51203 bearing: complete specifications & equivalents
The 51203 is a ISO 104 radial bearing sized 17 x 35 x 12 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 17 mm |
| Outer diameter D | 35 mm |
| Width B | 12 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Basic dynamic load rating C | 17.2 kN |
| Basic static load rating C0 | 27.3 kN |
| Fatigue load limit Pu | 1.23 kN |
| Limiting speed (mechanical) | 4800 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 54 g |
| Manufacturer designation | NTN 51203 |
| Standard | ISO 104 |
| Catalog version | NTN Ball and Roller Bearings CAT.No.2203E |
| Last verified | 2026-10-01 |
Source: catalog reference (NTN Ball and Roller Bearings CAT.No.2203E) · extracted 2026-10-01
Parameters extracted from the official NTN Ball and Roller Bearings catalogue (NTN Ball and Roller Bearings CAT.No.2203E); NTN® is a trademark of NTN Corporation, used here for identification only — mpnbase is not an authorized NTN distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at ntnglobal.com.
Dimensions in inches & 51200-series range
| Bore d | 0.669 in |
|---|---|
| Outer diameter D | 1.378 in |
| Width B | 0.472 in |
| ISO bore code | 03 |
| Radial section height (D−d)/2 | 9 mm |
| Smallest bore in 51200-series | 10 mm |
| Largest outer diameter in 51200-series | 520 mm |
| Width range across 51200-series | 11–112 mm |
| Bore position in 51200-series (smallest first) | 4 of 39 |
| Bore above smallest 51200-series size | 7 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 51203-SKF | SKF | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
Interchange levels: full standard interchange · partial verify parameters · verify manual confirmation required. Links resolve to the equivalent model's specification page where available.
How the 51203 model number is decoded
In NTN Ball and Roller Bearings CAT.No.2203E, 51203 appears as a medium-section 51200 thrust family bearing with a 17 mm shaft washer, a 35 mm housing diameter and 12 mm of stack height. Two fields make up the code — 512 is the dimension series (medium-section 51200 thrust family) and the bore code 03 maps to the 17 mm bore by the standard table. The ball-and-cage set is separable from both washers, which is why the mounting order (shaft washer first, then cage, then housing washer) matters. Within the thrust-ball plan, 511 is the light section and 514 the heaviest for a given bore; moving up a series raises the C0a from 27.3 kN class figures at the cost of OD and height. Keep the support faces square: a housing face out of perpendicular by a few hundredths over the 35 mm OD already tilts the raceway and concentrates the 27.3 kN static capacity on a few balls.
Typical equipment & failure modes
Common applications
- bench drill-press and tapping-machine spindles
- automotive steering-gear and clutch release points
- small crane hook and swivel-joint thrust supports
- valve actuator and gearbox thrust seats
Known failure patterns
- ball and raceway smearing from gyroscopic spin when the bearing runs fast under too little axial load — the minimum-load rule exists exactly for this
- fretting between shaft washer and shaft when the seat is cut loose — the washer creeps, the centre line shifts and the balls start running off-track
- early fatigue from thrust reversal hammering the unloaded raceway, because the bearing has no contact surfaces for the opposite direction
- washer face corrosion under long standstill in humid service, later printed into the raceway as evenly spaced pitting
- cage breakage after repeated shock reversal banged the ball set against the pocket bridges
Selection pitfalls — read before you substitute
- Small thrust washers of 17 mm bore bend if the shaft shoulder is rough — check burrs before seating, and never press on the housing washer to drive the shaft washer home.
- The 512 medium section is the default choice: balanced height and capacity for general vertical-shaft duty at 17 mm bore.
- Radial load is the silent killer here: a belt pull or gear force on the same shaft must land on a radial bearing, because 51203 has no radial capacity at all.
- A housing washer pressed tight in its seat cannot self-centre; the resulting eccentric running shows up as cage wear long before raceway fatigue.
- Radial load is the silent killer here: a belt pull or gear force on the same shaft must land on a radial bearing, because 51203 has no radial capacity at all.
- The Ca 17.2/C0a 27.3 kN ratings and 1.23 kN Pu here are NTN Ball and Roller Bearings CAT.No.2203E figures for 51203; raceway geometry legitimately differs between manufacturers, so re-verify ratings and washer identification before substituting another make.
Frequently asked questions
What are the dimensions and catalogue ratings of NTN 51203?
51203 is a single-direction thrust ball bearing to ISO 104: 17 mm shaft-washer bore, 35 mm outside diameter, 12 mm stack height (0.669 x 1.378 x 0.472 in). From NTN Ball and Roller Bearings CAT.No.2203E: axial Ca 17.2 kN, C0a 27.3 kN and a 1.23 kN fatigue load limit Pu; the grease limiting speed is 4,800 r/min and the oil rating 6,800 r/min; mass about 54 g.
How is the 51203 designation structured?
512 is the dimension series (medium-section 51200 thrust family); the bore code 03 maps to the 17 mm bore by the standard table. The 35 mm OD and 12 mm height follow the ISO 104 boundary plan for that series.
Can 51203 carry radial load or reversing thrust?
No. The raceways contact the balls at 90 deg to the shaft, so the rating is purely axial and single-direction. Radial load must be carried by a separate radial bearing, and reversing thrust needs a second thrust bearing or a double-direction execution.
Why does NTN specify a minimum axial load for 51203?
At the 4,800 r/min class speed, centrifugal and gyroscopic effects make an under-loaded ball set skid and smear instead of roll. The catalogue minimum-load formula gives the axial preload — from the machine weight, springs or a dead-weight — that keeps the contact rolling.
What should be checked before substituting another make for 51203?
The 17/35/12 mm envelope is ISO 104 interchangeable, but washer bore codes, raceway osculation and ratings are maker-specific. Re-verify Ca 17.2 kN / C0a 27.3 kN, the speed figures and the shaft/housing washer identification against the donor catalogue.
Reviewed & verified by the MPN Base engineering desk against source catalog NTN Ball and Roller Bearings CAT.No.2203E on 2026-10-01. Read our editorial & correction policy.