Genset valve clearance sets how long a valve stays shut and how far it opens, and a small error at the rocker burns a valve. Check the specification against the cold setting, then compare supplier offers before the head service. Tesla Power writes the figures on the engine plate; send the model to the technical desk and we confirm.
- Set the clearance on a cold engine, because the figure on the plate assumes ambient temperature.
- Use the intake and exhaust figures separately, since they are rarely the same number.
- Turn the engine by hand to the firing order rather than cranking it on the starter.
- Retorque the rocker locknut and check the clearance again after tightening.
What does genset valve clearance control?
It controls the gap between the rocker and the valve stem, and that gap decides how long the valve stays closed and how far it opens. Too little gap holds the valve off its seat and it burns. Too much gap shortens the opening and the engine loses breathing at the top of the speed range. Neither fault shows on a gauge until the damage is done. A set that has just had a head service is the one to check first, because the figures were set by hand.
The gap exists because the valve train grows as the engine warms, and the setting is made cold so that the running gap lands in the right place. A clearance set on a hot engine comes out tight when the engine cools and then tighter still when it next runs. Tesla Power prints the cold figure on the engine plate, because a figure copied from a workshop manual for a different build is a common error.
Which figure applies to intake and exhaust?
The exhaust valve usually carries a larger clearance than the intake, because it runs hotter and grows further. A typical pair on an industrial diesel is 0.30 mm on the intake and 0.45 mm on the exhaust, and the two are not interchangeable. Setting both to the intake figure leaves the exhaust tight when hot. A plate that carries one figure for both valves is a plate that will be read wrongly by the next technician.
The figures also change with the build and the camshaft, so the engine plate wins over a general rule. Tesla Power Equipment prints both figures with the firing order beside them, because a technician setting eight valves needs the order as much as the gap. A plate with one figure on it is a plate that will be read wrongly on a twin cam engine.
| Item | Cold figure | Note |
|---|---|---|
| Intake valve | 0.30 mm | Set on the base circle only |
| Exhaust valve | 0.45 mm | Runs hotter, so it needs more |
| Rocker locknut | Torque from the plate | Recheck the gap after tightening |
| Check interval | 1,000 h or 2 years | Sooner after any head work |
Why must the check be done cold?
Metal grows with temperature, and the head, the pushrods and the rocker shaft all grow at different rates. A clearance measured on a warm engine is therefore smaller than the same clearance measured cold, and setting it warm leaves the valve held open when the engine reaches working temperature. The error is small in millimetres and large in consequence. A bay that has just run a load test is not a cold bay, so the check waits for the next morning.
Cold means ambient, so a set left in the sun is not cold and a set in a warm bay is not cold either. Let the engine stand overnight where the check is going to be made, and measure the bay temperature as well. Tesla Power writes the ambient beside the figures on the service sheet, because a clearance set at 35 °C in a yard and a figure written for 20 °C are not the same measurement.
How is the clearance adjusted?
Turn the engine to the firing order, confirm the valve is fully closed on the base circle, and slide the feeler gauge in with a light drag rather than a push. Loosen the locknut, turn the adjusting screw until the gauge drags evenly, hold the screw and torque the locknut. Then check the gap again, because tightening the nut moves the screw on most designs. A feeler gauge that has been bent to reach a valve reads wide, and a wide gauge sets a tight clearance.
The feeler gauge has to be the right width and flat, and a worn gauge reads tight. Use a gauge set with the exact figure rather than stacking two blades. Yangzhou Tesla Power checks every clearance twice on the test bed and records both readings, because a clearance that changes between the two checks points to a worn rocker or a stretched stud.
What happens when the clearance is tight?
A tight exhaust valve does not close fully, so hot gas leaks past the seat and burns a channel through it. The first symptom is often a misfire at light load and a loss of compression rather than a noise, which is why the fault is found late. By then the head has to come off and the seat has to be recut. A burnt seat shows as a dark channel on the valve face and a matching mark on the seat, and it is visible once the head is off.
A tight clearance also shows as a rough idle and a smell of unburnt fuel, because the cylinder does not seal on the compression stroke. A set that starts badly and clears as it warms is a candidate for a clearance check before anything else. A clearance that closes by 0.10 mm over 1,000 h is normal bedding in rather than a fault. Tesla Power adds that check to the service card beside the figures.
How often should it be checked?
Check at the first service after commissioning and then every 1,000 h or two years, whichever arrives first. A set on continuous duty reaches the figure in a few months and a standby set reaches the calendar limit with very few hours. Both limits belong on the service card, and the shorter one wins. A set that has run hot needs the check regardless of the hours, because the head may have moved on its gasket.
The check is also worth doing after any head work, any injector change and any overheating event, because the head may have moved. On a 500kVA set at 0.8 pf and 400kW the clearance closes by about a third of the setting over that interval: 0.10 mm / 0.30 mm = 33%. A figure that has closed more than that points to a worn seat.
What should a buyer put on the enquiry?
Send the engine model, the build year, the duty cycle and the ambient the set runs in. Add whether the head has been worked on and whether the site keeps a service record. Those five items let a supplier quote the right figures, and they show whether the offer has quoted a single clearance for both valves.
The wording on the offer matters too. Genset valve clearance manufacturers often quote one figure for the whole head, and Chinese genset suppliers sometimes ship a rocker cover gasket that does not seal after the first check. Ask for genset valve clearance for a cold engine, genset valve for a standby set and genset for a rental fleet as three separate cases. Tesla Power prints the figures, and the 1000kW container build shows the rocker assembly.
What does the handover file record?
The file carries the intake figure, the exhaust figure, the firing order, the ambient at the check and the two readings per valve. One page covers the head service when those five items sit under the engine serial number. Add the conformity assessment for the head service when the installation is part of a permit package.
Hand the page to the technician who sets the clearances rather than to the buyer who placed the order. the Yangzhou factory describes the bay where the check is made. the commissioning notes lists the frames that use a twin cam head, the build range explains how the engine is barred over by hand, the ISO 8528-1 duty definitions sets the rating basis the service interval comes from, and the NFPA 110 maintenance routine covers the load steps the set was accepted on.
Frequently asked questions about genset valve clearance
It can be measured warm and it should be set cold, because the figure on the plate is a cold figure. A warm measurement is useful as a comparison rather than as a setting. If the engine has to be warm to reach the rockers, let it cool before the adjustment.
Normal bedding in closes the gap slowly, and a worn rocker tip or a stretched stud closes it faster. A gap that opens rather than closes points to a receding valve seat. Record both readings each time so the direction of the change is visible.
It usually does, and it can also be an injector or a worn rocker bush. A light tapping that fades as the engine warms points to the clearance. A heavy knock that stays constant points elsewhere. Check the clearance before replacing any part.
It can be adjusted, and the rest of the head should still be checked, because a single tight valve usually means the others have moved too. Setting one and leaving seven is how a head fails on the cylinder nobody checked. Do the whole head in the firing order.
The screw moves and the clearance closes or opens within a few hours, and the valve then burns or the rocker hammers. Torque the nut to the plate figure and recheck the gap afterwards. A nut tightened by feel is the most common cause of a repeat failure.
A high sulphur fuel leaves more deposit on the seat and the valve face, so the clearance closes faster and the interval shortens. Check the clearance earlier on that site and record the fuel figure with the reading. Tesla Power shortens the card when the enquiry names a high sulphur fuel.