Most hollow shaft encoders are shipped with the bore machined to a single standard tolerance, H7, with US suppliers commonly calling out –0.0000 to +0.0005 inch on the inside diameter, and that single spec decides the fit class you have to honour on the mating motor shaft [S1][S4].
The shaft tolerance itself is not the encoder supplier's variable, it lives in the host machine, and the practical installation limits are radial runout under 0.007 in (0.18 mm) and axial endplay within ±0.030 in (±0.76 mm) for typical thru-bore designs [S3]. Choosing the wrong fit class, not choosing the wrong encoder, is the root cause of most field failures called in as encoder faults.
H7 Bore and h6 Shaft: The Default Clearance Pair
ISO 286 H7/h6 is the workhorse fit for hollow shaft encoder installations, and it is the answer on roughly 80% of nameplates and catalogue pages [S1][S7]. A 25 mm H7 bore measures +0.000/+0.021 mm, and the matching h6 shaft measures +0.000/-0.013 mm, giving a guaranteed clearance of 0.013 to 0.034 mm at room temperature [S7].
That pair is defined as a sliding fit in most references and it makes encoder swap-out possible without pulling the motor, which is the whole point of a hollow bore architecture versus a modular shaft encoder [S5][S6]. For typical NEMA motor frame shafts, 0.375 in (9.53 mm) to 1.125 in (28.58 mm) bore, the same H7 envelope applies, and the supplier rarely offers a tighter alternative because the encoder's internal bearing set takes care of the disc-to-sensor air gap [S3].
Clearance vs Transition vs Press: What Each One Does to Service Life
A clearance fit (H7/g6) gives the loosest mechanical coupling and is preferred on applications where the encoder is mounted and dismounted repeatedly, such as motor rewind shops or rental fleets [S8]. A transition fit (H7/k6) is the practical middle ground: depending on the tolerance band, the pair can come up either with a slight clearance or a near-zero interference, and it locates the encoder concentrically without a keyway.
Press fits (H7/r6, H7/s6) are used only when the application requires zero slip under shock load, for example crane slew encoders or large mill drives, and the industry guidance from clutch and coupling makers is to keep interference below 0.001 in (0.025 mm) [S9]. Exceed that and you risk encoder bore deformation, bearing preload shift, and premature seal failure, problems that almost always show up as a noisy or hot bearing within the first 500 operating hours.
For hollow shafts specifically, bearing-seat guidance from SKF is explicit: when the shaft diameter ratio ci = Di/De exceeds 0.5, the standard solid-shaft tolerance bands must be tightened to preserve the same fit effectiveness, because the hollow section deforms under press load [S2]. That rule applies directly to any hollow-bore encoder installed over a stepped or hollow motor shaft.
Bore Diameter Range and Common NEMA / IEC Sizes
Stock hollow shaft encoder bores cover 0.250 in (6.35 mm) up to 1.500 in (38.10 mm) as standard catalogue offerings, with some manufacturers holding specials out to 2.0 in and beyond [S5][S6]. The most common in-service bores map directly to NEMA frame shafts: 0.375 in for NEMA 17 and 23, 0.500 in for NEMA 23 and 34, 0.625 in for NEMA 34, and 0.875 in to 1.125 in for NEMA 42 and 56 [S4].
IEC frame bores (11 mm, 14 mm, 19 mm, 24 mm, 28 mm, 38 mm) overlap the same envelope, and most encoder suppliers stock reducer sleeves so a single 1.125 in bore encoder can serve a 0.500 in, 0.625 in, 0.750 in or 0.875 in motor without a custom bore [S5]. For background on the broader rotary encoder family, the bore spec is the single dimension that varies most across product lines.
Radial Runout, Endplay, and Why They Replace Shaft Tolerance
For modular incremental encoders the motor shaft has to be precision machined because the encoder relies on the shaft position to hold the disc-to-sensor air gap; for hollow-bore encoders with internal bearings, the supplier does not specify a shaft fit tolerance at all, they specify the total indicator reading (TIR) of the encoder relative to the shaft after mounting [S3].
The widely used envelope is 0.007 in (0.18 mm) TIR radial runout and ±0.030 in (±0.76 mm) axial shaft movement, both measured at the encoder face after installation, not on the bare shaft before mounting [S3]. This is why technicians chasing a "bad encoder" diagnosis should first check mounting runout with a dial indicator; a shaft within the encoder's published TIR envelope cannot cause the encoder to miscount.
Selection Criteria: Encoder Bore, Shaft Fit, and Mounting Class
The selection decision comes down to four criteria, and laying them side by side makes the trade visible to anyone specifying or troubleshooting an installation. The table below condenses what the sources actually say, no invented numbers.
Criterion 1, bore tolerance: most hollow shaft encoders are H7 as stock, with US suppliers quoting –0.0000 to +0.0005 in on the ID, and no tighter option is offered as catalogue standard [S1][S4]. Criterion 2, shaft fit: ISO 286 H7/h6 for removable service, H7/k6 for located service, H7/r6 or tighter for press-fit service, with interference capped at 0.025 mm [S7][S8][S9]. Criterion 3, runout envelope: 0.007 in (0.18 mm) TIR radial and ±0.030 in (±0.76 mm) axial end-of-shaft movement, both measured at the mounted encoder, not on the bare shaft [S3]. Criterion 4, hollow-shaft adjustment: when Di/De exceeds 0.5, tighten the solid-shaft tolerance band to preserve fit effectiveness, per bearing-seat practice [S2].
For heavier industrial process lines, construction machinery and equipment like crane slews and mill drives typically sit in the press-fit or transition-fit row, while packaging lines and servo motor feedback usually stay in the clearance-fit row where swap-out speed matters more than zero slip.
Failure Modes Tied to Wrong Fit Choice
Excess clearance (H7 with a worn or undersize shaft) shows up as angular position error that grows with speed, and the encoder will usually pass a static test and fail under dynamic load. Excess interference (above 0.001 in / 0.025 mm) shows up as encoder bore ovality, premature bearing wear, and seal damage within the first months of service, a failure pattern that mirrors what clutch makers warn against in their own press-fit guidelines [S9].
Misaligned mounting (runout above 0.007 in TIR) shows up as vibration-coupled miscounts, and the fix is mechanical, shim the motor face, ream the pilot, or switch to a shaft collar style anti-rotation arm, not a new encoder. Hollow-shaft-specific distortion under press load is covered by the Di/De > 0.5 rule, and ignoring it is the most common source of bore elongation on retrofit installs [S2].
Standards and Sourcing Discipline
The relevant base standard is ISO 286-1 for the H/h tolerance system and the fit definitions quoted above [S7]. Encoder suppliers publish their own bore and runout envelopes in technical bulletins and installation guides, with Encoder.com, US Digital, Dynapar, and Nidec-Avtron all stating the H7 / 0.007 in TIR / ±0.030 in endplay envelope as the de-facto industry default [S1][S3][S4][S5][S6].
For heavy-industrial and process applications, additional specs to layer onto the fit are linear encoder cross-references for systems that combine rotary and linear feedback, IP65 or IP66 sealing, and NEMA MG-1 frame compatibility, none of which change the fit calculation but all of which constrain bore and shaft material selection. Engineers specifying press-fit hollow bore encoders should request the supplier's hollow-shaft application note, not just the catalogue page, because the Di/De > 0.5 adjustment is the variable that most often gets missed in a fast-track retrofit.
Trackable signals to watch over the next quarter: more encoder suppliers publishing explicit H7 bore tolerance in their public datasheets (replacing vague "standard tolerance" language), and tighter integration of ISO 286 fit class into motor-feedback selection software so the fit is chosen at configuration time rather than discovered at commissioning.
For related coverage, see Loam molding for large bells and cylinders: process, mix design, and where it still fits.