How To Adjust Main Bearing Clearance

You're staring at a crankshaft on the bench. Which means micrometer in one hand, dial bore gauge in the other. The spec sheet says 0.In real terms, 0015–0. 0025 inches. Your measurements keep coming back 0.0032.

Now what?

Most people freeze here. They've read the manual. They've watched the YouTube videos. But when the numbers don't match, the confidence evaporates. Main bearing clearance isn't a suggestion — it's the difference between an engine that lives 200,000 miles and one that spins a bearing at 3,000.

What Is Main Bearing Clearance

Main bearing clearance is the oil film thickness between your crankshaft main journals and the bearing shells pressed into the block and caps. Which means that's it. A few ten-thousandths of an inch of pressurized oil keeping metal from touching metal.

The crank spins. The bearings stay put. Now, oil gets pumped into the gap through holes in the block, rides the rotation like a wedge, and creates a hydrodynamic film. That's why no contact. No wear. In theory.

In practice, that clearance window is tight. Too loose and you lose oil pressure — the pump can't keep up, the film collapses, and you get metal-on-metal at high RPM. Too tight and the oil can't get in fast enough, the journal runs hot, the bearing material fatigues, and you're pulling the pan again inside of 5,000 miles.

Factory specs usually land somewhere between 0.0015 and 0.0020 — because tighter clearance builds oil pressure faster and stabilizes the crank at high RPM. Consider this: 0030 inches depending on the engine. Also, 0010 to 0. Performance builds often run tighter — 0.But tighter leaves zero margin for error.

The Two Numbers That Matter

You're really managing two clearances:

Vertical clearance — measured at 90 degrees to the parting line. This is your working clearance. The one that carries the load.

Horizontal clearance — measured across the parting line. Usually 0.0005–0.0010 tighter than vertical because the cap stretches slightly under bolt load. If horizontal reads loose, something's wrong with the housing bore or the cap itself.

Most guys only check vertical. Don't be most guys.

Why It Matters / Why People Care

Oil pressure is the obvious answer. But it's not the whole story.

A 350 Chevy with 0.0035 clearance might idle at 15 psi hot. Same engine at 0.Consider this: 0018 runs 45 psi. Consider this: that pressure difference isn't just a gauge reading — it's the lifeblood reaching your rod bearings, your cam bearings, your valvetrain. Every leak path in the system steals from somewhere else.

But there's more. Bearing clearance affects:

Oil temperature — Loose clearance shears more oil. More shear means more heat. Hot oil thins, pressure drops further, and you're in a death spiral.

Crank stability — At 7,000 RPM, a loose crank walks. That walking hammers the thrust bearing, fatigues the block webs, and eventually cracks something expensive.

Bearing life — The bearing material (usually aluminum or copper-lead overlay on a steel back) needs a stable film. Boundary lubrication — metal touching metal during startup or low RPM — wipes the overlay. Once it's gone, the steel back eats the journal.

Noise — That knock you hear at hot idle? Often just loose mains. Tighten the clearance, the knock vanishes.

I've seen builders chase "low oil pressure" for months — new pump, new pickup, different oil, external cooler — when the real problem was mains worn to 0.0040. Fix the clearance, pressure comes back. Every time.

How It Works (or How to Do It)

Adjusting main bearing clearance isn't one procedure. It's a decision tree. Where you are in the tree depends on what you're building and what you found.

Step 1: Measure Everything First

Before you order a single bearing, you need real numbers. Now, not "the book says. " Real numbers.

Crank journals — Micrometer. Measure each main journal at multiple points around the circumference and along the width. You're looking for:

• Diameter (is it within spec?)
• Taper (difference front-to-back on the same journal)
• Out-of-round (difference at different clock positions)

Any journal over 0.Even so, don't try to "bearing your way out of it. And 0005 taper or 0. Now, period. 0003 out-of-round needs grinding. " You'll just buy bearings twice.

Housing bores — Dial bore gauge. Block mains and cap bores. Measure at parting line (horizontal) and 90° to it (vertical). Check each bore in at least three positions top to bottom.

Write it all down. On top of that, sketch the block. Even so, label each main. Future you will thank present you.

Step 2: Understand Bearing Sizing

Bearings come in standard and undersizes. The marking on the shell back tells the story:

• STD — standard thickness
• 0.010, 0.020, 0.030 — undersize by that many inches (journal ground smaller)
• X or 0.001 — sometimes used for "extra clearance" shells, usually 0.001 thinner than standard

Here's where it gets tricky. But **Bearing clearance changes roughly 0. Because of that, 0005 inches per 0. Worth adding: 001 inch of bearing thickness change. ** But it's not perfectly linear because of housing bore distortion, cap stretch, and crush.

Crush matters. When you torque the caps, the bearing shells are slightly larger than the bore — they "crush" in, creating radial pressure that holds them in place and improves heat transfer. If you mix shells wrong, you lose crush. Typical crush is 0.001–0.Still, 002 inches per shell half. Lose crush, the bearing spins. Spun bearing = new block.

Step 3: The Mix-and-Match Game

This is where experience earns its keep.

Say your #3 main vertical clearance measures 0.Spec is 0.Which means 0020 max. 0032. You need to remove ~0.0012 clearance.

Options:

• Grind the crank 0.010 under and run 0.Also, 010 undersize bearings — removes ~0. 010 from journal diameter, gains ~0.In real terms, 005 clearance per side. Too much. This leads to - Run one standard upper, one 0. 001 extra-clearance lower — gains ~0.0005. On top of that, not enough. - Run 0.001 undersize bearings (if available) — gains ~0.0005 per side. Closer.
• Polish the journal — removes maybe 0.Think about it: 0001–0. In real terms, 0002. Band-aid.

The real answer often involves selective fitting. Which means most bearing manufacturers (King, Clevite, ACL, Federal-Mogul) sell shells in 0. 0005 increments.

For more on this topic, read our article on if you add more enzyme the reaction will or check out journal of physical chemistry letters impact factor.

The Real‑World Solution: Selective Fitting and “Bearing Stack‑Up”

When the numbers don’t line up with a single off‑the‑shelf set, you become a little bit of a machinist‑detective. The goal is to arrive at a combination that gives you the target clearance while preserving the engineered crush and maintaining proper cap geometry.

1. Identify the exact clearance shortfall – Use the micrometer reading from the previous step and subtract the spec‑maximum clearance. In the example above, a 0.0032 in. vertical clearance where 0.0020 in. is the limit means you need to shave roughly 0.0012 in. off the bearing thickness on that journal.

2. Choose the bearing series that offers the smallest incremental thickness – Most OEM‑approved suppliers (King, Clevite, ACL, Motorcraft, etc.) sell “precision” shells in 0.0005‑in. increments. If you can’t find a 0.001‑in. undersize set, you’ll have to mix a standard upper with a 0.001‑in. extra‑clearance lower, or vice‑versa, to achieve the desired net effect.

3. Calculate the net effect of each half – Remember that bearing thickness changes clearance on both sides of the journal. If you install a 0.001‑in. thinner lower shell, you gain about 0.0005 in. of clearance on that side; a 0.0005‑in. thinner upper adds another 0.0005 in. The sum of the two halves must equal (or slightly exceed) the required reduction.

4. Account for cap stretch and housing bore distortion – When the main caps are torqued to spec (usually 70–80 lb‑ft for a 350 small‑block, but always verify the factory service manual), the bearing shells are forced into a slight oval shape. This “crush” typically amounts to 0.001–0.002 in. per half. If you remove too much material, you’ll end up with insufficient crush, which can lead to bearing spin or premature wear.

5. Practical mixing strategy

   • Step A: Start with a standard upper half on the journal.
   • Step B: Install a 0.001‑in. extra‑clearance lower half. This gives you roughly 0.0005 in. of clearance gain.
   • Step C: If still short, replace the upper with a 0.0005‑in. undersize half. The combined gain is now about 0.0010 in., which is close to the 0.0012 in. target.
   • Step D: Verify the final clearance with a feeler gauge or a dial bore gauge after the caps are torqued. If it’s still a hair off, you can polish the journal a few tenths of a thou with a fine abrasive cloth—just enough to shave the remaining clearance without compromising the journal’s integrity.

6. Document every decision – Write the exact bearing part numbers, the measured clearances before and after assembly, and the torque values used. Future rebuilds (or a different engine builder) will thank you for the paper trail.

Step 4: Installing the Crank and Checking End‑Play

With the correct bearing set in place, the crank goes back into the block:

1. Apply a thin film of assembly lube to the bearing surfaces. Do not use heavy grease; a light oil‑based lubricant is sufficient for initial rotation.

2. Insert the crank and hand‑rotate it a few turns. You should feel a smooth, resistance‑free motion. Any binding indicates a mis‑aligned bearing or an out‑of‑round journal.

3. Measure end‑play with a dial indicator placed against a crank journal flange. The spec for most small‑block V8s is 0.004–0.006 in. of axial play. If it’s out of range, you may need to shim the thrust bearing or adjust the timing gear set.

4. Torque the main caps to the manufacturer’s spec in the proper sequence (usually a criss‑cross pattern). Use a calibrated torque wrench; over‑torquing can stretch the caps and alter bearing crush, while under‑torquing can allow movement and lead to bearing wear.

5. Re‑check clearances after final torque. Place a dial bore gauge in each main bore and measure again at the parting line and 90° to it. The numbers should now sit within the target range (typically 0.0018–0.0022 in. for a street‑performance build).

Step 5: Break‑In and Monitoring

Even with perfect clearances, the first few minutes of operation are critical:

• **Run the engine at idle for 10–

5. Break‑In and Monitoring

Even with perfect clearances, the first few minutes of operation are critical:

• Run the engine at idle for 10–15 minutes. This allows the oil to circulate fully and the bearings to seat properly. Avoid high RPMs; the goal is to gently seat the new components without shock loading.

• Gradually increase throttle. After the initial idle, blip the throttle to about 2,500 RPM for a few seconds, then return to idle. Repeat this cycle three or four times. This helps the rings and bearings wear in evenly.

• Monitor oil pressure and temperature. Use a gauge to ensure oil pressure stays within the manufacturer’s specifications. Rising temperature is normal, but if it spikes, shut the engine down immediately to check for excessive friction or inadequate cooling.

• Listen for unusual noises. Any knocking, ticking, or grinding could indicate a problem with clearance, lubrication, or component seating. Address these before continuing.

• Change the oil and filter after the first 30–50 miles. Break-in generates fine metal particles. A fresh oil change removes these contaminants and protects the newly seated components.

Conclusion

Achieving the perfect bearing clearance is a blend of measurement, strategy, and patience. Document your process, trust your tools, and don’t rush the final checks. Remember, the extra time spent dialing in these details pays dividends in longevity and smooth operation. By following a systematic approach—assessing the existing condition, selecting the right components, verifying clearances post-assembly, and executing a careful break-in—you lay the foundation for a reliable, high-performance engine. A well-assembled bottom end is the heart of any engine build, and it deserves the same care and precision you’d give to a masterpiece.