What is the difference between concentricity, runout and true position?

What does concentricity control, and why did ASME Y14.5-2018 drop it?

Concentricity, as defined up to ASME Y14.5-2009, requires every median point of diametrically opposed surface elements to fall inside a cylindrical zone centered on the datum axis. ASME Y14.5-2018 removed the symbol because that check is hard to verify, rarely matches the design intent, and can almost always be replaced by runout, position or profile.

Think about what the definition asks for. The inspector has to collect many pairs of points on opposite sides of the feature, compute the midpoint of each pair, and confirm that the whole cloud of midpoints sits inside the zone. An oval or three-lobed diameter can pass easily, because its opposite points balance out even though the surface is far from round. Concentricity therefore controls how evenly mass is distributed around the axis, not how the surface runs when it turns or how it fits a mating part.

That is rarely what a designer means. In most cases the real requirement is “this diameter must not wobble against the bearing seat” (a runout question) or “this pilot must fit into the housing bore” (a position question). The 2018 edition of the ASME Y14.5 standard removed both concentricity and symmetry and points users to those other controls instead. The edition was reaffirmed in 2024, so it remains the current US reference.

Existing drawings do not become invalid. A drawing is read against the edition of Y14.5 it names, so a print that cites Y14.5-2009 and shows the concentricity symbol still means exactly what it meant when it was released. The trouble starts when a drawing shows the symbol but names no edition, or names 2018 and still uses the symbol. In that case the supplier has to ask which rule applies before quoting.

When is position the right callout for a coaxial feature?

Use position when the feature has to assemble with something, such as a pilot, spigot or locating diameter that slides into a mating bore. Position can take the maximum material condition modifier, which adds bonus tolerance as the feature departs from its tightest size and allows a simple functional gauge.

When position is used coaxially, its zone is a cylinder that shares the datum axis, and what it tolerances is the axis of the feature’s actual mating envelope. That is a fit-oriented definition: it asks whether the feature can go together with its partner, not how it turns. Applied at maximum material condition (MMC), a pilot made smaller than its largest allowed size gets extra positional tolerance, because the smaller part still fits. The requirement can then be checked by a receiver gauge that either accepts the part or does not, which is fast on a production line.

Position is the wrong tool for rotating function. A seal lip or bearing reacts to every high spot as the shaft turns, and position does not see out-of-roundness. For those surfaces, runout describes the behavior you actually care about.

Which callout should you use on shafts and valve stems?

On most shafts, control bearing journals and seal diameters with runout to a common datum axis made from the two bearing seats, and use position only for features that locate or assemble. On valve stems, tie the packing or seal diameter to the guide diameters with runout, and let form and size limits handle the rest.

• Bearing journals. Make the two journals datum features A and B and reference the common axis A-B. Total runout to A-B on the journals themselves covers taper and straightness, which matter for bearing life.
• Seal diameters. Circular runout to A-B, because the lip reacts to wobble at each section, not to the average axis.
• Shoulders and thrust faces. Circular runout if the face only has to run true; total runout if the whole face must be square to the axis, as for a bearing abutment.
• Pilots and spigots. Position at MMC to the datum axis, so fit is guaranteed and a functional gauge can be used.
• Valve stems. Runout on the packing diameter relative to the guide diameters, since a stem that wobbles through the packing wears it unevenly and leaks. The thread and the disc or plug seat usually follow from position and profile.
• Balance-critical parts. If mass distribution was the reason for a concentricity callout, say so and specify a balance grade per ISO 21940-11 or its equivalent rather than asking for midpoint data.

Whatever you choose, pick datum features that the part will actually sit on in the assembly. A runout callout to a center-drilled hole makes sense only if the part turns on those centers in service; if it runs on its journals, the journals should be the datums.

What should a drawing sent to an overseas turning shop say about coaxial controls?

Name the standard and its edition in the title block, use the control that matches the function, and state how the datum axis is established. With those three things on the print, a shop in Vietnam or anywhere else can plan inspection before it quotes.

• Edition. “Dimensioning and tolerancing per ASME Y14.5-2018” or “per ISO 1101”, not just “GD&T”.
• Legacy concentricity. If a released drawing under Y14.5-2009 keeps the symbol, say whether runout may be used as a substitute check. Runout is usually the more restrictive of the two, but a substitute check should be agreed in writing, not assumed.
• Datums. Show which diameters or faces form the axis. Two journals forming A-B read differently from a single short diameter, which cannot stabilize an axis on its own.
• Inspection report. Say whether you need FIM readings, CMM data or a gauge attribute result for each coaxial feature, so the report format is priced in.

The RFQ package guide lists the rest of what to send. When the drawing is ready, send it for a quote; coaxial callouts that need discussion are raised in the reply, one by one.