Tappet

The tappet converts cam shape into controlled valve movement

As the cam lobe rises, the follower transmits lift through a bucket, pushrod, rocker or finger. On the base circle, the system retains a defined clearance or hydraulic preload so the valve can close fully.

Contact geometry and lubrication determine whether the lobe and follower survive.

Common tappet and follower layouts

Hydraulic lash adjustment

Pressurised oil fills a plunger chamber through a check valve. Slow movement takes up expansion and wear; rapid cam lift traps oil so the adjuster acts almost solid.

Air, varnish, wear or low supply pressure allows collapse and noise.

Solid clearance

Valve lash provides room for thermal expansion and complete seating

Measure between specified surfaces with the cam on its base circle and engine at the stated temperature. Select shims or adjustable screws to achieve the range.

Never set all engines to one generic clearance.

Selection details

Cam and follower contact

Flat tappets often have a slight crown and run on a tapered lobe so they rotate, spreading wear. Buckets and finger pads have their own surface treatment. A used follower develops a pattern matched to one lobe.

Do not swap used followers between positions unless the manufacturer explicitly allows it.

Roller followers

The roller reduces sliding friction but its pin/bearing must remain aligned. Check roughness, axial play, flat spots and blue heat marks. A failed anti-rotation guide can turn the wheel across the lobe.

Replace related cam and guides after roller failure as the repair procedure requires.

Noise patterns

Oil pressure and viscosity

Hydraulic adjusters rely on gallery pressure and correct flow through small feeds. Oil too thick delays cold filling; too thin when hot increases leak-down. Sludge blocks passages.

Verify oil level, approval, filter and calibrated hot/cold pressure before replacing tappets.

Oil aeration

Overfill, low level, pickup air leaks or crank windage can introduce bubbles that compress inside adjusters. The result can be several noisy lifters rather than one failed part.

Inspect oil condition and correct the aeration source.

Valve-clearance measurement

Set the crank/cam position defined for each valve. Insert clean feeler gauges parallel to the contact and judge the specified drag. Record every inlet/exhaust value before changing shims.

Temperature and sequence matter. Do not rotate an interference engine with shims/buckets missing.

Shim calculation

Use the engine's formula relating measured clearance, target and existing shim thickness. Measure shims with a calibrated micrometer if markings are unclear. Choose available grade within the preferred target, not merely the broad limit where guidance gives one.

Keep shims seated and never stack two.

Hydraulic adjuster leak-down tests

Some procedures apply controlled force and time plunger movement in clean oil. Others prohibit disassembly or manual compression. Follow the exact adjuster design.

A hard pumped-up unit can hold a valve open if installed without the specified preparation.

Cam lift measurement

Use a dial indicator or scan/running compression method as directed to compare lift. Reduced lift can come from worn lobe, collapsed follower or bent mechanism. Check base circle and runout.

One good clearance reading does not prove full cam lift.

Spring, valve and guide checks

A broken spring, sticking valve or excessive guide friction overloads the follower. Inspect installed spring height and coil condition; perform leak-down/compression when a valve may not seat.

Do not replace tappets while leaving a bent valve or seized guide.

Switchable lifters and oil control

Deactivation systems use commanded oil pressure to move locking pins. Dirty oil, failed solenoid, leaking manifold and electrical faults can mimic lifter failure. Scan codes and command/status data before dismantling.

Updated parts and software may apply; use current technical information.

Removal planning

Set timing, use crank/cam locks and follow cam-cap loosening order. Caps are line-bored and remain by position. Store every follower upright in a labelled tray.

Protect open oilways and prevent shims falling into the engine.

Inspecting bores and oil feeds

Check follower bore scoring, varnish, diameter and oil-hole alignment. Clean galleries with approved methods and verify flow without forcing debris deeper. A damaged bore changes leak-down.

Machine work may be required if the head/block is worn.

New cam and follower pairing

Flat-tappet designs often require new lifters with a new cam and a dedicated break-in lubricant/procedure. Mixing a new lobe with a patterned used follower can cause rapid wear.

Roller and OHC policies vary; follow engine/cam supplier instructions.

Assembly lubrication

Apply specified oil or high-pressure cam assembly paste to lobes and contact faces; keep oil holes clear. Do not fill a hydraulic adjuster or compress it unless instructed.

Torque carriers/caps progressively in sequence to avoid cam fracture.

Timing and hand rotation

Restore chains/belts, tensioners and variable-timing units with locks. Turn the crank slowly through the stated revolutions and recheck marks and valve clearances.

Any hard stop requires investigation, not more leverage.

Priming and first start

Build oil pressure by the documented pre-lube or ignition-disabled cranking procedure. Start at the specified speed; some new cam systems require immediate controlled break-in, while others need idle.

Stop for persistent heavy knock, absent pressure, misfire or abnormal cam correlation.

Bedding and recheck

Use the specified oil, rpm/load and initial service interval. Cut open the filter or inspect debris if required. Recheck solid clearance after the stated mileage or thermal cycles.

Hydraulic ticking should follow an allowed bleed-down time, not continue indefinitely.

Emissions, MOT and urgency

Incorrect valve lift/clearance causes misfire, emissions and potential catalyst damage. Applicable warning lamps and emissions can affect UK MOT inspection.

Do not drive with severe valvetrain noise, low oil pressure, metal debris or a valve held open; failure can damage the engine suddenly.

Common mistakes

• Treating every top-end tick as a failed hydraulic tappet.
• Ordering without engine/cam and follower type.
• Swapping used followers between lobes.
• Setting solid lash at the wrong temperature/cam position.
• Stacking shims.
• Ignoring low pressure, wrong oil and blocked galleries.
• Forcing hydraulic plungers by hand without procedure.
• Starting after assembly without timing check and oil priming.

Practical tappet FAQs

Q: Is a tappet always hydraulic?
A: No; the term also covers solid buckets, rollers and followers.

Q: What does a hydraulic adjuster do?
A: It uses oil pressure to maintain near-zero running lash.

Q: Why do solid tappets need clearance?
A: Lash allows thermal expansion and full valve closure.

Q: Does a brief cold tick mean failure?
A: Not always; diagnose drain-down and oil pressure against limits.

Q: Can wrong oil cause tappet noise?
A: Yes, viscosity, aeration and deposits affect hydraulic operation.

Q: Can used followers change positions?
A: Usually not; preserve their matched lobe wear pattern.

Q: Can two shims be stacked?
A: No; fit one correct graded shim or bucket.

Q: Why check the camshaft?
A: A worn lobe can cause the same noise/lift loss.

Q: Should hydraulic tappets be prefilled?
A: Only if the exact procedure requires it.

Q: Do new flat tappets need bedding?
A: Often; follow cam/lifter lubricant and rpm instructions.

Q: Can a tight clearance burn a valve?
A: Yes, it can prevent full seating and heat transfer.

Q: When must the engine stop?
A: For heavy noise, low pressure, misfire, metal or binding.

Q: What verifies repair?
A: Correct timing, pressure, clearances/lift and stable quiet operation.