Valve clearance, firing order and engine timing are basic workshop settings, but the exact figures vary by engine. A small error in valve lash or timing can cause difficult starting, poor performance, smoke, overheating or mechanical damage.
Always use the workshop manual for the exact engine model and serial range. Valve clearances, timing marks, firing orders and injection timing procedures are not safe to guess from a similar-looking engine.
1. What Is Valve Clearance?
Valve clearance, often called tappet clearance or valve lash, is the small gap in the valve operating mechanism when the valve is fully closed.
The clearance allows for:
- Thermal expansion
- Correct valve seating
- Lubrication
- Normal wear
2. Why Valve Clearance Matters
Too Tight
- Valve may not fully close
- Low compression
- Hard starting
- Burnt valve or seat
- Power loss
Too Loose
- Excessive tappet noise
- Reduced valve lift/duration
- Accelerated rocker and valve-train wear
3. Cold or Hot Clearance?
Some engines specify valve clearance:
- Cold
- At normal operating temperature
- At a specified coolant temperature
Use the condition stated in the manual.
A hot clearance figure cannot simply be used on a cold engine unless the manual specifically allows it.
4. Identify Inlet and Exhaust Valves
Before adjusting, confirm which valves are inlet and exhaust.
Use:
- Workshop manual
- Port positions
- Rocker arrangement
- Valve sequence diagram
5. Position the Engine Correctly
Valve clearance is normally checked with the valve fully closed and the cam follower on the base circle of the cam.
This may be done by:
- Bringing a cylinder to compression TDC
- Using the manufacturer’s valve-adjustment sequence
- Using a specified crankshaft position
6. Compression TDC vs Exhaust TDC
A four-stroke engine reaches top dead centre twice in one cycle.
Compression TDC
Both valves are normally closed.
Exhaust/Overlap TDC
One valve is closing while the other begins to open.
Make sure you are using the correct TDC.
7. Use the Correct Feeler Gauge
The feeler gauge should pass through the clearance with the drag described by the manufacturer.
Keep gauges:
- Clean
- Flat
- Free from burrs
8. Adjuster Screw and Locknut Type
On many engines:
- Loosen locknut.
- Turn adjuster screw.
- Set clearance with feeler gauge.
- Hold adjuster still.
- Tighten locknut.
- Recheck clearance.
The setting can move while the locknut is tightened, so always recheck.
9. Shim-Adjusted Valves
Some engines use shims rather than screw adjusters.
The procedure may require:
- Measuring existing clearance
- Removing shim
- Measuring shim thickness
- Calculating replacement shim
Use the correct formula and shim range.
10. Hydraulic Tappets
Engines with hydraulic lash adjustment may not have a routine manual valve-clearance setting.
Do not adjust hydraulic tappets using a solid-tappet procedure.
11. Valve Clearance After Cylinder Head Work
Recheck valve clearance after:
- Head removal
- Valve replacement
- Valve-seat work
- Rocker-shaft work
- Pushrod replacement
12. Bent Pushrods
If one valve cannot be adjusted normally, inspect the pushrod.
A bent pushrod can indicate:
- Valve seizure
- Piston-to-valve contact
- Incorrect timing
- Previous over-revving
13. Firing Order
The firing order is the sequence in which cylinders produce their power strokes.
It matters for:
- Injector pipe identification
- Spark plug lead routing on petrol engines
- Valve-adjustment procedures
- Fault diagnosis
14. Cylinder Numbering
Do not assume No. 1 cylinder is always at the same end.
Manufacturers may number cylinders from:
- Front of engine
- Flywheel end
- Other defined reference point
Confirm the engine-specific convention.
15. Diesel Injection Timing
Mechanical diesel engines may specify injection timing as:
- Degrees before top dead centre
- Piston position before TDC
- Pump timing mark alignment
- Spill timing
- Dial indicator lift
The method is engine- and pump-specific.
Incorrect timing can cause hard starting, smoke, overheating, diesel knock, poor power and engine damage.
16. Mechanical Injection Pump Timing Marks
Timing marks may be found on:
- Flywheel
- Crankshaft pulley
- Timing gears
- Injection pump flange
- Pump drive gear
Use the exact manual to identify the correct marks.
17. Spill Timing
Some older diesel systems use fuel spill timing to determine the start of injection.
The procedure requires:
- Correct cylinder
- Correct crankshaft rotation direction
- Accurate timing marks
- Correct pump control position
18. Dial Gauge Injection Timing
Some rotary and inline pumps are timed by measuring internal pump movement with a dial indicator.
Use the specified:
- Gauge adaptor
- Reference position
- Plunger lift
- Crankshaft position
19. Electronic/Common-Rail Engines
Modern common-rail engines do not generally use the same manual timing adjustments as older mechanical pumps.
Timing may depend on:
- Crankshaft sensor
- Camshaft sensor
- ECU control
- Gear timing
- Calibration procedures
20. Petrol Ignition Timing
Older petrol engines may use:
- Distributor timing
- Static timing
- Timing light adjustment
- Centrifugal advance
- Vacuum advance
Use the specified engine speed and test conditions.
21. Timing Marks and Paint Marks
Do not rely solely on old paint marks made by a previous owner.
Confirm factory timing marks from the manual.
22. Timing Gears
During engine rebuilds, timing gears normally have alignment marks.
Check:
- Crank gear mark
- Cam gear mark
- Injection pump gear mark
- Idler gear marks
Some idler gears carry several different marks for different mating gears.
23. Timing Chains and Belts
Where used, check:
- Correct timing marks
- Tensioner position
- Direction of rotation
- Belt/chain condition
- Replacement interval
24. Rotate Engine by Hand After Timing Work
After setting mechanical valve timing:
- Turn the engine slowly by hand.
- Complete at least the specified number of revolutions.
- Recheck timing marks.
- Confirm no mechanical interference.
Do this before using the starter.
25. Valve Timing vs Injection Timing
These are different settings.
Valve Timing
Controls when valves open and close relative to crankshaft position.
Injection Timing
Controls when fuel injection begins.
An engine can have correct valve timing and incorrect injection timing, or vice versa.
26. Symptoms of Incorrect Valve Clearance
| Symptom | Possible relation |
|---|---|
| Hard starting | Too-tight valves reducing compression. |
| Valve noise | Excessive clearance. |
| Low power | Incorrect lash or valve damage. |
| Burnt valve | Valve not seating fully. |
27. Symptoms of Incorrect Diesel Timing
| Symptom | Possible relation |
|---|---|
| Hard starting | Timing too advanced or retarded. |
| Excess white smoke | Late injection or poor combustion. |
| Harsh diesel knock | Timing too advanced. |
| Poor power | Incorrect timing or fuel-system fault. |
| Overheating | Incorrect combustion timing among other causes. |
28. Check Timing After Pump Removal
If a mechanical injection pump has been removed, do not assume it can be refitted in approximately the same position.
Use the correct timing procedure before running the engine.
29. Record Settings
During service or rebuild, record:
- Valve clearance used
- Hot or cold condition
- Firing order
- Injection timing value
- Pump position
- Date and engine hours
30. Valve & Timing Checklist
| Item | Check |
|---|---|
| Engine identification | Exact engine/model/serial range confirmed. |
| Valve clearance | Correct inlet/exhaust values. |
| Temperature condition | Hot/cold requirement followed. |
| Engine position | Correct TDC or adjustment sequence. |
| Firing order | Correct for engine. |
| Cylinder numbering | Manufacturer convention confirmed. |
| Injection timing | Correct method and specification used. |
| Timing marks | Factory marks identified correctly. |
| Final check | Engine rotated manually where required. |
Related Agrimanual Manuals
Valve clearances, firing order, pump timing and valve timing must be taken from the correct engine or machine workshop manual.
- Search Agrimanual engine workshop manuals
- Search Agrimanual injection and fuel-system manuals
- Request a manual if your engine or machine is not listed
Frequently Asked Questions
Can I use the same valve clearance as another engine in the same family?
Only if the correct manual confirms it. Similar engines can use different valves, camshafts, rocker arrangements or settings.
Should valve clearance be checked hot or cold?
Use the condition stated in the workshop manual. Do not substitute one for the other.
Is cylinder No. 1 always at the front?
No. Cylinder numbering conventions differ by manufacturer and engine family.
Can I adjust diesel pump timing until the engine sounds right?
No. Set mechanical injection timing using the correct specified method and figures.
Why should I rotate the engine by hand after timing work?
It helps confirm that valve timing is correct and that there is no piston-to-valve or other mechanical interference before using the starter.
Valve clearance and timing are precision settings. Identify the exact engine, use the correct manual and verify the procedure before making adjustments.