Camshaft Journal, Lobe, and Lift Inspection

The short version — a worn cam lobe kills valve lift and shows up as a tick and a miss; worn journals kill oil clearance and show up as low oil pressure and a knock. You confirm both with a micrometer, plastigage, or a dial indicator — never by eyeballing it alone.

What the camshaft is actually doing

The camshaft's job is to turn rotary motion into the reciprocating motion that opens the valves. The lobes push against followers — lifters, rocker arms, or cam followers — to open valves against valve spring pressure at a precise point in the cycle. That timing isn't optional: the camshaft is timed to the crankshaft through a belt, chain, or gears so valve events happen in the correct sequence relative to piston position. Rotation direction and lobe orientation are what set up intake-before-exhaust and the correct cylinder firing order. If you're chasing a driveability complaint, remember the cam is a timing device first and a lift-generating device second — both have to be right.

Lobe lift is measured from the base circle (the heel) to the peak of the lobe. That raw lift number then gets multiplied by the rocker arm ratio, if the engine has one, to produce the actual lift seen at the valve. So a "small" difference at the lobe can turn into a bigger difference at the valve once the rocker ratio is applied — keep that relationship in mind when you're comparing lobe wear to a driveability symptom.

Inspecting journals and bearings

Camshaft journals ride in bearing bores — either machined right into the head or block, or held in separate bearing shells or caps — and they need correct oil clearance to get lubrication and support. Two ways to check that clearance:

• Plastigage across the journal, or
• Calculate it directly: bore diameter minus journal diameter.

Beyond just measuring clearance, look at the journal and bearing surfaces themselves for:

• scoring
• discoloration (a sign of overheating or the bearing running dry)
• pitting
• galling

Any of those tells you the bearing was starved for oil or contaminated at some point, even if the clearance measures in spec right now.

Worn journals or bearings mean excess clearance, and excess clearance shows up as low oil pressure, a knocking noise, and — if you don't fix it — accelerated wear that gets worse fast. Oil clearance is there to hold a film of oil under load; too much clearance and that film can't be maintained.

Checking lobe lift and condition

To evaluate the lobes, measure the base-to-nose dimension with a micrometer. Then compare intake lobes to intake lobes, exhaust lobes to exhaust lobes, and all of them against specification. A lobe that measures noticeably smaller than its neighbors — or smaller than spec — has worn or flattened.

A worn or flattened lobe reduces lift, and reduced lift produces a ticking or tapping noise, a loss of power, a rough idle, or a misfire on that specific cylinder. That's the classic symptom chain: less lift at the lobe means less valve opening means less airflow into (or exhaust out of) that cylinder — the engine runs like it's missing on that hole because, effectively, it is.

When you're measuring lift with a dial indicator instead of (or in addition to) a micrometer, support the camshaft evenly and rotate it slowly and squarely. If the cam is misaligned or binding while you turn it, you'll get a false reading — the indicator will report a number that has nothing to do with the actual lobe condition. Take your time here; a rushed or sloppy setup wastes the measurement.

Easy to mix up

• Journal/bearing wear vs. lobe wear symptoms — these get confused because both are "camshaft problems," but the symptoms point different directions:
• Journal/bearing wear → low oil pressure, knocking noise
• Lobe wear → ticking/tapping, loss of power, rough idle, misfire on one cylinder

If someone describes a knock and low oil pressure, don't go chasing lobe lift — go check clearance. If someone describes a specific-cylinder miss with a tick, don't go chasing bearing clearance first — go check that lobe.

• Plastigage vs. calculation — both get you oil clearance, but one is a physical measurement (plastigage) and one is arithmetic (bore diameter minus journal diameter). Know that either method answers the same question.

• Lobe lift vs. valve lift — lobe lift is measured at the lobe (base circle to nose); valve lift is that number multiplied by rocker ratio. A test question can ask for "lift" without specifying which — read carefully.

Check yourself

Question: A customer complains of a ticking noise, a rough idle, and reduced power, but oil pressure is normal. What's the more likely cause — a worn lobe or worn journals — and why?

A worn or flattened lobe. Reduced lift from a worn lobe produces exactly this symptom set: ticking/tapping, rough idle, loss of power, or a misfire on that cylinder. Worn journals/bearings would instead point toward low oil pressure and a knocking noise, which aren't present here.

Question: Technician A says you can determine journal-to-bearing oil clearance by subtracting journal diameter from bore diameter. Technician B says the only valid method is plastigage. Who is right?

Technician A is right, Technician B is wrong. Oil clearance can be measured with plastigage OR by calculating bore diameter minus journal diameter — both are valid methods.

Question: Why must the camshaft be rotated slowly and squarely, supported evenly, when checking lobe lift with a dial indicator?

Because misalignment or binding during rotation will produce a false reading on the dial indicator — the number you get won't reflect the true lobe lift. Even, square support and slow rotation are needed to get an accurate measurement.