How to measure a hydraulic cylinder to order the right seals

Most of the requests we receive start like this: "I have a leaking cylinder, but I don't know the seal codes". That is normal: on many machines the cylinders have been repaired several times, the original seals have no legible markings and documentation is not available. The good news is that in most cases the right measurements are enough to identify the correct spare parts.

The tools you need

• A caliper (preferably digital) for diameters and heights;
• An inside micrometer or bore gauge to measure the tube accurately, if available;
• Clean cloths and degreaser to clean grooves and surfaces;
• Pen and paper (or a smartphone) to note the measurements and photograph the parts.

Step 1: dismantle and clean

After releasing the pressure and safely dismantling the cylinder, separate head, rod and piston. Remove the old seals noting the order and direction of each one: a photo before dismantling is the easiest way. Clean the grooves well: oil and dirt residues distort measurements.

Step 2: measure the rod and the head

1. Rod diameter (d): measure it in an unworn area, at several points. Rods almost always have standard diameters (for example 30, 40, 50 mm).
2. Rod seal groove: measure the groove bottom diameter (D) and its height (L).
3. Wiper housing: outer diameter, height and any steps.
4. Guide ring grooves in the head: diameter and width.

Step 3: measure the tube and the piston

1. Tube bore (D): the most important measurement for piston seals. Measure it at several points and depths to also check for ovality.
2. Piston seal groove: groove bottom diameter (d) and width (L).
3. Guide ring grooves on the piston: groove bottom diameter and width.

Step 4: do not rely too much on the old seals

Used seals may have swollen, flattened or worn. Measuring them helps identify the profile, but the reference dimensions must always be those of the grooves and metal surfaces. If the part is badly damaged, keep it anyway: its shape and section help us recognise the type of seal.

Step 5: gather the operating information

Besides the dimensions, note down:

• type of machine and function of the cylinder;
• approximate working pressure;
• single or double acting;
• oil temperature and environment (dust, water, outdoors);
• type of fluid, if different from mineral hydraulic oil.

Step 6: use the search by dimensions

On our website, in the products section, you will find the search by dimensions: enter the inner diameter, outer diameter and groove height and the system shows compatible profiles with price and availability. You can filter by category (rod, piston, wipers, guides, O-rings) and compare different materials.

Check the surfaces too

Before ordering, check the condition of the rod and tube. Deep scores, flaking chrome or an oval tube will make even new seals leak. In these cases it is worth regrinding or replacing the damaged parts.

Any doubts? Send us the measurements

If you cannot find the profile or the dimensions do not match standard ones, write to us with the measurements you took and a few photos of the grooves and old seals. Our technical support will tell you which solutions are available.

Frequently asked questions

Are the dimensions in millimetres or inches?

Most European cylinders use metric sizes, but on machines of American or British origin you will also find inch sizes. If the values do not match "round" millimetre numbers, try converting them to inches.

How precise does the measurement need to be?

For rod and bore you need the hundredth-of-a-millimetre precision of a good caliper. For grooves a tenth is usually enough, but the more precise you are, the easier it is to find the exact profile.

Can I just send you photos?

Photos help a lot in recognising the type of seal, but without dimensions it is not possible to identify the exact spare part. Send us both.