s measured from the center of one end fitting to the center of the other end fitting when the strut is fully extended.
- If the old strut is still attached and fully extended, measure directly from the center of its top mounting point to the center of its bottom mounting point.
- If the old strut is removed, fully extend it manually. Then, measure from the center of the ball socket or eyelet on one end to the center of the ball socket or eyelet on the other end.
- Record this measurement precisely. Most manufacturers list this as the primary length.
It’s vital to measure to the center of the attachment points, as the actual metal body of the strut will be shorter.
Step 3: Measure the Compressed Length (Center-to-Center)
While extended length is often sufficient for ordering, the compressed length is important for ensuring the new strut won’t interfere with anything when the hatch or hood is fully closed.
- Carefully compress the old strut as much as possible. This might require some force.
- Once fully compressed, measure from the center of one end fitting to the center of the other.
- Record this measurement. It’s often provided as a secondary specification by manufacturers.
If you can’t fully compress the old strut due to damage, you’ll need to estimate this or rely on the extended length and stroke data from a compatible replacement.
Step 4: Measure Rod and Body Diameter
The diameter of the shaft (the thinner, shiny part) and the body (the thicker cylinder) are important for fitment and can sometimes indicate the strut’s force rating.
- Use your digital caliper or ruler to measure the diameter of the rod (shaft).
- Measure the diameter of the main body cylinder.
- Record both measurements.
These measurements ensure the new strut will fit into any existing brackets or clearances without rubbing or getting stuck.
Step 5: Identify End Fittings and Mounting Brackets
Gas struts come with various end fittings, and matching these is just as important as matching the length. Common types include:
- Ball Sockets: These snap onto a ball stud. Measure the diameter of the ball stud (e.g., 10mm, 13mm).
- Eyelets/Clevis Forks: These have a hole for a bolt. Measure the inner diameter of the hole.
- Threaded Studs: Some struts have a threaded rod that bolts directly to a bracket.
- Blade/Flat Mounts: Less common, but used in specific applications.
Note the type of fitting on both ends of your old strut and the size of any accompanying ball studs or bolt holes. Take pictures if it helps!
What If Your Old Gas Strut is Missing or Broken?
Sometimes, the strut is so damaged it’s impossible to measure, or it’s completely missing. This is a common scenario, especially with custom builds or older vehicles. Don’t panic; there’s still a way to figure out the right replacement.
This is where a little investigative work comes in, using the application itself to determine the necessary dimensions.
Measuring Without an Existing Strut
- Identify Maximum Extension: Open the hood, hatch, or cover to its desired maximum open position. This is the position you want the new strut to hold. Use your prop rod to hold it securely.
- Locate Mounting Points: Find the exact locations where the old struts were attached. These are usually visible as ball studs or bolt holes.
- Measure Maximum Distance: With the object held at its desired maximum height, measure the straight-line distance between the center of the top mounting point and the center of the bottom mounting point. This will be your target extended length.
- Estimate Minimum Compressed Length: Close the hood/hatch completely. Measure the straight-line distance between the two mounting points. This gives you the maximum possible compressed length the new strut can have without interfering with closure. The actual compressed length of the new strut should be slightly less than this measurement to ensure full closure.
When measuring the distance between mounting points without a strut, make sure the surface (hood, door) is fully supported at its desired open angle. This is your “operating” extended length.
Understanding Force Rating and End Fittings
Length isn’t the only critical factor. The force rating, often expressed in pounds (lbs) or Newtons (N), indicates how much weight the strut can support. This is vital for proper function and safety.
You’ll usually find the force rating printed on the body of the old strut. It might look like “100N” or “250LBS.”
Finding the Force Rating
- Check the Old Strut: Most gas struts have the force rating printed directly on the cylinder body. Look for numbers followed by “N” (Newtons) or “LBS” (Pounds).
- Vehicle Specifications: If the old strut is unreadable or missing, consult your vehicle’s service manual, an online parts catalog using your VIN, or a specialty gas strut supplier. They often have cross-reference charts.
- Estimate (Last Resort): If all else fails, you can try to estimate by carefully weighing the object (e.g., hood, hatch) it needs to support. Generally, you’ll need struts with a combined force rating slightly higher than half the weight of the object, as the pivot point helps. However, this is an advanced method and often requires trial and error, so use it with caution.
Always try to match the force rating as closely as possible. Too weak, and it won’t hold; too strong, and it could make closing difficult or stress hinges.
Matching End Fittings
As mentioned earlier, end fittings are crucial. If your existing mounting points are ball studs, measure their diameter. If they are bolt holes, measure the bolt hole diameter.
Many replacement struts come with standard ball socket fittings, but others may require specific eyelets or clevis mounts. Some aftermarket struts come with interchangeable end fittings, which can be a lifesaver for custom applications or when you can’t find an exact match.
Installation Tips and Post-Measurement Checks
Once you have your new struts, proper installation is key. Always refer to your vehicle’s service manual or the strut manufacturer’s instructions, but here are some general tips.
After installation, a few simple checks will confirm you’ve got the right fit and function, ensuring your new gas struts perform as expected.
Installation Best Practices
- Replace in Pairs: Even if only one strut is failing, it’s best to replace both at the same time. Struts wear out together, and a new one paired with an old one can lead to uneven lifting and premature failure of the new strut.
- Lubricate Ball Studs: A little grease on ball studs can make snapping on new ball sockets easier and reduce wear.
- Orient Correctly: Most gas struts are designed to be installed with the fatter cylinder body at the top and the thinner rod (shaft) at the bottom when closed. This allows the internal oil to lubricate the seal. Check manufacturer recommendations.
- Test Carefully: After installation, slowly open and close the hood/hatch a few times to ensure smooth operation and full extension/compression without binding.
Post-Measurement Checks
- Full Extension: Does the hood/hatch open to its full, desired height without straining the hinges?
- Full Closure: Does it close completely without resistance from the struts?
- Stable Hold: Does it stay securely open without sagging or slowly closing?
- Smooth Operation: Is the movement smooth throughout the entire range, or does it feel jerky?
If any of these checks fail, re-evaluate your measurements and the force rating. It’s possible you need a slightly different strut, or there might be an issue with the mounting points themselves.
Common Mistakes to Avoid When Replacing Gas Struts
Even experienced DIYers can make simple mistakes when dealing with gas struts. Knowing these pitfalls can save you time, money, and headaches.
Avoiding these common errors will make your gas strut replacement project much smoother and more successful, whether it’s for a daily driver or a hardcore off-road rig.
- Not Supporting the Load: As mentioned, never rely on a failing strut. Always use a prop rod or an assistant.
- Ignoring Force Rating: Getting the length perfect but the force wrong is just as bad as getting both wrong.
- Mixing Old and New Struts: Always replace them in pairs for balanced performance and longevity.
- Measuring Incorrectly: Failing to measure center-to-center or using a flexible tape measure that sags can lead to inaccuracies. Double-check your measurements.
- Forgetting End Fittings: The best strut in the world won’t work if the end fittings don’t match your vehicle’s mounting points.
- Overlooking Obstructions: Ensure the strut has a clear path of movement and won’t hit anything as the hatch/hood opens and closes.
Frequently Asked Questions About Gas Strut Measurement
What if I can’t find the exact same length strut?
Aim for the closest match possible. A difference of 5-10mm (about 1/4 to 1/2 inch) in extended length is often acceptable, but ensure the compressed length still allows full closure and the force rating is correct. If the difference is larger, you might need to adjust mounting points or look for a different strut.
Do gas struts need to be lubricated?
The internal components are sealed and self-lubricating. You generally don’t need to add lubricant to the shaft or body. However, a little grease on the ball studs or pivot pins where the strut attaches can help with installation and prevent squeaks.
How do I know if my gas strut is failing?
Common signs include a hood or hatch that won’t stay open, sags slowly, or is difficult to lift. You might also hear a hiss or see oil leaks around the shaft, indicating a seal failure. Extreme temperatures can also affect their performance temporarily.
Can I replace just one gas strut?
While technically possible, it’s highly recommended to replace gas struts in pairs. Even if only one has visibly failed, the other is likely equally worn and will soon follow. Replacing both ensures balanced support and extends the lifespan of your new components.
Is there a difference between gas struts for a car hood versus an RV compartment?
Functionally, they are the same: they provide lift assistance and hold a load. However, the specific lengths, force ratings, and end fittings will vary significantly based on the weight and size of the object they need to support. Always measure for the specific application.
Mastering how to measure gas strut length is a fundamental skill for any DIY mechanic or off-road enthusiast. It ensures safety, proper function, and avoids the frustration of ill-fitting parts. By taking accurate measurements, identifying the correct force rating, and choosing the right end fittings, you can confidently replace those failing lift supports and restore your vehicle’s functionality.
So, grab your tape measure, secure that hood, and get to work. Your arms (and your head!) will thank you. Stay safe and stay comfortable!
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