Checking Bearing Clearance and Alignment — What the Numbers Are Telling You

The short version — Plastigage is the go-to method for checking radial oil clearance (torque the cap to spec first, never turn the crank with it in place), while a straightedge or bore gauge checks whether the main bores are still in line; too much clearance knocks and drops oil pressure, too little starves the bearing and cooks it.

What bearing clearance actually is

Bearing clearance is the oil-film gap between the bearing shell and the journal it rides on. That gap is not wasted space — it's what lets a wedge of oil build up and float the journal so metal never touches metal. Too much gap and the oil just squirts out the sides instead of building pressure. Too little gap and there isn't room for enough oil to get in and carry heat away. Both directions are a problem, just from opposite ends.

• Too much clearance → low oil pressure, a knocking noise, and higher oil consumption because the oil escapes faster than the pump can replace it.
• Too little clearance → not enough oil flow reaches the journal, so the bearing overheats and wears fast — or seizes outright.

That's the core logic behind every reading you take: you're not chasing a number for its own sake, you're confirming there's a proper floating oil film.

Measuring clearance: two ways to get there

Plastigage (crush-gauge) method is the one you'll use and be asked about most:

• Lay a strip of the gauging material across the journal.
• Torque the cap down to the specified value and sequence — torque and sequence (often center-out) must be correct before you take the reading, because clearance changes with how the bearing seats under proper torque.
• Remove the cap and compare the flattened width of the strip to the printed scale to read clearance directly.
• Do not rotate the crankshaft with the material in place. Rotating it smears the strip and the reading becomes meaningless — you'd be measuring a smear, not a clearance.

Micrometer/bore-gauge method gets you the same answer a different way:

• Micrometer the journal's outside diameter.
• Measure the assembled bearing bore's inside diameter with a bore gauge, dial gauge, or telescoping gauge.
• Subtract journal OD from bore ID — the difference is your clearance.

Either method should agree with spec. If Plastigage looks off, the micrometer method is your cross-check (or vice versa).

Journal condition and case alignment — the other half of the job

Clearance numbers only mean something if the journal itself is round and straight, and if the bores holding the bearings are still in line.

• Taper and out-of-round are checked with a micrometer at multiple points around and along the journal. If the journal tapers or has gone out-of-round beyond spec, that's not a bearing problem — it means the crankshaft needs machining or replacement, because no bearing swap fixes a bad journal shape.
• Main bore alignment (line bore condition) is checked by laying a precision straightedge along the main bearing bores, or by measuring each saddle with a bore gauge/dial indicator. If the straightedge rocks, or a feeler gauge slips light underneath it, the bores are not true — the case is twisted or "tweaked" and needs correction before you trust any clearance reading taken in it.
• Thrust (end-play) clearance is a separate spec from radial oil clearance. It's controlled by the thrust bearing/flange, not the main bearing shells, and it's measured with a dial indicator or feeler gauge. Don't cross-reference an end-play number against a radial clearance spec — they're checking two different things.

Cleanliness and visual diagnosis

Before final assembly, bearing surfaces, journals, and oil passages have to be spotless. Any grit left behind embeds itself in the soft bearing material and grinds away at the journal from the inside — a clearance that measured perfect on the bench won't stay that way if you buttoned it up dirty.

When you pull old bearings, read the wear pattern before you just toss them and grab new ones. Wear tells a story:

• Uneven wear across the bearing → points to misalignment.
• A wiped or scored surface → oil starvation got to it.
• Embedded debris → contamination got past cleaning or filtration.
• Discoloration from heat → overheating, often tied to insufficient clearance or oil starvation.

Diagnosing the cause matters because if you just drop in new bearings without fixing what killed the old ones, the new ones fail the same way.

Easy to mix up

• Radial oil clearance vs. thrust (end-play) clearance — different specs, different tools (Plastigage/micrometer vs. dial indicator/feeler gauge), different parts controlling them (bearing shell vs. thrust bearing/flange). Don't apply one spec to the other.
• Too much clearance vs. too little clearance symptoms — excessive clearance gives you low pressure, knock, and oil consumption; insufficient clearance gives you low oil flow, overheating, and rapid wear/seizure. If you see knocking, think excessive, not tight.
• A crankshaft problem vs. a bearing problem — taper/out-of-round on the journal means the crank needs machining or replacement; it's not something a new bearing shell can correct on its own.

Check yourself

Question: While checking clearance with Plastigage, you bump the crank and it rotates a few degrees before you remove the cap. What should you do?

Redo the check. Rotating the crank with the gauging material in place smears it, and a smeared reading isn't valid — the crank must not be rotated while the material is in place.

Question: Technician A says main bearing clearance should always be checked before the cap bolts are torqued to spec, since torquing might change the reading. Technician B says the cap must be torqued to the specified value and sequence first, because clearance depends on how the bearing seats under proper torque. Who is right?

Technician B. Fasteners must be torqued to spec and in sequence (often center-out) before you take a clearance reading — torque affects seating and therefore the clearance itself.

Question: A straightedge laid across the main bearing bores rocks slightly, and a feeler gauge slips underneath it in the middle. What does this indicate, and what should you check separately from this issue?

It indicates the main bores are not in true alignment — the case is twisted or "tweaked." This is a separate issue from thrust (end-play) clearance, which is checked independently with a dial indicator or feeler gauge against the thrust bearing/flange, not against the straightedge check.