How to Measure a Mechanical Seal: Shaft, Bore and Dimensions

Why Measuring Matters

A mechanical seal is a precision component: seal faces are lapped to micron-level flatness, and the difference between a seal that fits and a seal that does not is often one millimeter. Ordering by guesswork — "it looks about 25mm" — is the fastest way to buy a seal you cannot use. Correct measurement is the difference between a 15-minute order and a 3-week return cycle.

The good news: you only need four numbers. Shaft diameter, seal bore, installed height, and (if readable) the old seal's part number. With those, any competent supplier can match a replacement — and a good one will confirm the match before shipping.

Tools You Need

• • Digital calipers (150mm range is enough for most pump seals) — accurate to 0.01mm
• • A clean, lint-free cloth to wipe the shaft before measuring
• • A steel ruler as backup (digital calipers can fail)
• • A notepad and camera — record every number and photograph the old seal

Do not use a tape measure for seal dimensions. Tape measures are fine for pipe runs, but a 0.5mm error on a tape is the difference between a seal that seats and one that leaks.

Measuring Shaft Diameter

The shaft diameter is the most important number — it fixes the seal size. Measure at the point where the seal sits on the shaft, not at the coupling end (the shaft may have different diameters in different zones).

Method: wipe the shaft clean, close the caliper jaws on the shaft, and read the measurement. Take two readings at 90° apart — shafts can be slightly oval, and the smaller reading is the one that matters. Standard pump seal shaft sizes are typically 12, 14, 16, 18, 20, 22, 25, 28, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110 and 120 mm. If your reading is 24.9mm, the seal is a 25mm — not a custom 24.9.

Measuring Seal Bore

The seal bore (also called the stuffing box or seal chamber bore) is the hole in the pump casing where the seal sits. Measure the internal diameter of this bore with the caliper jaws opened outward (inside jaws).

Standard seal chamber bores follow API 610 / ISO 3069 sizes: 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120, 125, 140, 160 mm and beyond. The bore determines which gland and stationary face fit. A common mistake is measuring the bore opening at the face rather than the true chamber — measure the machined bore where the gland seats.

Measuring Installed Height

Installed height (the distance from the gland face to the rotating face contact point, measured along the shaft axis) matters for seals with specific compression requirements — especially component seals where the spring must be set to a working length. If you are replacing a cartridge seal, the installed height is pre-set and you only need the part number or shaft + bore.

For component seals, measure the old seal's free height (assembled but uncompressed) and note the gland-to-face distance when installed. Take a photo of the seal sitting in the pump before removal — it is worth a thousand words to a supplier.

Reading Dimensions Off the Old Seal

Many seals carry their size printed or stamped on the gland plate or sleeve — for example "25-40" meaning 25mm shaft, 40mm bore, or a full part number like "M7N-25" or "58U-40". Photograph the stamping clearly before cleaning. The part number is the single most valuable piece of information: it lets a supplier identify the exact configuration, including face materials and spring design.

If the stamping is worn, measure the outside diameter of the stationary face and the inside diameter of the rotating face — these usually correspond to standard sizes. Wipe the faces first; a millimeter of grime will ruin your reading.

Metric vs Inch: Avoiding the Classic Mistake

Mechanical seals are made in both metric and inch sizes, and mixing them up is one of the most common order errors. A 1-inch shaft measures 25.4 mm — very close to a 25 mm seal. If your calipers read 25.3 mm and you round to 25, you may be buying a metric seal for an inch pump, or vice versa.

How to tell which system your pump uses: check the pump nameplate (many European and Asian pumps are metric; US pumps are often inch), look at the old seal's part number (a "1.000" or "1-1/4" indicates inches), and measure precisely — a true inch shaft reads 25.4 mm, a true metric 25 shaft reads 25.0 mm. If in doubt, give your supplier both readings: the exact millimeter value and your guess at inches. A good supplier will resolve the ambiguity before quoting.

Seal Chamber Types: Know What You Have

The seal chamber (stuffing box) comes in a few common configurations, and knowing yours avoids ordering a gland that will not seat. A standard bore is a straight cylindrical chamber. A tapered bore widens toward the impeller, which changes how the gland and stationary face sit. A cartridge-style chamber is machined to accept a cartridge seal directly. Large API pumps may have a chamber with a throat bushing or a specific gland bolt pattern.

When you photograph the chamber, include a straight-on view of the gland face and a side view showing the bore profile. Note the gland bolt spacing and hole size as well — these are part of the fit. If the chamber has been re-machined or sleeved in the past, say so; it changes the effective bore dimension and the correct seal.

Common Measuring Mistakes

• • Measuring a worn or pitted shaft zone — the seal sits on the unworn area
• • Measuring with the caliper at an angle — always square to the shaft axis
• • Confusing inches with millimeters (a 1-inch shaft is 25.4mm, not 25)
• • Measuring the bore at a burred or chamfered edge
• • Trusting a single reading — always measure twice, 90° apart
• • Ignoring the old seal's part number because it "looks different"

What to Send Your Supplier

Send: pump brand and model, shaft diameter, seal bore, a photo of the old seal (both faces and the stamping), and — if known — the media, temperature and pressure. That is everything a competent supplier needs to confirm the right seal. If a supplier quotes without asking for these, treat it as a warning sign: they may be guessing, and you will be the one paying for the return.

We confirm the fit before you order — send your numbers and we will match the seal, or tell you honestly what else we need.