How to Measure a Hydraulic Cylinder

Welcome back to RuggedMade. I'm Jared. Today we're continuing our look at hydraulic cylinders. We offer a range of hydraulic cylinders that are suitable for many applications. Our cylinders are used on all kinds of machines, from replacing a cylinder on a loader or a dump trailer to custom projects like making a DIY log splitter or a forge press. And we often hear from customers that the process of figuring out which cylinder to get is confusing. The purpose of this video is to help you select the right cylinder, and we're going to focus on getting some key dimensions and specifications.

The good news is you don't need to be a hydraulics engineer just to find a suitable cylinder. If you're just trying to get your machine back up and running or finish a fabrication project, this video is for you. We split this topic into two videos. The first video, which you can watch by clicking the link here or in the description below, is an introduction to the main parts of a cylinder. In part two, we're going to focus on taking the measurements that will help confirm that you have the right fit.

If you're replacing a cylinder and you don't have detailed specifications for the original, you will need to take some careful measurements of the old cylinder. If you're shopping for a cylinder for a custom project, well, you'll have to determine the specifications first. It can be helpful to know what off-the-shelf sizes are readily available, though, so check out ruggedmade.com for more information.

A couple of tools you'll need are a tape measure and a caliper. You can get a caliper for 10 or 20 bucks at your local hardware store these days, so there's really no reason not to have one. But if you don't already have one, I suggest getting one that not only shows inches in decimal but also fractions. You will not regret it. You really don't need a micrometer for this type of task, and a bore gauge can be useful for getting a really accurate measurement when we get to measuring the bore of the cylinder barrel, but it's usually not necessary. We're not machining this thing on a lathe or milling machine.

Most companies stick to fairly common dimensions and increments for the cylinders when engineering their products. You'll rarely see a mounting pin with a weird diameter. It'll be one and a quarter inches or one and half inches or 1.625 inches, which is 1 and 5/8, and cylinder stroke is typically going to be in 1 inch or 1/2 inch increments, unless it's a really proprietary cylinder for a unique piece of equipment.

The stroke is one of the most critical dimensions. It is also the one that seems to confuse the most people. The stroke is simply the distance that the rod and piston travel from the fully retracted to the fully extended positions. You measure the stroke before you disassemble the cylinder. Use a tape measure to measure the distance from pin center to pin center when the rod is fully retracted to when it's fully extended, and the difference between these two lengths is the stroke. With the ports open, you can usually pull the rod out. If you're measuring a cylinder that you just took off a piece of equipment, remember that it may be full of oil. When you push or pull on the rod to measure the fully extended and retracted lengths, hydraulic fluid might squirt out of the ports, so we suggest draining it over a bucket first.

For some larger cylinders that you can't pull out manually, compressed air can be used to extend and retract the rod. Just use caution. The rod can move surprisingly quickly, even with just 100 PSI of air pressure from a hydraulic shop compressor. This cylinder measures 14 and 1/4 inches pin center to pin center retracted, and it measures 18 and 1/4 inches pin center to pin center extended. The difference is 4 inches, and that is our stroke. Now, these pins are 1-inch pins. So, if you're having a little trouble getting measurement to the dead center or if the pins aren't there to the dead center of the hole, it's pretty easy to just go to the outer edge of the pins. You just want to be on the same side for both. So, we're going to go from the outer edge of that pin to the outer edge of this pin, and we get that 18 and 1/4 inches for the extended, and we would get 14 and 1/4 [inches] for the retracted. We'd still get our 4 inch stroke.

When a clevis mount is threaded onto the end of the rod like it is with this one, it should be threaded on almost completely in order to create a strong connection. However, this design does allow for a small amount of fine-tuning of the mounting, the final mounting position of this clevis. This can be helpful when you need a little more or a little less overall length to make the cylinder fit your machine. Just keep in mind this doesn't change the stroke; it's just slightly changing the mounting dimensions. In this case, it's still going to be a 4 inch stroke, even if you maybe unthreaded this maybe a quarter inch.

We've been talking about pin center to pin center. For cylinders with different mounting types, just use a consistent reference point, like on these trunnion mounts. So, you'd be measuring from the center of the trunnion mount to the center of the rod end hole.

Bore refers to the inner diameter or ID of the barrel. The piston travels inside the barrel as fluid pushes on each side of it, as you can see in this cutaway cylinder. The bore ID, which also correlates to the surface area of the piston, determines how much force the cylinder can generate. When someone says they have a 4 inch cylinder, they're usually referring to the bore ID. If you know the wall thickness, you can determine the ID by measuring the OD and subtracting the wall thickness. If you want to do it that way, just remember that there are two walls in the outer diameter, so you need to back the thickness of both of them out in order to get the ID.

Best practice is to disassemble the cylinder. Don't attempt to disassemble a cylinder unless it's been completely disconnected from the hydraulic system and drained of oil. Now, we're going to remove this gland from this threaded cylinder. Threaded glands on welded cylinders typically require a special wrench. Here are a couple of examples, and the threads are often secured with thread lock or another type of sealant, so you may need to apply some heat, and keep in mind that heat could damage the seals. You will need to secure the cylinder due to the amount of force that is usually required to unscrew these glands. Sometimes the best way is to just leave the base end in the piece of equipment pinned in place, and that'll give you the leverage that you need.

Once you've loosened the gland, you can gently pull it out. Keep in mind some residual oil will probably drain out and make a mess. Then use your caliper to measure the ID of the barrel. So, this is the key dimension that you need to match. Now, you may not need to fully remove the whole rod and piston to get this measurement, but make sure that the rod is supported so that it stays aligned with the barrel. We don't want the edges of the piston damaging the inner surface of the barrel. The threaded portion near the opening of the barrel might be machined a little bit thinner than the actual ID where the piston sits inside the barrel. Generally, the dimensions are going to be so close that that'll tell you what you have. In this case, it's a 4-inch ID cylinder. If you want the most precise dimension, you can remove the rod completely and use a bore gauge and measure from a location further inside the barrel.

Tie rod cylinders use nuts to compress the segments. Loosening these nuts will allow you to separate the gland from the barrel. You should loosen and tighten the rod nuts evenly to prevent warping or misalignment.

Here we have our tie rod base end, and it is machined to fit inside the barrel and has an O-ring seal. That's a static seal. There's our gland end. There’s our gland, piston, and there’s our barrel. With a tie rod cylinder, it's very easy to measure the ID. So here we have a 2-inch bore ID.

Measure the rod outer diameter with calipers. If you already know the bore ID, there's no need to disassemble the cylinder to this extent. Okay, so that's our 2 and 1/4-inch OD rod, and this smaller cylinder has a 1 and 1/8 inch or 1.125-inch OD rod.

We've covered stroke, bore, and rod outer diameter; those are three of the most important measurements to get to match up a cylinder. But there are a few other things we need to look at.

Now let's figure out what kind of ports your cylinder has. Two of the most common thread standards used for high-pressure hydraulic connections are NPT or National Pipe Tapered, and SAE, Straight Thread O-Ring or O-Ring Boss. You also see that abbreviated as ORB. NPT is a tapered thread. A seal is created when the male and female threads wedge themselves together and deform slightly as the parts are tightened. It's supposed to be a dry-seal thread, but Teflon tape or a thread sealant is recommended to prevent leakage, especially if these parts have been used before. O-Ring Boss is a parallel thread. The threads do not create the seal; instead, they create a mechanical connection. The seal is created when this O-ring is compressed between the sealing face on the male end and the chamfer on the female end as the two parts are pulled together.

Once you've determined which type are on the cylinder that you're replacing, you need to figure out which size you need. And this is where things get a little confusing. The measurements of these physical dimensions do not have much in common with the official nominal inch size used to describe the parts. The nominal inch size is the number used to describe NPT ports and fittings when you go to purchase components. O-Ring Boss sizes are described by their nominal in size or more commonly their Dash size, typically written as SAE-8 or SAE-10. To make things even more confusing, the physical measurements of the male and female threads for the same nominal size are different.

So let's look at a few examples. You measure the size of a male NPT fitting at the midpoint. So this male fitting has an OD of 1 and 1/16 inches. Now, that correlates to a nominal inch size of 3/4 inch. The female thread ID at the opening of the port is 1 inch or technically, .98 inches, and its corresponding nominal inch size is 3/4 inch, which means these two are a match.

Let's look at another example. The ID of this port measures 25/32 inches or 0.77 inches, and that correlates to a nominal inch size of 1/2 inch. This male fitting OD measures 27/32 inches, which also correlates to a nominal inch size of 1/2 inch.

O-Ring Boss is not intuitive either. This female port measures 11/16 inches or 0.69 inches. The male fitting OD is 3/4 inch or 0.75 inch. Now, the nominal port size for both of these is 1/2 inch, and the corresponding thread size is 3/4-16, and the dash size is SAE-8.

As you can see, there are a lot of ways to describe just one size of O-Ring Boss connection, but don't shoot the messenger. We just want you to be aware that measuring the opening with a tape measure and thinking the raw physical measurement is the same as the nominal port size that you would use when you go to shop for these things doesn't match up. So this is one of the most common mistakes people make when ordering cylinders and fittings. Fortunately, most cylinders use a few common sizes. Nominal sizes such as 1/4 inch, 3/8 inch, half inch, 3/4 inch, 1 inch, and 1 and 1/4 inch are what you'll see most often when you're looking at pumps, valves, cylinders, and fittings.

Your new cylinder should have the same size ports. Ideally, it will have the same thread type in order to utilize your original fittings. However, you may not be able to find a cylinder that matches perfectly. If your original cylinder has O-Ring Boss ports and the only replacement cylinder you can find is an exact match in every way except that it's only available with NPT ports, that's not a big problem. It is usually easy and inexpensive to replace the fittings with ones with another thread type or even to use NPT to O-Ring Boss adapter fittings. And this will allow you to connect your original hoses to the different type of port threads on your new cylinder. The connection to the hoses, which is probably something like JIC 37° compression, would remain the same.

The ports on a cylinder can be oriented in various angles. The rod can rotate, so this orientation is usually indexed relative to the fixed mounting point at the base of the cylinder. Twelve o'clock is the default position on most cylinders, like on this cross tube example. However, some applications require that the ports be lined up at the three or the nine o'clock position, like on this example. This is usually done to allow the cylinder to fit into a tight area where the fittings would interfere with the frame or some part of the machine if they were installed at the 12 o'clock position.

One interesting feature of a tie rod cylinder like this is there's one port on the gland end, and you see that gives us the three o'clock or the nine o'clock orientation, but there are actually two ports on the base end. One would normally be plugged. If you wanted to change this from the three o'clock to a 12 o'clock orientation, as you saw earlier, we just can disassemble the cylinder, rotate that base 90°, and then we would have a 12 o'clock orientation. Some custom cylinders may have an extended port tube. You might see a 2 or 3-inch long tube here. Finding a replacement with the same length port tube can be difficult, but sometimes you can make a more standard cylinder like this work by using a long straight fitting like this. It would probably be a male to female, and you could thread that in, and that would pretty much give you that tube length that you need.

A cylinder needs to be mounted with at least two points of contact: one somewhere along the barrel and one at the end of the rod. So let’s take a look at some common mounting types. Here we've got our clevis. This is a cast clevis threaded onto the end. There’s a cast fixed clevis on the base. This snowplow cylinder uses pin eye on both ends. Here’s our cross tube. Here we’ve got a ball swivel. All of these examples have a mounting point at the rear end of the barrel but sometimes cylinders can mount in other ways. Here we have an example of a trunnion mount. As you can see, these ears are very close to the rod end of the cylinder. Another similar style would be a flange mount where you could weld a plate anywhere on the barrel and attach it to your machine with bolts. Also keep in mind, we looked at pin eye, cross tube, swivel ball, and clevis, but the cylinder doesn’t have to use the same type at both ends. There could be a mix of clevis and cross tube. Again, it really depends on how the cylinder needs to mount to the machine.

Check the dimensions of the mount. Check the length of the tube or the clevis, check the gap, and measure the diameter for the pin, and measure both ends because, in this case, the tubes are different lengths. Also, some mounts are designed to be greased. If you’re replacing a cylinder with a grease fitting like this, make sure that the new one has one as well. Most cylinders mount with some kind of pin like this, so you would want to measure the length and diameter of the pins. Here we’ve got a 4.6 inch pin in length and 1 inch in diameter. It’s retained with a spring clip, so that would mount in this cross tube, in this pin eye, and also in that clevis, and this one would be held in with two spring clips. Some pins are going to be held in with cotter pins. That one would take one cotter pin and the other end of the machine with a flange. That one takes two.

Replace pins if they’re worn out. Keep in mind, your machine may also have a bushing that the pin sits in that needs to be replaced. You may find a cylinder that is almost perfect; maybe there's a dimension that is just a little bit different than the original that you're replacing. A cylinder that is not an exact match will often work just fine, and this can save a lot of money compared to buying an OEM cylinder, assuming one is even available. When it comes to the stroke, you do need to be careful. If the replacement cylinder has a longer stroke than the original, make sure you will not be overextending a part of the machine in such a way that it could damage itself or interfere with another part. In the case of a log splitter, for example, you wouldn't want to use a 26-inch stroke cylinder on a beam that was really originally meant for a 24-inch stroke cylinder. That would allow you to push the blade right into the foot. But on a tractor, maybe all that happens with a longer stroke is the bucket curls a bit more, and it doesn't cause any issues. On the other hand, a shorter stroke would mean less throw than the original, but that might not affect operation either.

Many of our customers install cylinders that have a larger bore than the original. More power is always better, right? Just keep in mind that the larger bore will allow the system to generate more force, and that could damage your machine. If you do use a cylinder with a larger bore, the cycle time will be slower. This assumes you're using the original pump with the same displacement or flow rate. That pump will have to move more fluid into and out of the new larger cylinder in order to move the piston, resulting in a slower speed.

If you need to replace a cylinder that's one of a matched pair, such as the lift cylinders on a boom, it's recommended that you replace both. If the two cylinders don't match, it could cause problems. If you're only going to replace one of a pair, ensure that the new one has the same specs as the old one.

Sometimes a minor modification will allow you to use a replacement cylinder that is not an exact match. For example, on this cross-tube cylinder, if the cross-tube mount is a little bit too long to fit the machine, you could grind it down, as long as you're not taking material away from the barrel. Or if the tube is too short, you might be able to use some washers to shim it, and that would keep it centered in its mount. Or if your original mounting pin is 3/4 inch in diameter and the replacement cylinder tube takes a 1-inch pin, you might be able to sleeve that down to accept the 3/4 inch pin. If the ports on the new cylinder are larger than the original, you can often use a reducer fitting. That covers how to measure a cylinder.

Armed with this information, we're confident you'll be able to select the right cylinder for your project. Check out our line of cylinders at ruggedmade.com. There are filters on the left-hand side of the cylinder page; those will help you narrow down what you're looking for based on key specifications. We also provide a detailed schematic for each cylinder. You can view these schematics in the diagrams and drawing section. We are continually expanding our lineup of cylinders; this includes more sizes and more types of cylinders for specific applications. If you don't see what you're looking for, contact our customer service department. Thanks for watching.