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.
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| Fig. 1: Removing the valve seat insert by milling. | Fig. 2: After milling a remainder of the insert can be seen. |
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.
Fig. 3: The milling head for machining the seat of the valve seat insert.
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.
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| Fig. 4: The valve seat inserts are cooled down in liquid nitrogen to facilitate fitting. | Fig. 5: The cracked partition – result of excessive overlap. |
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.
Fig. 6: The optimum fitting result with replaced valve seat insert
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”.