Choosing the right mechanical seal shaft size is one of the most critical steps in ensuring a leak-free, long-lasting pump operation. A wrong size means immediate failure, costly downtime, and potential damage to your equipment.
This guide will walk you through everything you need to know about selecting the correct shaft diameter—covering both inch-based (imperial) and metric sizes commonly used worldwide.
Browse our complete range of metal bellow mechanical seals from 12mm to 18mm shaft sizes.
The shaft size (also called shaft diameter, denoted as d₁) determines whether the seal can fit onto the rotating shaft of your pump. Even a 0.5mm mismatch can cause:
Seal slippage or spinning on the shaft
Premature wear of the sealing faces
Leakage along the shaft sleeve
Complete seal failure within hours of startup
Below is a practical reference table covering the exact sizes you'll encounter in the field.
| Shaft Size (mm) | Typical Applications |
|---|---|
| 12mm mechanical seal | Small circulating pumps, booster pumps, domestic water systems |
| 14mm mechanical seal | Compact centrifugal pumps, condensate pumps |
| 16mm mechanical seal | Light-duty industrial pumps, pool filtration pumps |
| 18mm mechanical seal | Small chemical dosing pumps, HVAC circulators |
| 19mm mechanical seal | Standard small water pumps, irrigation pumps |
| 20mm mechanical seal | General-purpose end-suction pumps, light slurry handling |
| 22mm mechanical seal | Medium-duty clean water & chemical pumps |
| 24mm mechanical seal | Multi-stage pumps, boiler feed pumps |
| 25mm mechanical seal | Very common in ANSI B73 standard pumps, API process pumps |
| 28mm mechanical seal | Industrial water treatment, medium-pressure applications |
| 30mm mechanical seal | Larger centrifugal pumps, cooling tower pumps |
| 32mm mechanical seal | Heavy-duty water supply, fire-fighting pumps |
| 35mm mechanical seal | Chemical process pumps, refinery service, often interchangeable with 1.375″ |
| 40mm mechanical seal | Large industrial pumps, mining dewatering, marine pumps |
| 45mm mechanical seal | High-flow pumps, large vertical turbines, heavy slurry handling |
| Search Term | Exact Inch Size | ≈ Metric Equivalent | Common Application |
|---|---|---|---|
| 3/4 mechanical seal | 0.750″ | 19.05 mm (~19mm) | Small end-suction pumps, domestic water boosters |
| 1.375 mechanical seal | 1.375″ (1⅜″) | 34.93 mm (~35mm) | ANSI chemical pumps, API 610 process pumps |
Remove any rust, scale, or old seal debris. Measure at the exact location where the seal will sit.
Measure the shaft diameter at two different points (90° apart) and take the average. This compensates for minor ovality or wear.
Compare your measurement against the table above. If you get 34.9 mm, choose a 35mm mechanical seal. If you get 19.1 mm, use a 19mm mechanical seal.
Remember: shaft size alone is not enough. You also need:
• Bore diameter (d₂) – the housing bore where the seal sits
• Operating length (L) – the working height of the seal assembly
Some search terms look like sizes but are actually OEM part numbers.
Look up the original Grundfos pump model
Check the seal kit number (e.g., 96511950)
Match the shaft size (usually 20–28mm depending on pump frame)
Send us a photo of your old seal next to a ruler and we'll help identify the correct replacement.
Use precise keywords to filter results faster:
Take a photo of the old seal next to a ruler
Note any markings on the rubber bellows or metal shell
If possible, keep the original packaging with part number
Many international pumps use metric shafts but imperial bores (or vice versa). Always confirm both before ordering.
Before purchasing a replacement seal, ask yourself these 4 questions:
If you answer "No" to any of the above, stop and verify first. A 5-minute measurement saves hours of rework and hundreds of dollars in lost production time.
Send us a clear photo of your current seal placed next to a ruler, including the shaft, bore, and overall length. We can help match it to the correct size.
We can help match it to the correct mechanical seal 12mm, 35mm mechanical seal, 3/4 mechanical seal, or any other size you need.
Our company is specialized in the production of sealing materials, mechanical seals , cartridge seals.
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