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How to measure a hydraulic cylinder for a seal kit
Bore and rod decide the kit. Everything else just confirms which cylinder you have. You need a vernier or digital caliper — fifteen minutes here saves a week of shipping the wrong thing back and forth.
- Bore
- Inside diameter of the tube
- Rod
- Outside diameter of the rod
- Stroke
- Extended minus retracted
Bore and rod set the seals. Stroke identifies the cylinder but changes nothing in the bag.
Step by step
- 01
Measure the bore inside the tube
Bore is the inside diameter of the barrel, not the outside. Slide a vernier or digital caliper inside the open end of the tube and read across the middle. If you cannot get in there, measure the piston OD instead and add about 0.010 in for seal squeeze.
- 02
Measure the rod on a clean, unworn section
Measure close to the gland, on a section that has not been running through the seal. The middle of a working rod polishes undersize, and measuring there sends you a kit that is 0.010 in too small.
- 03
Work out the stroke
Stroke is extended pin-to-pin length minus retracted pin-to-pin length. It is not the length of the rod, and it is not the length of the tube. Stroke does not change which seals fit — it only tells us which cylinder you have.
- 04
Note the gland style and the port type
Threaded external, threaded internal, bolt-on, snap-ring, wire-ring, or setscrew. Two cylinders with identical bore and rod take different kits if the glands differ. Note the port too: SAE ORB, NPT, JIC, BSPP or a Code 61 flange.
- 05
Measure the old seals and photograph them
Lay the old seals out next to a ruler and photograph them. Measure each one's ID, OD and height with a caliper. That set of numbers identifies a kit on its own more often than anything else you can send us.
The four mistakes that cause wrong orders
Nearly every returned kit traces back to one of these. None of them are careless. A worn cylinder just doesn’t measure like a new one.
You measured the rod seal ID, not the rod
A rod seal's inner diameter is the rod diameter, but a worn or stretched seal reads large and a compressed one reads small. Measure the rod itself. If the only thing you have is the old seal, say so when you send the numbers — we allow for it.
It is metric and reads as almost-inch
50 mm is 1.968 in, not 2 in. 40 mm is 1.575 in. 70 mm is 2.756 in. If your reading sits 0.020–0.040 in under a round inch number, you have a metric cylinder and an inch kit will not seal it. Kubota and Komatsu are metric throughout; Bobcat mixes both on one machine.
The bore is worn or scored, so it reads large
A grooved tube measures bigger than the cylinder was built. Measure the piston OD instead and add roughly 0.010 in. If the scoring is deep enough to catch a fingernail, a seal kit will not stop the leak — the tube needs honing or replacing.
A tape measure is not a caliper
Bore and rod need to be right to a few thousandths of an inch. A tape or a rule will not get you there. A 6 in digital caliper is cheap and it is the difference between one order and three.
Common metric bores in inches
| Metric | Reads as | Mistaken for |
|---|---|---|
| 40 mm | 1.575 in | 1.5 in |
| 45 mm | 1.772 in | 1.75 in |
| 50 mm | 1.968 in | 2 in |
| 60 mm | 2.362 in | 2.25 in |
| 63 mm | 2.480 in | 2.5 in |
| 70 mm | 2.756 in | 2.75 in |
| 80 mm | 3.150 in | 3 in |
| 90 mm | 3.543 in | 3.5 in |
Photo checklist for a request
The old seals laid out next to a ruler · the cylinder tag or stamped number · the gland, close up, so we can see how it retains · the whole cylinder for scale. Four photos and we can usually identify a kit without the machine model at all.