A stepped ejector pin breaks in a predictable place: at the step, where the thin tip meets the larger shank. That is a stress concentration and it is where the bending load peaks, so it is where the pin lets go. It is also, unhelpfully, the exact feature that carries the dimension you need in order to order a replacement.
This is a short field guide to recovering the size from what is left in your hand, and to what you can measure instead when the step itself is destroyed.
The four dimensions that define the pin
A stepped pin needs four numbers, not two. We write them as P × D × L – N:
- P — tip diameter, the small working end that contacts the part
- D — retention or shoulder diameter, the larger section that runs in the plate
- L — overall length
- N — retention length, from the head-side reference face to the step
The effective tip length is L − N. That is the free, unsupported part of the pin, and it is the number that decides whether the replacement survives or breaks in the same place a month later.
A common mistake is to order on the tip diameter alone. The same tip diameter is offered against more than one shank diameter — a Ø2 tip on a Ø3 shank and a Ø2 tip on a Ø4 shank are different parts and need different plate holes. Always give the P and D pair together.
If you have both broken halves
This is the easy case, and it is worth spending five minutes on before you throw the pieces away.
- Measure P and D with a micrometer, not a caliper, and take the reading away from the worn zone. The last few millimetres of the tip are often bell-mouthed or peened over from ejection, and the shank is often polished undersize where it has been running in the plate. Measure the tip near the step and the shank near the head.
- Butt the two pieces back together and measure L across the whole assembly. Add nothing for the material lost at the break — a clean fatigue fracture removes almost nothing, and guessing high here is how people end up with a pin that stands proud of the core face.
- Measure N from the underside of the head to the step. On a shouldered head, measure from the seating face, not from the top of the head.
Write down the head diameter and head thickness too. Head geometry is not in the P × D × L – N code, and it has to match the counterbore in the ejector plate.
If the step is gone
When the pin has broken exactly at the step, or when it has been in the tool long enough that the fracture face is battered, N is no longer measurable on the part. Measure the mold instead.
N exists to put the step inside the plate rather than inside the core. If the shoulder lands in the core bore, the pin will not seat: you will crush the step, open the hole, or both. So N is set by the plate stack, and the plate stack is still sitting on your bench.
Give us the thicknesses in order — ejector plate, ejector retainer plate, support plate, air gap, core plate — and the depth at which the pin has to change diameter, and we will work N back from that. A dimensioned sketch on paper is fine. So is a photo of the plate stack with a rule laid across it.
Measure the hole, not just the pin
If the pin has been running for a long time, the plate hole has worn too, and copying the old pin exactly reproduces the fit that failed. Two checks are worth the time:
- The tip bore in the core. If it has opened up, a new pin at nominal will flash immediately. You either ream the bore up to the next size and order the pin to suit, or you sleeve it. Deciding this before ordering saves a second shipment.
- Whether the step face has bedded in. A shoulder that has hammered a dish into the plate face means the pin has been taking impact, not just static load. Replacing the pin without addressing that will get you the same break.
Do not judge the material by the colour
Nitrided pins look dull grey and through-hardened pins look bright, but both look much the same after a year in a hot tool. What matters is the hardness distribution: a nitrided SKD61 pin carries a hard case of around 60 HRC over a tougher core of around 42 HRC. The case resists wear at the bore; the core stops the pin snapping like glass.
That has a practical consequence when you cut a pin to length. The nitrided case is hard and shallow, and cutting exposes soft core material at the cut face. At the head end that is fine and normal. At the working tip it is not, which is why finished-length pins should be ordered cut and faced rather than trimmed at the tip on the bench.
The minimum you need to send a supplier
If you take nothing else from this: a quotable enquiry for a stepped pin contains the tip diameter, the shank diameter, the overall length, the retention length or the plate stack it has to suit, the head form, and the quantity. Six items, and it can be a photograph of a caliper reading rather than a drawing.
What is not quotable is "the stepped pin from the 2-cavity tool". We have no way to guess P against D, and the pin that arrives will be wrong in a way that is not obvious until it is in the plate.
Related
- SKD61 nitrided stepped ejector pins, packs of 5 — standard P / D combinations, ready to order
- Custom stepped ejector pins — non-standard combinations, double-step and three-section pins
- Ejector pins and core components
If you are holding a broken pin and are not sure which dimension is which, email a photo with a caliper or a rule in the frame to sales@privleymold.com. That is usually enough for us to come back with a size, and it is faster than a drawing.