Bearing Bore Sizes Explained: How to Measure the Shaft and Order Correctly 

Imagine a plant maintenance technician ordering a "UC205" insert bearing for a broken 1 in. conveyor roller shaft, only to receive a 25 mm bore insert that refuses to slide on leaving the production line down for two extra days while waiting on a UC205-16. Getting your machinery back up and running requires a bore that matches your shaft diameter down to the fraction or millimeter, and part numbers only reveal that critical measurement if you know how to decode them. In this guide, we will walk you through reading metric and inch bore codes, taking step-by-step shaft measurements with micrometer precision, checking shaft tolerances, and using a printable checklist so you never place a wrong bearing order again. 

What Is a Bearing Bore Size and Why Does It Matter for Insert Bearings? 

Bore Diameter vs. Shaft Diameter vs. Outside Diameter 

Before you break out your measuring tools, it helps to keep three basic bearing dimensions distinct in your mind: 

• Bore Diameter (Inner Diameter / ID): This is the internal opening of the bearing's inner ring where your steel shaft passes through. 

• Shaft Diameter: The precise outer measurement of the actual steel shaft or axle sitting inside your machine. A precise shaft diameter measurement ensures proper component alignment. 

• Outside Diameter (OD): The external dimension across the bearing's outer ring or outer housing wall. 

To put it simply, your shaft diameter measurement and your insert bearing bore size need to be exact partners. The bore is the hole; the shaft is the solid metal fitting right inside it. 

Why Insert Bearings Use a Slip Fit 

Unlike standard deep-groove ball bearings that require a heavy press fit or thermal heating to force onto a shaft, insert bearings are designed for a smooth slip fit. They feature a slightly loose clearance, so you can slide them into place along the shaft during routine maintenance without heavy shop tools. 

Because the bearing glides on easily, it relies entirely on an external locking mechanism such as set screws, an eccentric locking collar, or an adapter sleeve to lock down tight onto the shaft metal. That is why your shaft's actual size, cleanliness, and surface condition directly dictate how long your bearing assembly lasts compared to traditional press-fit components. 

What Happens When the Bore Is Wrong 

If your insert bearing bore size is off by even a fraction of a millimeter, bad things happen fast on the factory floor. If the bore is too small, you cannot slide the bearing on without hammering it, which quickly cracks internal raceways or scores the shaft surface. If the bore is too large, the inner ring sits loosely on the shaft, causing the shaft to creep, chatter, fret, and loosen its set screws until the shaft spins inside the inner ring and destroys the entire assembly. (If you are already dealing with an erratic, slipping fit, read our full guide on diagnosing and preventing insert-bearing spin.

How Do You Read the Bore Size in an Insert Bearing Part Number? 

Metric Bore Codes: The "×5" Rule 

Standard metric insert bearings use a simple mathematical shortcut to display their bore diameter. When learning how to read bearing part numbers, remember that for any two-digit bore code ending in 04 or higher, you simply multiply the last two numbers by 5 to find the exact bore in millimeters. Knowing UC bearing bore sizes makes identifying these numbers simple: 

• UC204: 04 × 5 = 20 mm bore 

• UC205: 05 × 5 = 25 mm bore 

• UC208: 08 × 5 = 40 mm bore 

• UC210: 10 × 5 = 50 mm bore 

(Keep in mind that small bore codes below 04 use fixed metric standard sizes: 00 = 10 mm, 01 = 12 mm, 02 = 15 mm, and 03 = 17 mm). 

Inch Bore Codes: The Dash Number in Sixteenths 

When dealing with inch-dimensioned equipment, manufacturers add a dash number after the frame designation. If you are learning how to read bearing part numbers for inch sizes, this dash number tells you the bore diameter measured in sixteenths of an inch (1/16 in.): 

• UC205-16: 16 / 16 = 1 in. bore (25.400 mm) 

• UC206-20: 20 / 16 = 1-1/4 in. bore (31.750 mm) 

• UC207-23: 23 / 16 = 1-7/16 in. bore (36.513 mm) 

One Frame Size, Several Bore Options 

A very common misunderstanding in maintenance bays is thinking that a frame designation like "205" tells you the bore size. It does not. The "205" portion only specifies the outer ring diameter, ball size, and housing footprint. A single 205 frame size is built with multiple bore options to fit totally different shaft sizes. 

A 205 frame insert can come with a 25 mm metric bore (UC205) or inch options ranging from 3/4 in. (UC205-12) up to a 1 inch bore insert bearing (UC205-16). Understanding how to read bearing part numbers helps you avoid ordering mistakes. 

Prefix letters such as UC, UB, UE, or NA describe the inner-ring design, sealing arrangement, and locking mechanism. Understanding UC bearing bore sizes is essential when reviewing these variations. To learn more, read our guide to UC, UE, and UB insert-bearing series designations.

Bore Code Reference Chart 

Use this bearing bore size chart and insert bearing size chart to quickly translate frame numbers into metric and inch shaft diameters: 

Frame Size	Metric Bore (Code)	Common Inch Bores (Dash Code)	Inch Example in mm
204	20 mm (UC204)	1/2 in. to 3/4 in. (-08 to -12)	UC204-12 = 19.050 mm
205	25 mm (UC205)	3/4 in. to 1 in. (-12 to -16)	UC205-16 = 25.400 mm
206	30 mm (UC206)	1-1/16 in. to 1-1/4 in. (-17 to -20)	UC206-20 = 31.750 mm
207	35 mm (UC207)	1-1/4 in. to 1-7/16 in. (-20 to -23)	UC207-23 = 36.513 mm
208	40 mm (UC208)	1-1/2 in. to 1-9/16 in. (-24 to -25)	UC208-24 = 38.100 mm
209	45 mm (UC209)	1-5/8 in. to 1-3/4 in. (-26 to -28)	UC209-28 = 44.450 mm
210	50 mm (UC210)	1-13/16 in. to 2 in. (-29 to -32)	UC210-32 = 50.800 mm

This bearing bore size chart covers the most common UC bearing bore sizes across industrial applications. Consult an insert bearing size chart whenever you are replacing mounted assemblies. 

Is Your Shaft Metric or Inch? 

Near-Miss Sizes That Cause Ordering Errors 

Some of the most frustrating maintenance mistakes happen when evaluating a metric vs inch bearing bore. Certain metric and inch shaft sizes are so close that they look identical to the naked eye. If you measure quickly with a worn tape measure or cheap plastic tool, you can easily order the wrong system. 

Metric Shaft	Nearest Inch Size	Inch Size in mm	Difference
25 mm	1 in.	25.400 mm	0.400 mm
30 mm	1-3/16 in.	30.163 mm	0.163 mm
35 mm	1-3/8 in.	34.925 mm	0.075 mm
40 mm	1-9/16 in.	39.688 mm	0.312 mm
50 mm	2 in.	50.800 mm	0.800 mm

Analyzing metric vs inch bearing bore options prevents costly dimensional errors. 

How to Tell Which Standard Your Equipment Uses 

When you are on the fence about whether your machine runs on metric or inch dimensions, look for these practical clues: 

1. Equipment Origin: Equipment made in North America usually runs on fractional inch shafts. Machinery manufactured in Europe, Japan, or China almost exclusively uses metric shafting. 
2. Part Number Stamps: Wipe down the face of the old bearing's inner ring or rubber seal. A stamped dash code (like -16) is a dead giveaway for an inch bore, such as a 1 inch bore insert bearing. 
3. OEM Manuals: Check your machine schematics for standard bolt sizes or shaft callouts across other components on the same line. 
4. Micrometer Readings: Measure the cleaned shaft seat and compare the precise thousandths against both standard metric integers and fractional inch equivalents. Knowing how to measure a shaft for a bearing ensures accurate results. 

Why "Close Enough" Fails 

Trying to force "close enough" sizes together creates immediate operational issues when comparing a metric vs inch bearing bore. Putting a 1 inch bore insert bearing (25.400 mm) onto a 25 mm shaft leaves a 0.400 mm gap. That sounds tiny, but it creates massive slop, causing the set screws to wobble loose and destroying the shaft in short order. On the flip side, trying to install a 1-3/8 in. bearing (34.925 mm) onto a 35 mm shaft leaves the shaft 0.075 mm too large, meaning the bearing inner ring will physically lock up before it ever slides onto the seat. 

How Do You Measure a Shaft for a Bearing? 

Tools You Need 

Learning how to measure a shaft for a bearing requires the right tools. Put away flexible tape measures and steel workshop rulers they do not offer the precision needed for bearing fits. 

• Outside Micrometer (Preferred): This gives you the highest level of accuracy when checking tolerances down to thousandths of an inch or hundredths of a millimeter during shaft diameter measurement. 

• Digital Caliper (Acceptable): Great for a fast preliminary check, provided it is properly calibrated and zeroed. 

• Clean Shop Rags & Solvent: To strip away old caked-on grease, dirt, scale, and paint. 

• Fine Emery Cloth (400-Grit): To smooth down high spots, metal burrs, or ridges raised by old locking set screws. 

Step-by-Step Shaft Measurement 

Knowing how to measure a shaft for a bearing involves careful execution. Follow this 8-step process to get precise shaft diameter measurement readings every single time: 

1. Lock out and tag out the machine's power supply to ensure total safety before putting your hands near the drive shaft. 
2. Clean the bearing seat area completely using solvent and clean rags to remove all grease, rust scale, and paint. 
3. Remove burrs and high spots caused by old set screw marks or corrosion using your fine emery cloth until the steel feels smooth to the touch. 
4. Measure directly at the bearing seat—do not take readings on an unworn section of shafting further down the line where the bearing never actually sits. 
5. Take readings across two planes 90° apart (Plane A and Plane B) at your first position to verify the shaft is perfectly round rather than oval shaped. 
6. Repeat readings at three axial points (front, middle, and back of the seat) to check for tapering along the seat. 
7. Record all numbers and compare your smallest and largest readings against standard metric and inch tables, such as an insert bearing size chart. 
8. Measure at ambient temperature—never take final readings on a shaft that is hot immediately following machine shutdown, as thermal expansion will throw off your numbers. 

How to Interpret Your Readings 

Once you have written down your measurements, use this table to figure out your next move: 

What Your Readings Show	What It Means	What to Do
All readings match or sit slightly under a standard size	Shaft is within normal specification	Order that nominal bore size
Readings come out larger than standard size	Leftover paint, scale, burrs, or custom shafting	Clean thoroughly, sand down burrs, and re-measure
Readings fall significantly below tolerance limits	Shaft is worn down or undersized	Repair or replace the shaft before installing a new bearing
Readings vary across planes or positions	Shaft is out-of-round or tapered	Machining, reconditioning, or shaft replacement is needed

Knowing how to measure a shaft for a bearing ensures your shaft diameter measurement data guides the correct repair action. 

Measuring When the Old Bearing Is Still Installed 

If you cannot take the machine offline immediately to dismount the bearing, you can still gather clues. Wipe off the face of the old bearing housing and inner ring to find stamped part numbers. Next, measure the exposed shaft section right next to the bearing housing with your digital caliper. Always make sure to take your final, verified micrometer reading directly on the cleared bearing seat as soon as the old unit comes off before placing a large replacement order. 

What Shaft Tolerance Do Insert Bearings Need? 

Typical Tolerance Ranges 

Because insert bearings rely on a slide-on fit, your shaft needs to be turned to a specific bearing shaft tolerance—often referred to as an ISO h-tolerance. In simple terms, an h-tolerance means the shaft should measure right at the nominal size or slightly below it (never oversize). 

• ISO h7 Tolerance (High Speed / Precision): Used where smooth running and minimal vibration are critical. For a 25 mm shaft, a bearing shaft tolerance under h7 allows a range of 0 to −0.021 mm (24.979 mm to 25.000 mm). 

• ISO h8 or h9 Tolerance (Normal Duty): Common on standard industrial conveyors and farm machinery. An h9 bearing shaft tolerance on a 25 mm shaft allows a range of 0 to −0.052 mm (24.948 mm to 25.000 mm). 

• Adapter Sleeve Inserts: Tapered adapter sleeve units (like the UK series) can handle wider shaft variations because the sleeve pulls down tight around the shaft as you torque the locknut. 

Always double-check your exact bore tolerances and shaft fit guidelines against the manufacturer's engineering catalog for your specific series. 

What to Do with a Worn or Undersized Shaft 

If your measurements show that your shaft seat has worn down past acceptable bearing shaft tolerance limits, you have a few options: 

• Replace the Shaft: The most reliable option for high-speed or heavily loaded drives. 

• Recondition the Shaft: Have a machine shop turn down the shaft, spray-apply metal cladding, and grind it back to factory specs. 

• Switch to an Adapter Sleeve Insert: If space permits, switch to a tapered sleeve insert bearing that clamps firmly onto undersized shafting. 

• Never shim a set screw insert bearing with feeler stock or shim brass. The shim will slip out under vibration, imbalance the unit, and cause catastrophic failure. 

What Information Do You Need to Order the Right Insert Bearing? 

The "Before You Order" Checklist (Printable) 

Before submitting your order, verify your exact pillow block bearing bore size and details using this text checklist: 

• Exact Shaft Measurement: _________ (in. / mm) 

• Full Part Number off Old Bearing: ___________________________ 

• Insert Series Prefix (UC, UB, UE, NA, SA): __________________ 

• Locking Method: Set Screws (UC), Eccentric Collar (HC/SA), Adapter Sleeve (UK), or Concentric Clamp Collar 

• Housing Style (if buying a complete mounted unit): Pillow Block (P), 4-Bolt Flange (F), 2-Bolt Flange (FL), or Take-Up Unit (T) 

• Operating Environment & Seals: Standard Dust Contact or High Moisture / Washdown 

• Quantity Needed (+ Spare Stock): _________ 

Checking your pillow block bearing bore size before purchasing prevents costly downtime. 

Using Interchange and Cross-Reference Numbers 

Many bearing manufacturers share the exact same base frame and bore coding system. A 1 inch bore insert bearing in a 205 frame usually contains 205-16 inside the part number regardless of brand (for instance, UC205-16, YAR205-100, or GY1100KRRB). Knowing how to read bearing part numbers allows you to focus on matching the frame size and dash bore code rather than getting hung up on brand-specific prefix letters. 

Insert Only or Complete Mounted Unit? 

• Replace the Insert Only: If your cast iron or stainless steel housing is clean, structurally sound, and free of stress cracks, you can save money by simply replacing the internal bearing insert matching your pillow block bearing bore size. 

• Replace the Whole Mounted Unit: If the old housing is cracked, worn in the spherical seat, heavily corroded, or you are unsure of the bolt pattern, replace the complete assembly. Learn more in our detailed guide on matching an insert bearing to the correct housing.

What Are the Most Common Bore Sizing Mistakes? 

Assuming 25 mm and 1 in. Are Interchangeable 

Thinking metric and inch sizes that sit close together are identical leads straight to fitting failure when reviewing a metric vs inch bearing bore. A 0.400 mm difference guarantees your bearing will either seize on install or chatter loose while running. 

Ordering by Frame Size Alone (UC205 without the dash code) 

Ordering a generic "UC205" without specifying a dash extension gives you a standard 25 mm metric bore. If your conveyor uses a 1 inch bore insert bearing, that 25 mm bearing will not slide on. Knowing how to read bearing part numbers helps you avoid this trap. 

Measuring an Unworn Section of the Shaft 

Taking measurements a few inches away from where the bearing actually runs gives misleading numbers. Always measure directly across the worn bearing contact seat when performing a shaft diameter measurement. 

Ordering by Housing Number Without Checking the Insert 

Part numbers stamped on cast housings (like P205 or F205) only tell you the shape of the outer housing. They say nothing about the specific pillow block bearing bore size mounted inside it. Always check your UC bearing bore sizes or consult a bearing bore size chart. 

How Can Online Bearing Store Help You Order the Right Bore Size? 

Insert Bearings in Metric and Inch Bores 

At Online Bearing Store, we stock a vast selection of insert bearings across all standard metric sizes and inch dash sizes. Whether you need set screw locking, eccentric collars, or adapter sleeve styles, check our insert bearing size chart for the right fit. 

Complete Pillow Block and Flange Units 

When you need to replace the whole assembly, explore our catalog of mounted units. We carry pillow blocks, 2-bolt flanges, 4-bolt flanges, and take-up units built for heavy industrial environments. Verify your pillow block bearing bore size with our support team. 

Part-Number Verification from the Technical Team 

Unsure about a tricky shaft diameter measurement or a worn-off part number? Snap a photo of your bearing, note down your micrometer readings, and contact our technical support team. We will check our bearing bore size chart and verify the exact part number you need before you check out so you get the right order delivered every time. 

What Are the Most Common Questions About Bearing Bore Sizes? (FAQ) 

1. What does the -16 mean in UC205-16? 

The -16 is a dash code that specifies the bore size in sixteenths of an inch (16/16 in.), which equals a 1 inch bore insert bearing. Without the dash code, a standard UC205 comes with a 25 mm metric bore. Knowing how to read bearing part numbers simplifies checking these codes against a bearing bore size chart. 

2. Is a UC205 bearing 1 inch or 25 mm? 

A standard UC205 bearing has a 25 mm metric bore. If your machine uses a 1-inch shaft, you need to order part number UC205-16. Reviewing UC bearing bore sizes on an insert bearing size chart helps confirm this difference. 

3. Can I put a 1 inch bearing on a 25 mm shaft? 

No, you should never put a 1 inch bore insert bearing on a 25 mm shaft when evaluating a metric vs inch bearing bore. A 1-inch bore (25.400 mm) is 0.400 mm larger than a 25 mm shaft, creating slop that causes severe shaft chatter and rapid failure. 

4. Should I use a caliper or a micrometer to measure a shaft? 

Use an outside micrometer for your final shaft diameter measurement. While digital calipers work fine for quick rough checks, only a micrometer provides the exact precision required when learning how to measure a shaft for a bearing to separate near-miss metric and inch sizes. 

5. How do I find my bearing size if the part number is worn off? 

Clean off the shaft seat using solvent and perform a shaft diameter measurement across two planes 90° apart. Next, measure the outer ring width and housing dimensions, then check a bearing bore size chart or send your numbers to our tech support team. 

6. What is the difference between a UC205 and a UC205-12? 

A UC205 has a 25 mm metric bore, while a UC205-12 has a 3/4-inch bore (12/16 in. = 3/4 in.). Both bearings share the exact same 205 frame size and outer dimensions. Check an insert bearing size chart to compare UC bearing bore sizes. 

7. How are metric bearing bore sizes calculated? 

When learning how to read bearing part numbers for metric sizes 04 and larger, multiply the final two digits of the bearing part number by 5. For example, a UC208 bearing has a bore code of 08, making its bore 40 mm (8 × 5 = 40). Refer to a bearing bore size chart for quick confirmation. 

8. What does UC stand for in bearing part numbers? 

The UC prefix stands for an ultra-clean, set-screw locking insert bearing featuring a spherical outer ring and an extended inner ring on both sides. Knowing how to read bearing part numbers helps identify these UC bearing bore sizes easily. 

9. Why won't my new insert bearing slide onto the shaft? 

Your new bearing will not slide on if the insert bearing bore size is too small (e.g., 25 mm on a 1 in. shaft), or if the shaft exceeds proper bearing shaft tolerance due to raised burrs, paint, or corrosion. Learn how to measure a shaft for a bearing properly, clean the shaft thoroughly with 400-grit emery cloth, and re-measure with a micrometer. 

10. Can I replace just the bearing insert without buying a new housing? 

Yes, you can replace just the inner bearing insert as long as your existing cast housing is free of stress cracks, heavy rust, or seat damage. Make sure the new insert matches the original pillow block bearing bore size, frame size, and locking style found on an insert bearing size chart. 

Ready to Order the Correct Bore Size the First Time? 

Don't let a simple sizing mix-up slow down your operations. Head over to Online Bearing Store today to shop our huge inventory of metric and inch insert bearings and complete mounted units by insert bearing bore size—or send your shaft diameter measurement readings directly to our technical support team for fast, accurate part verification before you order!