Small cause, big effect

Not many components in engines are under as much load and tribological stress as valves and their associated parts. At temperatures up to 800ºC, the red-hot exhaust valves strike the valve seats more than 70 times per second and must withstand the hot and corrosive exhaust gases that flow around them. At every valve lift, enormous acceleration forces and retractive forces occur due to the strong valve springs.

Together with valve seat inserts and valve guides, valves form a closed system, which has to withstand extreme loads and stresses. Therefore, we not only have valves in our program, but also their tribological partners, valve seat inserts and valve guides. The combined development of these products in one system allows us the advantage of optimizing wear characteristics and minimizing component costs.

 

MAHLE manufactures high-quality valves for internal combustion engines at several production sites worldwide. Our program includes valves of diverse designs and types with stem diameters from 5 to 12 mm as well as valve lengths from 80 to 210 mm for passenger cars and commercial vehicles. Thanks to the expert technical know-how, the high productivity and the outstanding quality of these products, MAHLE is one of the world's leading valve manufacturers today.

We employ state-of-the-art production technology as part of our large-volume production to ensure our quality standards. For example, we use plasma powder methods in the production of forgings or high-speed grinding methods for mechanical finishing. In order to make valves resistant to the high loads, our valves have reinforced seats, are hardened, chrome-plated or nitrided as needed. A wide range of materials is available for every application. For special applications with extreme demands, sodium-filled hollow valves are available to reduce thermal loads.

The valve guides centre the valves in respect to the valve seats and compensate for the lateral forces acting on the valve shafts. In addition, they need to dissipate the generated heat towards the cylinder head. Depending on engine design, identical or different valve guides are used for the intake and exhaust side. The range of materials reaches from gray cast iron and brass to sintered materials of various alloys.

The valve seat insert has the task of sealing the combustion chamber against pressure loss. In addition it is responsible for heat dissipation. Another important task is to prevent impacting of the valve into the softer cylinder head material. Valve seat inserts are predominately made from chromium-steel alloys. Also sinter materials are now increasingly being used.

The greatest challenge is to determine the design of the valves. Our valves are designed for a wide variety of operating conditions in order to withstand the extreme mechanical, chemical and thermal loads and assure optimum heat dissipation.

Materials
Austenistic steels
Martensitic steels

Types
Mono-metal valves
Bi-metal valves

Valve seat
Plasma powder facing
Induction hardening

Valve tip induction hardening
Profile hardening
Through-hardening
Tip surface hardening

Hollow valves:
Stem size: > 6 mm
Bore sealing:
- Laser welding
- Friction welding
Sodium-filled

Fillet profile
Turned, ground
As forged

Head top surface
Machined
As forged
With or without spherical indentation

Valve tip geometry
1–3 grooves
Special designs

Valve length
80–210 mm

Valve head diameter
18–65 mm

Valve stem diameter
5–12 mm

Surface treatment
Salt bath nitrided
Hard chrome plated
(coating thickness: 3–35 µm)

Valve train components

DESPITE—OR EXACTLY BECAUSE OF THE INCREASING MILEAGE OF TODAY’S INTERNAL COMBUSTION ENGINES: THERE IS A BOOM IN VALVES. MORE AND MORE OFTEN, WORKSHOPS ARE CONFRONTED WITH VEHICLES THAT NEED THEIR VALVES REPLACED. HOWEVER, THERE ARE A FEW THINGS TO BE OBSERVED.

Valves are a classic example for workshop turnover due to neglected maintenance. This is because the reasons for valve damages are—apart from normal wear—in many cases old, torn timing belts, broken chains or belated oil changes. Another frequent reason for valve damage: driving errors such as inadvertent changing to a lower gear at high engine speeds—with the result of bent or even broken valves. And not to be ignored are the cases when valve damage occurs shortly after valve replacement—due to a (preventable) fitting error.

VALVES—MUST PROVIDE A PERFECT SEAL
Valves seal the combustion chamber and take care of optimal charge exchange. As they are constantly in motion—and this under difficult tribological conditions and under the effect of aggressive gases or exhaust gases—they are subject to natural wear. This can be accelerated due to extreme conditions such as mechanical or thermal overloads. Valves must therefore generally be replaced when any sign of damage is noticeable.

WHY IS IT DANGEROUS TO RECYCLE VALVES?
It is risky to “recondition” old valves. Often it is not possible to recognise the extent of valve damage with the unaided eye. The material of the valve may have been subjected to thermal overloads—or mechanical overloads have caused incipient cracks in the component. It is also often difficult to assess whether the seat facing and the hardened areas are strong enough for reworking (and whether the wear is then still permissible) or if the seat angle can really be ground correctly. This reconditioning makes no economical sense anyway—and any respectable workshop should not take the serious risk of engine damage due to old valves.

DISASSEMBLY—THE RIGHT TOOL IS IMPORTANT
In order to prevent irreparable damage to the cylinder head, the valve springs must be cushioned during dismantling—for instance, by using a valve spring compressor or valve spring tensioning device (fig. 1). The valve collets are best removed with a magnet rod.Tip: to avoid fitting the wrong parts it is recommended to compare the disassembled valves with the new ones before disposal—with respect to stem diameter, overall length, valve head diameter and seat angle.

ASSEMBLY: ALSO CHECK VALVE GUIDES AND SEAT INSERTS!
The conditions of the valve guides and valve seat inserts should be checked before the new valves are fitted. If heavy wear marks are present, they either have to be reworked or must be replaced. The new valve is then inserted from below through the valve guide.

REPLACEMENT OF VALVE GUIDES, REAMING OF THE BORES
Is the slack between the valve shaft and valve guide too large? In that case, the valve guides have to be replaced together with the valves. After pressing or shrinking in the new valve guides it is recommended to ream the bores. Only this assures that they have the right diameter, are cylindrical and burr-free—and can provide good performance together with the new valves. Reaming is a highly accurate and fine finishing method that should only be carried out by hand—with the aid of cutting oil for lubrication (fig. 2).

WORN VALVE SEAT INSERT?
In that case, it should be reworked or replaced with a new valve seat insert. Guide and sealing surfaces of the valve seat insert must be perfectly aligned—only then can the new valve provide a good seal.

SEAL AND RECESS
After the new valve has been inserted, the correct valve recess must be checked. In the next step, the valve stem seals should be fitted with the aid of the assembly sleeve. (The assembly sleeve is provided by most seal manufacturers together with the valve stem seal and prevents damage to the very sensitive sealing lip of the seal.)

VALVE SPRING NOT STRAIGHT?
To continue the assembly, make sure that the valve spring is positioned correctly in the cylinder head. There is a risk that the valve springs are fitted at an angle due to the design of some engines. During start-up of the engine afterwards, the spring can move with one side onto the block. The resulting large bending moment on the valve spring retainer can lead to tear-off or breaking of the valve—with the result of engine damage (fig. 3).

NEW VALVES—NEW COLLETS
A valve collet seats itself to the valve during operation. Old valve collets do therefore not fit exactly on new valves. Especially, when valve collets of individual valves are re-fitted randomly, there is a risk of uneven loads, bending stress, pressure peaks and therefore of broken valves in the grove area (fig. 4). In short: new valve collets protect from engine damage. To facilitate the work process in workshops and to assure even safer repairs, we are adding valve collets to our programme. MAHLE Original valves are then available with the matching collets.

FREE TO ROTATE
Valves with multiple groves need to be able to rotate in their collets. The rotation of the valves during engine operation supports the seal and heat dissipation, reduces wear at the valve base and prevents deposit build-up at the sealing surface. It should therefore be checked that the valves can rotate—a valve that cannot rotate can impact permanently into the cylinder head.

ARE THE VALVES REALLY GAS TIGHT?
A vacuum test provides certainty. Alternatively, the seal of the valve can be checked using a liquid of low viscosity (e.g. petrol or brake cleaner). This is done by turning the cylinder head facing upward and then pouring the liquid onto the valve heads. If the liquid drains off, the valves are not sealing correctly (fig.5).

VALVES—SUPPLIED ALREADY GROUND
Today it is no longer necessary to grind-in the valves as was usual in the past. The sealing surfaces of the valves are produced with high precision by MAHLE—and thanks to accurate, pre-set tools, the valve seats are machined precisely to provide perfect seals with the valves.

HYDRAULIC VALVE CLEARANCE ADJUSTMENT? PLEASE WAIT!
If hydraulic valve lifters are used for valve actuation, it is recommended to leave the engine for 12 hours after assembly before it is started again. This time is necessary to allow any excess oil to drain from the hydraulic valve lifters. Alternatively, the hydraulic valve lifters can be emptied before fitting.

FULL PROGRAMME—AS A SET AND SOLO
The MAHLE Original valve programme comprises numerous passenger car and commercial vehicle applications in different materials and designs according to the demands of the original equipment market. All valve types are also available individually—which provides a good basis for customer and demand oriented repairs.

Fig. 1: Helpful during valve disassembly: a valve spring tensioning device.

Fig. 2: The bore of the valve guide should be reamed by hand with the aid of cutting oil.

Fig. 3: Caution—if the spring is not fitted straight, the valve can be damaged and cause engine failure.

Fig. 4: Worn valve collets do not fit new valves and are therefore often the cause of valve breakage.

Fig. 5: For safety reasons: a vacuum test verifies the seal of the valves.

VALVE SEAT INSERTS ARE THE WEARING PRODUCTS IN ENGINE REPAIR. HERE A COUPLE OF TIPS—FROM THE PRACTICE FOR THE PRACTICE.

They look just like any ring. But they can do a lot more. And that’s what they need to do. They have to perform a very special task in the engine under accordingly high loads. Together with the valves, they must seal the combustion chamber. They have to prevent impacting of valves into the cylinder head. And they must absorb the combustion heat and transfer it to the cylinder head. Due to the different material properties of aluminium and steel alloys, valve seat inserts are almost indispensable for aluminium cylinder heads in order to assure sufficient sealing of the combustion chambers.

A LOOK AT THE PRODUCTION PROCESS
Valve seat insert are either produced in a centrifugal casting process or by sintering. Centrifugal casting is the more conventional method—a special casting process in which the molten steel alloy is poured into a casting mould that is rotating around its central axis. Due to the centrifugal force, the melt is pressed into the outer wall of the mould. When the melt solidifies it takes-on the shape of the mould and forms a hollow casting with the interior profile of the casting mould.

In the sinter process, powdered material is first formed so that a minimum cohesion between the powder particles is achieved. The pre-pressed so-called “green compact” is then further condensed and hardened by applying a heat treatment below its melting temperature. Depending on demands and applications in the engines, different powder materials are used in the sintering process. The choice of powder mixture depends, for instance, on the subsequent operating temperature of the insert and the required wear resistance. This makes it possible to custom design the inserts exactly for their later use in the engine.

1

Fig. 1: Removing the valve seat insert by milling.

2

Fig. 2: After milling a remainder of the insert can be seen.

B

REPLACEMENT: USING BRUTE FORCE – AND A GENTLER ALTERNATIVE
Before the new valve seat inserts can be fitted, the old inserts have to be removed. The widespread method of welding an old valve to the worn valve seat inserts and then to knock the insert from its seat in the cylinder head with a hammer appears rather brutish—often causing unforeseeable consequential damage to the cylinder head. The gentler version: removing of the worn insert from its seat by milling (fig. 1 and 2), while the cutter head is centered by the valve guide. Afterwards, the seat of the insert can be finished to the appropriate oversize of the new valve seat insert if necessary (fig. 3). After machining, the cylinder head is cleaned from chips in the washer.

3

Fig. 3: The milling head for machining the seat of the valve seat insert.

C

THE FITTING PROCESS—A HOT AND COLD AFFAIR
There exist several methods for fitting valve seat inserts to the cylinder head.

  • The valve seat insert is pressed into its seat in the cylinder head at room temperature.
  • A valve seat insert remains at room temperature and is pressed into a pre-heated cylinder head.
  • The insert is cooled in liquid nitrogen (fig. 4) and is then pressed into the cylinder head which remains at room temperature.
  • The cylinder head is heated and the valve seat insert is cooled down—the optimum joining method using almost no force.

4

Fig. 4: The valve seat inserts are cooled down in liquid nitrogen to facilitate fitting.

5

Fig. 5: The cracked partition—result of excessive overlap.

D

THE OVERLAP—AN IMPORTANT DETAIL IN THE REPAIR PROCESS
In order to fix the valve seat insert into the cylinder head, the correct overlap must exist between the valve seat insert and its seat in the cylinder head. If this overlap is to large, the strong deformation of the aluminium due to the pressing-in of the valve seat insert can lead to plastic deformations in the cylinder head – with the possible consequence of insufficient fixation of the insert. In addition, an excessive overlap can lead to stress cracks especially in the narrow area between the valve seat inserts (fig. 5). When the overlap is too small, there is a risk that the valve seat inserts fall out of the seat during operation.

THE FINAL STAGE: THE MACHINING PROCESS
After fitting, the inserts need to be machined. This is because the contact surface with the valve has to match exactly the angle of the valve (fig. 6). After the final machining process, the cylinder head needs to be cleaned carefully again—to prevent chips from getting into the engine during assembly afterwards.

6

Fig. 6: The optimum fitting result with replaced valve seat insert

E

MAHLE ORIGINAL VALVE SEAT INSERTS—A GOOD CHOICE
MAHLE Original valve seat insert are made from a variety of materials. Valve seat inserts made from cast iron alloys provide good wear resistance and hardness at high temperatures. Valve seats made from steel alloys with high chromium content are distinguished by appropriate wear and impact resistance as well as good thermal stability and form stability at high temperatures. Sintered metal alloys with their very good wear resistance against abrasion and impact stress are mainly used in modern high-performance n/a engines and turbo engines for petrol, diesel and gas operation. Take care that the replacement is of adequate material and alloy when replacing inserts. MAHLE Original offers you a wide range of high-quality valve seat inserts for many applications. You can find detailed information in the current catalogue “Valve train components”.

They decide how much fresh gas mixture can reach the combustion chamber. They control when the combusted mixture is expelled.They have to keep a tight seal—and this consistently. And they have to work reliably as long as the engine is running. In short: Valves have to perform a tough job.

After all, the valve heads with their sealing surfaces hit the valve seats up to 70 times per second. The inertia forces and the forces from the valve springs and combustion pressure represent a great challenge for the components. Especially the exhaust valves, which are heated up considerably by the exhaust gases with temperatures up to 800 °C. In the short time during which the sealing surfaces are in contact, as much heat as possible must be transferred from the valve seat to the cylinder head. However, not only the heat of the exhaust gases is attacking the valves, but also the aggressive chemical components, which can cause deposits and corrosion.

It's up to the material
To ensure that valves in combustion engines work reliably even under extreme conditions, first-class materials have to be used. MAHLE valves are made from high-grade alloyed materials that are matched optimally to the demands. The materials are developed in close cooperation with the engine makers.

Depending on the demands, different materials may be combined to make use of the positive properties of each material and to provide the optimum support for the different areas of the valve. For example, in a MAHLE bimetal valve, the valve head is made from highly alloyed steel, which is connected with the valve shaft in a way that is resistant to chemical and thermal attack. The actual valve shafts have to be hardened and of high strength.

According to demands, MAHLE valves are optimised further to reduce wear and increase service life. This may be achieved by hardening the valve seat or by build-up welding with special alloys (stellite) to hard-face the sealing surface. The valve shafts are often nitrided or chrome-plated to improve their sliding and wear properties.

Keep cool with liquid metal
A special highlight for engines under extreme thermal loads is the sodium filled hollow bore valve. This type of MAHLE valve has a deep bore that reaches from the valve head into the valve shaft. The bore is filled two thirds with sodium and closed afterwards. When the engine is operating, the sodium becomes liquid and turns into an excellent heat conductor. Due to the up and down movements, the heat is transported from the valve head to the valve shaft by the liquid sodium, where the heat is dissipated via the valve guide to the cylinder head and subsequently to the coolant circuit.

At a glance: The sodium filled hollow valve

MAHLE valves—news from the production lines
MAHLE is the world leader in the serial production of sodium filled hollow bore valves and manufactures valves with shaft lengths from 80 to 210 mm on state-of-the-art flexible manufacturing facilities. Flexible technologies ensure high quality standards and allow economical production of even small batch sizes. Also our customers in the aftermarket benefit from our close cooperation with the international automotive industry. We can offer a comprehensive valve program for car and truck engines, which has now been extended by more than 400 types.

In addition to a significant widening of the valve range for vehicles from Audi, BMW, Citroën, Ford, Opel, Peugeot, Renault, VW and Volvo, especially the program for Italian manufacturers such as Alfa, Fiat and Lancia has been expanded considerably. Also in the commercial vehicle sector (Deutz, MWM, Perkins, etc.), we have added to our range.