US20200173317A1

US20200173317A1 - Cylinder head assembly having a hybrid valve seat insert

Info

Publication number: US20200173317A1
Application number: US16/208,615
Authority: US
Inventors: Edward J. Keating; Brian W. Geiser
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2018-12-04
Filing date: 2018-12-04
Publication date: 2020-06-04
Also published as: CN111271185A; DE102019115844A1

Abstract

A cylinder head assembly for an internal combustion engine includes a main body, a valve seat insert, and at least one flow passage extending through the main body. The main body may be formed from a first material and defines a recess configured to cooperate with an associated cylinder bore and piston to form a combustion chamber. The flow passage may extend through the main body from the recess. The valve seat insert may be disposed within the recess proximate to an end of the flow passage. The valve seat insert includes a thermally conductive layer of powdered metal having an upper side disposed adjacent to the main body, a hardness layer of powdered metal, and a machining layer of powdered metal.

Description

TECHNICAL FIELD

The present disclosure relates to a cylinder head assembly wherein the valve seat insert is formed by at least three layers of different material.

BACKGROUND

Wear resistance is a prime requirement for valve seat inserts used in internal combustion engines. In an effort to achieve a combination of good heat and corrosion resistance and machinability coupled with wear resistance, exhaust valve seat inserts have been formed as a cast metal ahoy. Cast metal alloys are generally preferred over other materials given that valve seat inserts for internal combustion engines must exhibit high wear resistance at elevated temperatures for prolonged periods of time. Therefore, it is desirable for the valve seat insert to exhibit high hardness characteristics which include but are not limited to high creep strength and high thermal fatigue strength even under repeated impact loading at elevated temperatures.
Valve seat inserts may be formed using powdered metal given that powdered metal has low compressibility. Therefore, processes such as double pressing, double sintering, and high temperature sintering are used to achieve a desired density level. Accordingly, a valve seat is generally made of a metal different from and harder than that of the cylinder head itself so as to allow for high abrasion resistance, high heat resistance, and secure sealing. Thus, when subjected to machining after the valve seat is inserted into the cylinder head unit, it is very difficult to machine and finish with high accuracy since the material used for forming the valve seat insert exhibits high hardness characteristics.
It is understood that a traditional valve seat is first inserted into the cylinder head or engine block wherein the valve seat insert is installed into the cylinder head using an interference fit for positive retention. Once properly positioned within the cylinder head, the valve seat insert is then finish machined to achieve the desired insert height.

SUMMARY

The present disclosure provides a cylinder head assembly for an internal combustion engine where the cylinder head assembly includes a main body, a valve seat insert, and at least one flow passage extending through the main body. The main body may be formed from a first material and defines a recess configured to cooperate with an associated cylinder bore and piston to form a combustion chamber. A flow passage may extend through the main body from the recess. The valve seat insert may be disposed within the recess proximate to an end of the flow passage. The valve seat insert may be a sintered component which includes a thermally conductive layer of powdered metal having a first side disposed adjacent to the main body, a hardness layer of powdered metal, and a machining layer of powdered metal. The machining layer of powdered metal provided in the valve seat insert has a second side exposed to the associated cylinder bore and piston.
The thermally conductive layer of powdered metal may, but not necessarily, have a thermal conductivity which is greater than about 350 Wi(m-K). The hardness layer of powdered metal, may but not necessarily, have a hardness which is greater than about Rc 35. The machining layer of powdered metal may but not necessarily include, but not be limited to steel alloys. It is understood that the hardness layer of powdered metal may be disposed between the machining layer of powdered metal and the thermally conductive layer of powdered metal.
In another embodiment of the present disclosure, a first transition region and a second transition region may be further included in the valve seat insert. The first transition region may be defined between the thermally conductive layer and the hardness layer of powdered metal while the second transition region may be defined between the hardness layer of powdered metal and the machining layer of powdered metal. The first transition region includes a first mixture of powdered metal from the thermally conductive layer and hardness layer while the second transition region includes a second mixture of powdered metal from the hardness layer of powdered metal and the machining layer of powdered metal. It is understood that the first and second transition regions formed as the powdered metal from the adjacent layers mixed with each other prior to undergoing the sintering process.
In general, it is understood that, prior to machining and after sintering the layers of powdered metal, the valve seat insert may be provided in the form of a ring. In another embodiment of the present disclosure, the valve seat insert may further include a secondary hardness layer disposed along an inner surface of the ring wherein the secondary hardness layer is positioned between the thermally conductive layer and the machining layer. Similar to the hardness layer, the secondary hardness layer of powdered metal may, but not necessarily, have a hardness which is greater than about Rc 35.
Regardless of the various configurations for the various layers in the valve seat insert, a portion of the machining layer is configured to be machined away from the valve seat insert in order to shorten the height of the valve seat insert after the valve seat insert is installed into the cylinder head.
The present disclosure and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present disclosure will be apparent from the following detailed description, best mode, claims, and accompanying drawings in which:

FIG. 1A is a cross-sectional view illustrating a cylinder head assembly according various embodiments of the present disclosure;

FIG. 1B is an enlarged view of the valve and valve seat insert of FIG. 1A.

FIG. 2 illustrates an enlarged view of the valve seat insert of FIG. 1A relative to the flow passage.

FIG. 3A illustrates a first embodiment of a valve seat insert of the present disclosure before the valve seat insert is finish machined.

FIG. 3B illustrates a second embodiment of a valve seat insert of the present disclosure before the valve seat insert is finish machined.

FIG. 4A illustrates the first embodiment of a valve seat insert of FIG. 3A after the valve seat insert is finish machined.

FIG. 4B illustrates the second embodiment of a valve seat insert of FIG. 3B after the valve seat insert is finish machined.

FIG. 5 illustrates a third embodiment valve seat insert wherein the transition regions are shown by example only.

Like reference numerals refer to like parts throughout the description of several views of the drawings.

DETAILED DESCRIPTION

Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present disclosure, which constitute the best modes of practicing the present disclosure presently known to the inventors. The figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the present disclosure that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the present disclosure and/or as a representative basis for teaching one skilled in the art to variously employ the present disclosure.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present disclosure. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the present disclosure implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It is also to be understood that this present disclosure is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present disclosure and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and the comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, un-recited elements or method steps.
The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.
The terms “comprising”, “consisting of”, and “consisting essentially of” can be alternatively used. Where one of these three terms is used, the presently disclosed and claimed subject matter can include the use of either of the other two terms.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this present disclosure pertains.
The following detailed description is merely exemplary in nature and is not intended to limit the present disclosure or the application and uses of the present disclosure. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
The present disclosure provides an engine head assembly 10 wherein the valve seat inserts 18 are formed from various layers of powdered metals. The powdered metal blend/layers of the present invention may be used for valve seat inserts for engine valves. It should be immediately apparent that the powdered metal part in accordance with the present invention is equally suitable to other applications as well. An engine valve train component such as a valve seat insert constructed with the powdered metal arrangement according to the present invention may be employed as an intake valve seat insert as well as an exhaust valve seat insert component.
Referring to FIG. 1A, there is illustrated a cylinder head assembly 10 according to various embodiments of the present disclosure wherein the cylinder head assembly 10 is generally designated for use in an engine 11. Cylinder head assembly 10 includes a main body 24 (which may, in itself, be referred to as the cylinder head) and a plurality of valves 12 wherein each valve 12 is reciprocatingly received within the internal bore 15 of a corresponding valve stem guide 14. The valve stem guides 14 and the valves are disposed within the main body 24 as shown in FIG. 1A. The valve stem guide 14 is a tubular structure which is inserted into the main body 24 while the hybrid valve seat insert 18 is disposed within a recess 27 (see FIG. 1B) defined by the main body 24. The present invention is not intended to be limited to any specific structure since modifications and alternative structures are provided by various manufacturers. These valve assembly drawings are being provided for illustrative purposes to facilitate a better understanding of the present invention.
Referring now to FIG. 1B, each valve 12 in the cylinder head assembly 10 includes a valve sealing face 16 interposed between the cap 26 and fillet 28 of the valve 12. Valve stem 20 is located normally upwardly of neck 28 and usually is received within valve stem guide 14. As shown, valve seat insert 18 is mounted within the recess 27 defined by the main body 24 of the engine 11. Preferably, the insert 18 is annular in shape with a cross-section shown, and cooperatively receives the valve sealing face 16 as shown in FIGS. 1A-1B. The construction of the valve seat insert 18 and the position of the valve seat insert 18 relative to the cooperating recess 27 defined in the main body 24 at the mouth 22 of the flow passage 23 will now be described with reference to FIG. 2.
Referring now to FIG. 2, a second, enlarged cross-sectional view of the cylinder head assembly 10 is shown where the flow passage 23 intersects with the valve stem guide 14 and the valve seat insert 18. As will be described herein, the valve seat insert 18 has a metallurgical construction. This valve seat insert 18 is retained in the main body 24 by an interference fit. A result of this interference fit is that a portion of the material of the main body 24 may be elastically deformed upon installation of the valve seat insert 18 into the recess 27. It is understood that once the valve seat insert 18 is positioned within recess 27, then each valve seat insert 18 may undergo a finish machining process so as to reduce the height 54 (see FIG. 2) of the valve seat insert 18 to shortened height 54′ (FIG. 2). It is also understood that the finish machining process is also performed in order to properly align the sealing face 16 (FIG. 1B) of the valve 12 against the conical surface 30 of the valve seat insert (FIG. 13). Proper alignment improves the combustion performance of the engine 11. It should be noted that the alloy of the main body 24 is generally of the same chemical composition and same physical structure throughout, except for being slightly work hardened in the area adjacent to the valve seat insert. An example, non-limiting alloy for the main body 24 may be an aluminum alloy, such as but not limited to Alloy 319 and Alloy 356 aluminum alloys or other light alloys may be utilized.
Referring now to FIGS. 3A-5, example, non-limiting valve seat inserts 18 are shown according various embodiments of the present disclosure. FIGS. 3A-3B illustrate example, non-limiting valve seat inserts before undergoing the finish machining process while FIGS. 4A-4B illustrate the valve seat inserts of FIGS. 3A-3B after each respective valve seat insert undergoes the finish machining process as shown in FIGS. 4A-4B. Once the valve seat insert 18 undergoes the finish machining process, each valve seat insert 18 of the present disclosure may be formed with a cylindrical inner surface 32 that is relatively short in axial length and which merges into a tapered conical surface 30. As shown, the tapered conical surface 30 (FIGS. 4A-4B) is disposed between the second side 34 (combustion side 34) of the valve seat insert. The conical surface 30 is the seating surface which interfaces with the sealing face 16 of the valve 12 as shown in FIG. 1B.
Therefore, with reference back to FIG. 1A, the present disclosure provides a cylinder head assembly 10 for an internal combustion engine 11 wherein the cylinder head assembly 10 includes a main body 24, a valve seat insert 18, and at least one flow passage 23 (FIG. 2) extending through the main body 24. With reference to FIG. 2, the flow passage 23 may be an intake passage while flow passage 23′ may be an exhaust passage. The main body 24 may be formed from a first material and may define a recess 27 (FIGS. 1B and 2) configured to cooperate with an associated cylinder bore 21 (FIG. 1A) and piston (not shown) to form a combustion chamber 25. As shown in FIG. 2, the flow passage 23, 23′ may extend through the main body 24 from the corresponding recess 27. The valve seat insert 18 may be disposed within the corresponding recess 27 proximate to an end 19, 22 of the corresponding flow passage 23, 23′. In FIG. 2, the valve seat insert 18 is shown in both the pre-machined state 56 and the post-machined state 56′. The valve seat insert 18, 56 which has not yet been machined is shown in phantom as element 56 while the valve seat insert 18, 56′ which has undergone the machining process is shown in solid as element 56′.
Referring now to 3A-5, the valve seat insert 18 may be a sintered component which includes at least three different layers of powdered metal so as to achieve the desired wear resistant characteristics, thermal conductivity characteristics and ease of machining characteristics. As indicated, FIGS. 3A-3B illustrate example, non-limiting embodiments of the valve seat insert 18, 56 before the valve seat insert 18, 56 has been machined. FIGS. 4A-48 illustrate the valve seat inserts 18 of FIGS. 3A and 3B respectively after the valve seat inserts 18, 56′ have been machined. As shown in FIGS. 3A-43, the first layer provided in the valve seat insert 18 may be a thermally conductive layer 40. The thermally conductive layer 40 of the valve seat insert 18 enhances the combustion performance for the corresponding cylinder. As shown in FIGS. 3A-4B, the thermally conductive layer 40 of powdered metal defines a stem side 38 or first side 38 of the valve seat insert 18 which is disposed adjacent to the main body 24—in particular, the base 29 of recess 27 defined by the main body 24 as shown in FIG. 1B. It is understood that the stem side 38 may also be referenced as the first side 38 of the valve seat insert 18 in the present disclosure.
In addition to the thermally conductive layer 40, the valve seat insert 18 of the present disclosure includes at least one hardness layer 42, 42′ of powdered metal in addition to a machining layer 44 of powdered metal. (See FIGS. 3A-3B). The hardness layer 42 of powdered metal is provided to improve the wear resistance characteristics of the valve seat insert 18. With reference to FIGS. 3A-3B, the hardness layer 42 may be disposed between the thermally conductive layer 40 and the machining layer 44 wherein a portion of the hardness layer 42 defines a second region 31 of the inner cylindrical wall 32 of the valve seat insert 18. As shown in the example, non-limiting valve seat inserts shown in FIGS. 3A-3B, the machining layer 44 may, but not necessarily, define a third region 35 of the inner cylindrical wall 32 before the valve seat insert 18 undergoes the finish machining process. However, in FIGS. 4A-4B, the valve seat inserts 18 of FIG. 3A and 3B are shown respectively after such valve seat inserts 18 have respectively undergone the machining process. In contrast to FIG. 3A, FIG. 4A illustrates the valve seat insert 18 of FIG. 3A after the finish machining process is completed—wherein the third region 35 (FIG. 3A) of the inner cylindrical wall 32 has been be removed via the finish machining process. As a result of the finish machining process, tapered conical surface 30 (FIG. 4A) is defined in the valve seat insert 18. Similarly, in contrast to FIG. 3B, FIG. 43 illustrates the valve seat insert 18 of FIG. 3B after the valve seat insert 18 has undergone the finish machining process. Similarly, the third region 35 (FIG. 3B) of the inner cylindrical wall 32 may be removed via the finish machining process thereby leaving the tapered conical surface 30 (FIG. 4B) which is defined in the valve seat insert 18. Given that the machining layer 44 is relatively soft compared to the other layers in the valve seat insert 18 and is limited to the third region 35 of the inner cylindrical wall, then the machining layer 44 is configured to enable easy removal of such material from the valve seat insert 18 in order to achieve the desired shortened height 54′ (see FIGS. 4A-4B) in the valve seat insert 18. The original height of the valve seat insert is established to enable efficient installation of the valve seat insert into the cylinder head during mass production. The resultant reduced height is advantageous to efficient engine operation. Therefore, as shown in FIGS. 4A-4B, the hardness layer 42 of the machine finished valve seat insert 56′ may define the second side 34 (FIGS. 4A-4B) of the valve seat insert 18 wherein the second side 34 (see FIG. 1B) is exposed to or faces the associated cylinder bore 21 and piston (not shown). As indicated, the machining layer 44 is initially formed as part of the valve seat insert 18 at the second side 34 of the valve seat insert 18 (during the sintering process) to ease the subsequent machining process—wherein accurate surface cuts on the valve seat insert are achieved. As indicated, the valve seat insert 18 interfaces with the sealing face 16 of the valve 12 as shown in FIG. 1B. As a result of implementing a special machining layer 44 in the sintered valve seat insert 18, the machining tool is subject to decreased wear and tear as the desirable shortened height 54′ (FIGS. 2, 4A-4B) of the valve seat insert 18 is achieved via the machining process (not shown).
With reference to the non-limiting examples shown in FIGS. 3A-5, the thermally conductive layer 40 of powdered metal enables efficient thermal management within the combustion chamber and accordingly improves combustion performance of the chamber. The thermally conductive layer 40 may, but not necessarily, have a thermal conductivity which is greater than about 350 W/(m-K). The thermally conductive layer 40 may, but not necessarily, be formed from one or more high copper content sintering materials. Also shown in the non-limiting examples of FIGS. 3A-5, the hardness layer 42 of powdered metal in the valve seat insert 18, may but not necessarily, have a hardness which is greater than about Rc 35. As shown in the non-limiting example of FIGS. 3A-3B, the hardness layer 42 may extend up the inner cylindrical wall and/or the hardness layer 42 may span across the width 53 (FIG. 3A) of the valve seat insert 18 while being initially positioned in between the thermally conductive layer 40 and the machining layer 44 during the sintering process. However, as shown in FIGS. 3B and 4B, the hardness layer 42 may only be positioned between a portion of the thermally conductive layer and a portion of the machining layer 44. Thus, in the arrangement shown in FIG. 3A-4B, the hardness layer 42 may be primarily configured to define a second region 31 of the inner cylindrical wall 32. Accordingly, the second region 31 of the inner cylindrical wall machining layer 44 of powdered metal may but not necessarily include, but not be limited to highly alloyed steel sintering materials, or the like. As previously described, the hardness layer 42 of powdered metal may, but not necessarily, be initially disposed between the machining layer 44 of powdered metal and the thermally conductive layer 40 of powdered metal when the insert 18 is sintered as shown in the non-limiting examples of FIGS. 3A-B. The aforementioned layers of powdered metal may be arranged as described in the non-limiting examples shown in FIGS. 3A-5 so that the desired material characteristics for each layer are provided in pre-determined regions of the valve seat insert 18.
Referring now to FIG. 5, a transition region 48 may be further included any of the embodiments of the valve seat insert 18. The transition region 48 may be defined between the thermally conductive layer 40 and the hardness layer 42 of powdered metal. The transition region 48 includes a mixture 62 of powdered metal from the thermally conductive layer 40 and hardness layer 42. It is understood that the transition region 48 may be formed as the powdered metal from the adjacent layers mix with each other prior to undergoing the sintering process.
In general, it is understood that, prior to machining but after sintering the layers of powdered metal, the valve seat insert 18 may be provided in the form of a ring-like configuration as shown in FIGS. 3A-5. Moreover, in yet another embodiment of the present disclosure, the valve seat insert 18 may further include a secondary hardness layer 42′ (shown in phantom in FIGS. 3A and 4A) disposed along an inner surface 32 of the ring (separate from hardness layer 42) wherein the secondary hardness layer 42′ is also positioned between the thermally conductive layer and the machining layer. Similar to the hardness layer 42, the secondary hardness layer 42′ of powdered metal may, but not necessarily, have a hardness which is greater than about Rc 35. However, it is also understood that the secondary hardness layer may be integral to the hardness layer 42 so as to form a single layer which may have a non-linear configuration—as shown in solid.
Regardless of the various configurations of the hardness layer(s) 42, 42′ and the thermally conductive layer 40, the machining layer 44 defines the second side 34 (FIGS. 3A-3B) of the valve seat insert 18 prior to the machining (finishing) process. The machining layer 44 is configured to be removed from the valve seat insert 18 via a machining tool or the like in order to shorten the height 54 (FIGS. 3A-3B) of the valve seat insert 18 to shortened height 54′ (see FIGS. 2, 4A-4B) after the valve seat insert 18 is disposed within the recess 27 of the main body 24 (or cylinder head 24). As indicated, the cylinder head 24 or main body 24 may be formed via a casting process, and the cylinder head may define a recess 27 which further defines a base 29 which may contact the first side 38 of the valve seat insert 18 upon installation (see FIG. 1B).
With reference to the valve seat insert 18, it is understood that the valve seat insert 18 contacts with the poppet-type intake and exhaust valves of the engine 11 as shown in FIG. 1B. Therefore, the valve seat insert 18 of the present disclosure must demonstrate good wear resistance. It is understood that the hardness layer at the previously described non-limiting example predetermined regions in the valve seat insert enable the valve seat insert 18 to exhibit good wear resistance. In addition, since the valve 12 itself is cooled primarily by the transfer of heat from the poppet valve head to the cylinder head 24 (or main body 24) through the valve seat insert 18, high thermal conductivity of the valve seat insert 18 is also important. The high thermal conductivity is enabled via the thermally conductive layer 40 provided in the valve seat insert 18 according to the various embodiments of the present disclosure.
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.

Claims

What is claimed is:

1. A cylinder head assembly for an internal combustion engine comprising:

a main body formed from a first material and defining a recess configured to cooperate with an associated cylinder bore and piston for forming a combustion chamber;

at least one flow passage extending through the main body from the recess; and

a valve seat insert disposed within the recess proximate to an end of the at least one flow passage

wherein the valve seat insert includes a thermally conductive layer of powdered metal having an upper side disposed adjacent to the main body, a hardness layer of powdered metal, and a machining layer of powdered metal having a lower side exposed to the associated cylinder bore and piston.

2. The cylinder head assembly as defined in claim 1 wherein the thermally conductive layer of powdered metal has a thermal conductivity which is greater than 350 W/(m-K), the hardness layer of powdered metal has a hardness which is greater than Rc 35, and the machining layer of powdered metal includes free machining steel alloy.

3. The cylinder head assembly as defined in claim 2 wherein the hardness layer of powdered metal is disposed between the machining layer of powdered metal and the thermally conductive layer of powdered metal.

4. The cylinder head assembly as defined in claim 2 further comprising:

a transition region defined between the thermally conductive layer and the hardness layer of powdered metal;

wherein the transition region includes a mixture of powdered metal from the thermally conductive layer and hardness layer.

5. The cylinder head assembly as defined in claim 2 wherein the valve seat insert is formed by a sintering process.

6. The cylinder head assembly as defined in claim 5 wherein the valve seat insert defines a ring-like configuration.

7. The cylinder head assembly as defined in claim 6 further comprising a secondary hardness layer disposed along an inner surface of the ring wherein the secondary hardness layer is positioned between the thermally conductive layer and the machining layer.

8. The cylinder head assembly as defined in claim 6 wherein all or a portion of the machining layer is configured to be machined away from the valve seat insert.

9. The cylinder head assembly as defined in claim 7 wherein a portion of the machining layer is configured to be machined away from the valve seat insert.