Crankshaft Inspection: End Play, Journals, Keyways, Oil Passages, and Reluctor Ring

The short version — Most test questions on this task hinge on end play measurement method, what causes excessive vs. too-tight end play, and knowing that oil passage blockage starves the rod bearings because that gallery is their only oil source.

How the crankshaft is located and why end play matters

The crankshaft's job is to take the piston's back-and-forth motion and turn it into rotation through the connecting rods. Main bearings hold the crank centered radially, but a separate thrust bearing (or thrust flange) controls how far it can slide front-to-back — that fore-aft movement is end play.

To check it:

• Mount a dial indicator so it reads axial (front-to-back) movement of the crank.
• Pry or push the crankshaft fully one way, zero or note the reading, then push it the other way and read the total travel — that's your end play measurement.
• Alternatively, feeler gauges can be inserted between the thrust bearing and the thrust flange to get the same clearance number without a dial indicator.

Why it matters: too much end play lets the crank hammer back and forth against the thrust surfaces every time the clutch is worked or the transmission shifts, accelerating wear. Too little end play binds the crank and can cause overheating or seizure at the thrust surface.

Excessive end play is almost always traced to a worn or damaged thrust bearing surface, or a worn/damaged thrust flange on the crank itself. If you find too much play, check both those surfaces — one or the other is usually chewed up.

End play that's too tight (not enough clearance) usually comes from a thrust bearing that was installed misaligned — not from crank wear. This is a key distinction: too much play points to worn parts, too little play points to an installation error.

Journals, keyways, and surface condition

Both main journals and rod journals need to be measured, not just eyeballed:

• Use a micrometer at multiple points around each journal — this catches out-of-round wear (ovality).
• Measure at both ends of the journal, too — this catches taper, where one end has worn more than the other.
• Both checks also tell you if the journal is simply undersize from wear.

Beyond the micrometer, inspect journal surfaces visually and by feel for:

• Scoring or scuffing
• Discoloration (bluing) — a sign the journal got too hot, usually from a lubrication failure
• Pitting
• Ridges

Any of these findings points to inadequate lubrication, a bearing that was failing, or debris circulating in the oil.

Cracks are checked for visually across the crankshaft as part of the same inspection — a cracked crank is not repairable and must be replaced.

The keyway (where the key locks the harmonic balancer or timing sprocket to the crank) gets its own check. Look for:

• Elongation
• Cracking
• Rounded edges

Any of these lets the balancer or sprocket rotate slightly independent of the crankshaft — that throws off ignition/injection timing and can cause vibration problems that seem unrelated to the keyway at first glance.

The thrust flange and sealing surface also get inspected as part of this same process — since the thrust flange is what end play depends on, and the sealing surface is what keeps oil where it belongs, damage to either one means further wear or leaks down the road.

Oil passages and the reluctor ring

The crankshaft has drilled oil galleries that run from the main journals to the rod journals — this is the only path pressurized oil takes to reach the rod bearings. These passages must be checked for clear, unobstructed flow, free of sludge or debris. If a passage is even partially blocked, the rod bearing downstream is running on reduced oil pressure, which is exactly how spun bearings happen.

On engines equipped with one, the reluctor ring (also called a tone wheel) is a toothed or notched ring on the crankshaft that the crankshaft position sensor reads to determine engine speed and position — this signal drives ignition and injection timing. Inspect it for:

• Missing, bent, or damaged teeth
• Corrosion
• Looseness on the crankshaft

Any of these problems can cause an erratic or lost crank position signal — which on a running engine shows up as misfires, stalling, or a no-start, even though nothing else about the crank is wrong.

Easy to mix up

• Excessive end play vs. tight end play — excessive points to a worn thrust bearing or worn/damaged thrust flange (crank-side or bearing-side wear); tight end play points to a thrust bearing installed out of alignment (an assembly error, not wear).
• Out-of-round vs. taper — measuring around the journal at one point catches ovality (out-of-round); measuring at both ends of the journal catches taper. You need both to fully characterize journal wear.
• Keyway damage vs. reluctor ring damage — both cause timing-related symptoms, but a bad keyway lets the balancer/sprocket slip relative to the crank (a mechanical timing problem), while a damaged reluctor ring corrupts the sensor signal itself (an electrical/signal timing problem).

Check yourself

Question: What two methods can be used to measure crankshaft end play?

A dial indicator mounted to read axial movement while the crank is pried/pushed fully in each direction, or feeler gauges inserted between the thrust bearing and the thrust flange.

Question: Technician A says excessive crankshaft end play is usually caused by a worn thrust flange or worn thrust bearing surface. Technician B says end play that is too tight is usually caused by crankshaft journal wear. Who is right?

Technician A only. Excessive end play traces to worn/damaged thrust surfaces (bearing or flange). Too-tight end play is typically caused by a misaligned thrust bearing installation, not journal wear.

Question: Why must oil passages inside the crankshaft be checked for blockage, specifically?

Because these drilled galleries are the only path oil takes from the main journals to the rod journals — any sludge or debris blocking them starves the rod bearings of pressurized oil, which can lead to bearing failure.