In short. The cylinder head of a diesel generator engine seals combustion, carries the valves and injectors, and Head and valve parts are identified from the engine serial and the head casting number, supplied as a bare head, a complete assembly or individual valve-train items with the scope itemised, and selected only after the head has been inspected and the cause of failure, usually overheating or a fuel fault, has been found.
What the head does and why it fails
The cylinder head sits across the hottest boundary in the engine. On one face it sees combustion gas at well over a thousand degrees for part of every cycle; a few millimetres away it carries coolant that must stay below boiling point and oil that must stay clean. Between those faces it holds the inlet and exhaust ports, the valve seats, the valve guides, the injector bore and, on many engines, the pre-combustion chamber or injector sleeve. The head gasket seals all of this against a block face that moves slightly with every firing stroke.
Most head failures in generator service follow one of three events. Overheating, from a failed thermostat, a blocked radiator, a slipping fan belt or hot-air recirculation in a badly ventilated room, distorts the head and cracks it between valve seats or into the injector bore. A fuel fault, such as a dribbling injector or wrong timing, burns the exhaust valve and sometimes the seat. Long light-load running or poor fuel carbons the valve stems and guides until a valve sticks and strikes the piston. In each case the head is the component that shows the damage, and the cause is somewhere else. Our cooling-system parts and injectors and fuel pumps guides cover the two systems most often implicated.
Identification: casting number and build
The engine serial number identifies the head family, but the same engine model can have been fitted with different heads for different emission revisions, different injector types (mechanical, unit injector or common rail) and different valve counts (two or four valves per cylinder). The casting number on the head itself, usually raised on the side or the rocker face, together with the engine’s build code, pins down the exact assembly. For individual valve-train parts the existing valve head diameter, stem diameter and overall length, and the guide and seat dimensions, are read from the parts book for that head.
History matters here as much as anywhere. A head that was skimmed at a previous overhaul has lost thickness and may be at or near its minimum; a second skim may push the piston-to-valve clearance or the injector protrusion out of limits. Valve seats that were cut before may have less remaining depth. Ask for the previous repair record, and where there is none, measure the head thickness before planning any machining.
Bare head, complete head, or valve-train parts
The commercial descriptions of head parts overlap and cause expensive misunderstandings. The quotation should state exactly what is included.
| Supply description | Normally includes | Normally excludes | Suited to |
|---|---|---|---|
| Bare cylinder head | Casting with guides and seats fitted (sometimes seats only) | Valves, springs, collets, seals, injector sleeves, gasket | Workshop that will fit and lap its own valves |
| Complete head assembly | Casting with valves, guides, seats, springs, retainers, collets and stem seals fitted and lapped | Head gasket, rocker gear, injectors, bolts, manifold gaskets | Fast turnaround or a cracked head |
| Valve-train parts | Valves, guides, seats, springs, stem seals, rocker arms, pushrods, tappets as ordered | Machining and fitting | Top overhaul of a sound head |
| Head gasket set | Head gasket, rocker cover gasket, manifold gaskets, injector seals, often valve stem seals | Head bolts unless listed, lower-engine gaskets | Any head removal |
Head bolts deserve a specific mention. On most current diesel generator engines they are torque-to-yield fasteners, tightened in stages to a torque and then an angle, and they are single use. A gasket set does not always include them. Reusing them is one of the common reasons a new gasket fails.
Inspection and repair limits
A removed head is cleaned, crack tested (dye penetrant or magnetic particle for cast iron, pressure testing for suspected internal cracks), checked for flatness with a straight edge and feeler gauge along both diagonals and both axes, and measured for thickness. Valve guides are checked for stem clearance, valve seats for width, recession and concentricity, valve stems for wear and valve heads for margin thickness. Springs are checked for free length and load. Every one of these has a limit specific to the engine, and the limits come from the engine manufacturer’s manual, not from a generic table.
- A head beyond its flatness limit can be skimmed once if thickness allows; beyond minimum thickness it is replaced.
- Worn guides are replaced and reamed to size, after which the seats must be recut concentric to the new guides.
- Seats can be recut while recession stays within limit; replaceable seat inserts are renewed when they do not, integral seats may be machined for an insert where the manufacturer permits it.
- Burnt or pitted valves are replaced, not refaced, on most small and medium diesels because the margin is already thin.
- Valve recession is checked after any seat work, because excess recession changes the compression ratio and the injector spray relationship.
Valve clearance after assembly is set to the figure for that engine, hot or cold as the manual specifies. There is no single value that applies to all generator engines, and a wrong clearance either burns the valve or makes it noisy and reduces lift.
Head gasket failure is usually a symptom
A failed head gasket shows as coolant loss with no external leak, white exhaust, bubbles in the radiator top tank, oil in the coolant or coolant in the oil, or combustion pressure in the cooling system. Fitting a new gasket without asking why the first one failed means the second one fails too. The usual causes are overheating that distorted the head, a block or head face outside flatness limits, liner protrusion outside limits on wet-liner engines, head bolts that were reused or tightened out of sequence, or a gasket that was the wrong thickness grade for the piston protrusion. Each of these is checked when the head is off. Our top overhaul page describes the full procedure.
Common mistakes
- Ordering a head by engine model alone; the casting for a different emission revision or valve count arrives and the injectors or manifolds do not fit.
- Skimming a head without measuring its thickness or checking its history; it ends up below minimum, the compression ratio rises and the injector protrusion is wrong.
- Fitting a new gasket with reused torque-to-yield bolts; the clamp load is low and the gasket fails under the first sustained load.
- Replacing a burnt exhaust valve without testing the injector in that cylinder; the new valve burns within a few hundred hours.
- Returning a repaired head to an engine with an unresolved overheating cause; the radiator, thermostat or room ventilation fault cracks the new head as well.
What to send us
Send the engine plate photograph with serial and build code, a photograph of the head casting number, the symptom that led to the enquiry (coolant loss, low compression, burnt valve, crack, routine overhaul), the hour-meter reading, and any previous record of skimming or seat work. If the head is already off, photographs of the gasket faces and a note of flatness and thickness measurements let us advise on repair versus replacement. Use the spare parts enquiry to start.
Questions engineers ask
Can a cracked cylinder head be repaired?
Small cracks between valve seats on cast-iron heads are sometimes repaired by specialist welding or pinning, but for a generator engine that must start reliably the usual recommendation is a replacement head. A crack into the injector bore or a water jacket is a replacement in every case.
How many times can a head be skimmed?
Until it reaches the manufacturer’s minimum thickness, which for many engines allows one light skim and sometimes none. Measure the thickness and compare with the limit before any machining, and record the result for the next overhaul.
What is included in a complete cylinder head?
Normally the casting with guides, seats, valves, springs, retainers, collets and stem seals fitted and lapped. Head gasket, head bolts, rocker gear, injectors and manifold gaskets are usually separate. Always ask for the itemised list.
Why did my new head gasket fail again?
Most often because the cause of the first failure was not corrected: a distorted head or block face, liner protrusion out of limit, reused head bolts, wrong tightening sequence, or a cooling fault that is still overheating the engine.
Is there a standard valve clearance for diesel generator engines?
No. Each engine has its own inlet and exhaust clearance, specified hot or cold, and it must be taken from that engine’s manual. Clearances vary widely between makes and even between revisions of the same model.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.