The Short Answer
Size from the seat, not the bore. The ball must seat fully on the sealing surface without passing through it, and lift freely without binding in its guide. Those two constraints define a window, and the correct size sits inside it with room for thermal expansion.
The Two Constraints
Lower limit — the seat
The ball must be meaningfully larger than the seat's inside diameter so it lands on the sealing face rather than dropping into or through it. A ball only marginally larger than the seat bore contacts on a very small band, concentrating load and accelerating the cold-flow flat that eventually ends its life.
Upper limit — the cage
The ball must move freely in its guide or cage to lift off the seat under forward flow and return under reverse flow. Too large and it binds — sometimes only once it warms up, which produces an intermittent fault that is difficult to diagnose because the valve tests fine cold.
The Thermal Expansion Allowance
This is the step most often skipped, and it is the reason PTFE balls sometimes bind in service having passed bench testing.
PTFE expands roughly ten times as much as steel per degree. In a steel valve body, that means the clearance you measured at room temperature shrinks as the assembly heats — the ball grows faster than the housing around it. A ball with adequate clearance at 20°C can bind at 120°C.
Practical approach: establish your clearance at ambient, then check what happens at your maximum operating temperature before committing. If the valve runs hot, deliberately size toward the smaller end of the acceptable window rather than the middle.
Measuring an Existing Valve
When replacing a ball whose size you do not know:
- Measure the old ball if it is still there — but treat it as an approximation. A worn ball is undersize and out of round, so do not simply order what you measure.
- Measure the seat inside diameter with a small-hole gauge or pin gauge. This is your firm lower reference.
- Measure the cage or guide bore for the upper limit.
- Check the lift height — how far the ball can travel. This affects flow capacity and, in a vertical valve, cracking pressure.
- Photograph everything next to a scale and send it with the enquiry. We can usually identify a standard size from the seat dimension alone.
Density and Cracking Pressure
In a vertical check valve, the ball's weight sets the cracking pressure — how much flow is needed to lift it. Material density therefore matters as much as diameter:
- Stainless steel: ~8.0 g/cm³
- Solid PTFE: 2.14–2.20 g/cm³
- Delrin (POM): ~1.41 g/cm³
- Nylon: ~1.14 g/cm³
- Polypropylene: ~0.90 g/cm³ (floats)
Swapping steel for PTFE in the same valve cuts ball mass to roughly a quarter, so the valve opens far more easily. Sometimes that is exactly what you want. Sometimes it means the valve no longer closes reliably against low back-pressure. Decide deliberately rather than discovering it on commissioning.
What to Send Us
- Seat inside diameter
- Cage or guide bore
- Operating temperature range
- Media and pressure
- Orientation — vertical or horizontal
- Photograph of the old ball next to a ruler, if available
With those, we can recommend a diameter and a grade rather than just quoting whatever number you send us.