JP2013176784A - Method for casting cylinder head, and method for manufacturing port core used for casting cylinder head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the positional accuracy of intake and exhaust ports being molded on a cylinder head by casting, and also to improve its yield.SOLUTION: A method for casting a cylinder head includes a step S1 for acquiring a permissible clearance based on the relation between the minimum clearance formed between the upper portion of a port core and a casting die for forming intake and exhaust ports and the amount of a floating state when charging molten metal into the end of the combustion chamber side of the port core, a step S2 for measuring the minimum clearance about the port core using a jig for reproduction which can reproduce the minimum clearance, a step S3 for judging whether it is good or bad to use the port core for casting the cylinder head based on a measuring result of the minimum clearance and the permissible clearance, and a step S4 for utilizing the port core for casting the cylinder head judged as a good article by the good and bad judgment step.

Description

  The present invention relates to a method for casting a cylinder head and a method for manufacturing a port core used for casting a cylinder head.

  When molding a cylinder head of an engine by casting, a corresponding molding die such as a water jacket or an intake / exhaust port (referring to one or both of an intake port and an exhaust port; the same applies hereinafter) is cast from the die after casting. A so-called undercut portion that cannot be taken out can be molded by previously placing a core having a corresponding shape in a mold.

  In general, this type of core is disposed with a gap of an appropriate size provided between the core and the mold in order to avoid unnecessary interference with the mold due to thermal expansion. On the other hand, the core for molding the intake / exhaust port is disposed in the mold by placing a base plate provided on the side of the cylinder head of the core, for example, at a predetermined position of the lower mold. In this case, the end portion on the combustion chamber side of the core is simply placed on the combustion chamber forming portion provided in the lower mold (see FIG. 2A described later). Therefore, when the molten metal flows into the cavity provided in the mold during casting, the core may be lifted by the inflow pressure at that time or by the buoyancy received by the core from the molten metal, and the position may shift (described later). (See FIG. 2 (c)). In this case, it is difficult to accurately form a portion (intake / exhaust port) corresponding to the core at a predetermined position inside the casting.

  Here, for example, in Patent Document 1 below, a weight that can move up and down with respect to the upper mold is provided, and when the mold is clamped, the lower surface of the weight is brought into contact with the core for forming the intake / exhaust port. In addition, a casting mold is disclosed that can hold down a core for forming an intake / exhaust port with its own weight during pouring.

JP-A-7-155896

  By the way, since the core used for this type of casting is generally formed by heating and solidifying a mixture of sand and resin, it is more fragile than a mold. Therefore, as described above, an appropriate gap is provided between the core and the mold in order to avoid interference with the mold due to thermal expansion of the core. Nevertheless, as described in Patent Document 1, if the weight is brought into contact with the core at the time of clamping, the significance of providing the gap is lost, and the core may be damaged by the contact. It is not preferable. In particular, since the core has the above-described structure, dimensional variations are inevitably generated, and therefore, the contact state with the weight is apt to vary. Therefore, when the actual dimension of the core is larger than the design dimension, the core is excessively pressed when the mold is clamped, which may cause damage. Even if the damaged part is a part that does not directly affect the molding (baseboard part), this kind of damage is caused because the damaged part may be mixed into the cavity, resulting in deterioration of casting quality. It is the actual situation that we want to refrain as much as possible from the configuration and operation leading to.

  For example, a method of ensuring the dimensions of the core for forming the intake / exhaust port by measuring the dimensions of the intake / exhaust port formed in the rough material (cast) after casting by sampling inspection is also conceivable. In this method, since inspection is performed after casting, there is a merit that it is not necessary to send defective products to the subsequent process (secondary processing process), but in the end, the ratio of rework and disposal is not reduced. There is a problem that the yield is still low.

  In view of the above circumstances, the technical problem to be solved by the present invention is to improve the positional accuracy of the intake and exhaust ports formed in the cylinder head by casting and to improve the yield.

  The solution of the technical problem is achieved by the cylinder head casting method according to the present invention. That is, in this casting method, a cylinder that casts a cylinder head and forms an intake / exhaust port in the cylinder head by placing a port core for forming the intake / exhaust port in a casting mold and pouring hot water. In the head casting method, allowable based on the relationship between the minimum clearance formed between the upper part of the port core and the casting mold and the amount of floating at the end of the port core on the combustion chamber side during pouring The process of obtaining clearance, the process of measuring the minimum clearance for the port core using a jig that can reproduce the minimum clearance, and the port core where the measured value of the minimum clearance is included in the allowable clearance And a process used for casting.

  The present inventors manage the minimum clearance among the clearances between the upper portion of the port core and the casting mold that influence the amount of floating at the time of pouring the combustion chamber side end of the port core. We have found that the amount can be controlled. That is, since the position accuracy of the intake / exhaust port in the cylinder head affects the engine performance (for example, fuel efficiency), the allowable deviation of the port position based on the allowable range of the engine performance (for example, the required standard for fuel efficiency). The amount, in other words, the allowable range of the floating amount of the end of the port core on the combustion chamber side is determined. Here, the inventors have found that there is a certain correlation between the lift amount of the port core and the minimum clearance formed between the upper portion of the port core and the casting mold, Based on this correlation, a minimum clearance tolerance range (referred to as "allowable clearance", hereinafter the same) is established corresponding to the allowable range of the port core lifting amount, and the minimum clearance of the port core is determined based on this allowable clearance. As a result, it has been found that the above-mentioned lifting amount can be controlled.

  Therefore, in the present invention, first, based on the relationship between the amount of floating when pouring the combustion chamber side end of the port core and the minimum clearance formed between the upper portion of the port core and the casting mold, Acquire the allowable clearance and measure the minimum clearance for the port core. Then, the previously obtained allowable clearance and the measured value of the minimum clearance for each port core are compared, and the port core whose measured value is included in the allowable clearance is used for casting the cylinder head. Thereby, only the port core in which the floating amount during pouring falls within an allowable range can be selected and used for casting the cylinder head. Therefore, the position accuracy (dimension guarantee accuracy) of all intake / exhaust ports molded by casting can be controlled by keeping the amount of deviation of the intake / exhaust ports from the specified position within the allowable range without any special measures being taken on the casting mold. It becomes possible to improve. In addition, since it is possible to determine whether the port core is used for casting before the cylinder head is cast by using the port core, it is possible to determine whether the port core is used for casting. It is possible to greatly reduce the ratio of (casting) and improve the yield.

  The cylinder head casting method according to the present invention is characterized in that the minimum clearance of the port core is measured using a reproduction jig capable of reproducing the minimum clearance.

  As described above, according to the present invention, by measuring the minimum clearance for each port core, only the port cores that can be used for casting of the cylinder head can be selected from all manufactured port cores. Therefore, it is necessary to measure the minimum clearance for each port core. Here, as a means of actually measuring the minimum clearance, a method of directly measuring the minimum clearance in a state where the port core is placed in a casting mold to be actually used and the mold is clamped (a state where pouring is possible) is performed. However, in the first place, the measurement location is very difficult to visually recognize when the mold is clamped. Even if the measurement can be performed, the port core is heated more than necessary for the time required for the measurement operation, so that the surface layer portion of the core may collapse to cause a reduction in casting quality. Furthermore, since the cycle time increases by the time of the measurement work, a reduction in productivity is inevitable. In this respect, in the present invention, a reproduction jig capable of reproducing the minimum clearance is prepared separately from the casting mold, and the minimum clearance is measured for each port core using the reproduction jig. The minimum clearance can be measured for all the port cores manufactured without using the actual casting mold. Therefore, the minimum clearance measurement operation can be easily performed. Further, since the port core that can be used for casting can be selected at a place different from the casting area, the casting mold can be used only for actual casting work. Therefore, high productivity can be secured by maintaining the cycle time. Moreover, since it is sufficient to carry only the port cores that satisfy the above selection criteria into the casting area, the amount of port cores carried into the casting area can be reduced, and the casting area can be saved. In addition, as a specific example of the “reproduction jig that can reproduce the minimum clearance”, for example, in a state where the port core provided on the casting mold is placed and the port core is placed on the placement surface And a clearance forming surface that forms a minimum clearance with the upper portion of the port core.

  Moreover, the solution of the technical problem is achieved by a method for manufacturing a port core used for casting a cylinder head according to the present invention. That is, this core casting method is a method of manufacturing a port core for casting a cylinder head and forming an intake / exhaust port in the cylinder head by pouring by pouring it in a casting mold. Obtaining a permissible clearance based on the relationship between the minimum clearance formed between the upper part of the port core and the casting mold and the amount of lifting at the end of the combustion chamber side of the port core during pouring; and Using a reproducible jig that can reproduce the minimum clearance, measure the minimum clearance for all manufactured port cores, and measure the port clearance to the cylinder head based on the minimum clearance measurement and allowable clearance. And a step of individually determining whether or not it is used for casting.

  According to this core manufacturing method, as with the cylinder head casting method according to the present invention described above, first, the amount of floating at the time of pouring the combustion chamber side end of the port core and the upper portion of the port core Based on the relationship with the minimum clearance formed between the casting mold and the casting mold, an allowable clearance is obtained and the minimum clearance is measured for all manufactured port cores. Then, based on the previously obtained allowable clearance and the measured value of the minimum clearance for each port core, whether or not the port core is used for casting of the cylinder head is individually determined. Thereby, only the port core in which the floating amount at the time of pouring falls within the allowable range can be selected, and finally the port core can be used for casting of the cylinder head. Therefore, it is possible to improve the positional accuracy of all intake / exhaust ports molded by casting by keeping the amount of deviation of the intake / exhaust ports from the predetermined position within the allowable range without any special measures on the casting mold. It becomes. In addition, by measuring the minimum clearance of all manufactured port cores using a reproducible jig that can reproduce the minimum clearance, the port core is used for casting at the port core manufacturing stage. It is possible to determine whether the thing is good or bad. Therefore, it is possible to significantly reduce the ratio of the rough material (casting) that requires reworking and disposal after casting and to improve the yield.

  As described above, according to the present invention, it is possible to improve the positional accuracy of the intake / exhaust ports formed in the cylinder head by casting and to improve the yield.

It is a flowchart which shows the flow of the casting method of the cylinder head which concerns on one Embodiment of this invention. It is principal part sectional drawing which shows the arrangement | positioning aspect and behavior of a port core at the time of casting, (a) is the state which arranged the port core in the casting metal mold | die, and clamped, (b) started pouring. (C) is principal part sectional drawing which respectively shows the state which the pouring work was completed. It is a graph which shows the relationship between the minimum clearance and the floating amount of a core port. It is a top view of the reproduction | regeneration jig | tool for reproducing the minimum clearance formed between the upper part of a port core and a casting metal mold | die. It is principal part AA sectional drawing of the reproduction jig | tool shown in FIG.

  Hereinafter, an embodiment of a method for casting a cylinder head according to the present invention and a method for producing a port core used for the casting will be described with reference to FIGS. In this embodiment, a case where a cylinder head is cast by a low pressure casting method will be described as an example.

  FIG. 1 is a flowchart showing a flow of a cylinder head casting method and a port core manufacturing method used in the casting according to an embodiment of the present invention. Among these, as shown in FIG. 1, the casting method of the cylinder head includes a minimum clearance formed between the upper portion of the port core for forming the intake / exhaust port and the casting mold, and the port core. Based on the relationship between the amount of floating at the end of the combustion chamber side during pouring, the minimum clearance of the port core is measured using the process S1 for obtaining the allowable clearance and the reproduction jig capable of reproducing the minimum clearance. Based on the step S2, the measured value of the minimum clearance and the allowable clearance, the step S3 for individually determining whether the port core is used for casting of the cylinder head, and the good / bad determination step, it was determined to be a non-defective product. And step S4 in which only the port core is used for casting the cylinder head. Moreover, the manufacturing method of the port core used for this casting comprises the process of S1-S3 among the processes of S1-S4.

(S1) Allowable Clearance Acquisition Step A range (allowable clearance) that can be allowed as the minimum clearance formed between the port core and the casting mold at the start of casting is acquired. Hereinafter, the minimum clearance will be described first. FIG. 2 shows an example of a casting mold. As shown in FIG. 2A, the casting mold 10 according to this illustrated example is provided on the lower mold 11 that is a fixed mold, the upper mold 12 that is a movable mold, and the lower mold 11 in this embodiment. A slide mold 13 that is slidable with respect to the lower mold 11 is mainly provided. In this embodiment, the slide mold 13 is slidable in a direction orthogonal to the mold clamping direction of the lower mold 11 and the upper mold 12 (left and right direction in the figure). The lower mold 11 is provided with a mounting surface 14 for mounting the port core 1 for forming the intake / exhaust port.

  The port core 1 includes a port molding portion 2 that molds an intake / exhaust port of a cylinder head that is a cast product, and a skirting board that is integrally provided on a side surface side (right side in FIG. 2) of the port molding portion 2. Part 3. In this embodiment, the skirting board part 3 is integrated with these port molding parts 2 across a plurality of port molding parts 2 (for example, three) arranged in the casting mold 10 according to the number of cylinders of the engine. Is formed. In this embodiment, the protrusion 4 is provided on the upper part of the skirting board 3.

  Cylinder head casting is started using the casting mold 10 and the port core 1 configured as described above. First, the base plate portion 3 of the port core 1 is placed on the placement surface 14 of the lower die 11, and the mold is clamped with the slide die 13 moved to the combustion chamber side of the cylinder head. As a result, a cavity 15 having a shape corresponding to the cast product is formed in the casting mold 10, and a minimum clearance C is formed between the protrusion 4 and the lower surface 16 of the slide mold 13. In this state, the skirting board part 3 of the port core 1 is placed on the placement surface 14 of the lower mold 11, and the combustion chamber side end 2 a of the port molding part 2 is in the corresponding lower mold 11. The state is in contact with the combustion chamber forming portion 17.

  Thereafter, although not shown in the figure, by adding a low-pressure gas to the molten aluminum in the furnace (also referred to as a crucible) disposed below the casting mold 10, the liquid level of the molten metal is pushed down, thereby the furnace. The molten metal is poured into the cavity 15 of the casting mold 10 through a conduit (also referred to as stalk) connected to, and a communication hole 18 provided in the lower mold 11. In this way, when the pouring operation into the cavity 15 is started, as shown in FIG. 2B, the port core 1 placed on the placement surface 14 of the lower mold 11 is Due to the inflow pressure of the molten metal M flowing into the cavity 15 from the communication hole 18, or due to the buoyancy that the port core 1 receives from the molten metal M, the port chamber 1, particularly the combustion chamber side end 2 a of the port molding portion 2 is lowered. The port core 1 starts to rise with respect to the lower mold 11 after being separated from the combustion chamber molding portion 17 of the mold 11. When the pouring operation into the cavity 15 is completed, as shown in FIG. 2 (c), the port core 1 is rotated clockwise starting from, for example, the cylinder head side surface end of the skirting board 3. Casting is performed in a state in which the protrusion 4 forming the minimum clearance C with the slide mold 13 is in contact with the slide mold 13. In this state, the surface contact state between the port core 1 and the mounting surface 14 is eliminated, and the contact state is approximately linear or close to this. Therefore, the intake / exhaust port of the cylinder head, which is a cast product, is formed in a state of being shifted in the lifting direction by the amount that the combustion chamber side end 2a of the port forming portion 2 is lifted.

  Here, the amount of lifting at the combustion chamber side end 2a of the port core 1 and between the upper part of the skirting board portion 3 of the port core 1 and the casting mold (here, the protruding portion 4 and the slide die 13). When the relationship with the minimum clearance C to be formed was determined, the result shown in FIG. 3 was obtained. Here, the horizontal axis in FIG. 3 is the minimum clearance C [mm], with the right side being positive and the left side being negative with respect to zero. In addition, the vertical axis in the figure is the floating amount F [mm], with the upper side being positive and the lower side being negative with respect to 0 as a boundary. From this figure, it was found that there is a positive correlation between the minimum clearance C and the lift amount F. Therefore, for example, the maximum value of the displacement amount of the intake / exhaust port position that can be allowed based on the required standard regarding the fuel efficiency performance of the engine, that is, the allowable value Fa (upper limit value) of the lift amount of the combustion chamber side end 2a of the port core 1 Is determined, based on the regression line RL based on the above-described correlation, a minimum clearance allowable value Ca (an allowable clearance upper limit value) corresponding to the floating amount allowable value Fa is obtained. Here, in practice, when the value of the minimum clearance C is negative, there is a concern about interference between the port core 1 and the casting mold 10 (slide mold 13). [Mm] is set, whereby the allowable clearance is set to a range of 0 [mm] or more and Ca [mm] or less.

(S2) Minimum clearance measurement step As described above, the allowable clearance (the upper limit value of the allowable clearance) is acquired, and for example, all manufactured port cores (of course, the port cores that have clearly produced manufacturing defects are measured). The minimum clearance C is measured. Here, the dimensional accuracy level of the casting mold 10 is only an order that can be substantially ignored as compared with the dimensional accuracy level of the port core 1 in consideration of the difference in structure and manufacturing process. In view of this point, the minimum clearance C is measured for all manufactured port cores 1 using a reproduction jig that secures the dimensions of the casting mold 10 involved in the formation of the minimum clearance C. Here, as shown in FIGS. 4 and 5, the reproduction jig 20 has a positioning surface 21 that is positioned so that clearance can be measured by placing the port core 1, and has a substantially portal shape. A clearance forming portion 23 provided on the lower surface of the clearance forming portion 23, which is disposed across the port core 1 placed on top and forms a minimum clearance C between the port core 1 and the clearance forming portion 23. And a measuring unit 24 that measures the minimum clearance C formed between the clearance forming unit 23 and the port core 1. Here, the positioning surface 21 has a shape imitating the mounting surface 14 of the lower mold 11 that is actually used, and the positioning surface 21 can reproduce the mounting posture of the port core 1 and the posture at the start of pouring. 21 is formed. Further, with respect to the separation height H (see FIG. 5) between the positioning surface 21 and the clearance forming surface 22, the mounting surface 14 of the lower mold 11 and the port core 1 at the time of mold clamping (at the start of pouring) The dimension is set so that the separation height from the lower surface 16 of the slide mold 13 that forms the minimum clearance C between the protrusion 4 and the protrusion 4 can be reproduced. In addition to the dial gauge shown in the figure, a clearance gauge or other known gap measuring means may be used for the measurement unit 24.

  Using the reproducing jig 20 having the above-described configuration, the minimum clearance C is measured for all manufactured port cores 1. That is, first, the port core 1 to be measured is placed on the positioning surface 21 with the substantially gate-shaped clearance forming portion 23 removed. Then, by arranging the clearance forming portion 23 so as to straddle the port core 1, the minimum clearance C having the same dimension as that at the time of casting is reproduced between the protruding portion 4 of the port core 1 and the clearance forming surface 22. (See FIG. 5). Thereafter, the minimum clearance C is measured by the measuring unit 24 attached to the clearance forming unit 23. Here, the amount of deviation from the reference height of the port core 1 (design value of the separation distance from the surface abutting the positioning surface 21 to the top surface of the protrusion 4) is measured with a dial gauge. The minimum clearance C is calculated for each port core 1 by measuring the height dimension of each of the port cores 1 and subtracting this from the separation height H. In addition, you may measure the minimum clearance C for obtaining the relationship shown in FIG.

(S3) Port core pass / fail determination step In this way, for the port core 1 for which the minimum clearance C has been measured, the allowable clearance acquired in step S1 is compared with the measured value of the minimum clearance C. Whether the port core 1 is used for casting the cylinder head is determined. Specifically, when the value of the minimum clearance C obtained in step S2 is included in the allowable clearance, that is, here, when the value is 0 [mm] or more and the allowable clearance upper limit Ca [mm] or less. It is determined that the product is a non-defective product, and if it is less than 0 [mm] or exceeds Ca [mm], it is determined as a defective product. In this way, the port core 1 as a finished product is obtained by sorting with pass / fail judgment (or by performing post-processing if necessary thereafter).

(S4) Cylinder Head Casting Process Using Port Core Finally, the cylinder head is cast using only the port core 1 determined to be a non-defective product in the quality determination process. That is, although illustration is omitted, only the port cores 1 determined to be non-defective in step S3 among all the manufactured port cores 1 are carried into the casting area and arranged in a state where they can be charged. And as already stated, all the port cores 1 carried in are placed in the casting mold 10 to perform pouring. Here, since all of the port cores 1 used for casting include the measured value of the minimum clearance C at the start of pouring in the allowable clearance, casting is performed using these port cores 1. In this case, the floating amount F at the time of pouring of the combustion chamber side end portion 2a of the port core 1 falls below the allowable value Fa of the floating amount corresponding to the allowable value of the deviation amount of the intake / exhaust port position after molding. Therefore, the position accuracy of the intake / exhaust port can be suppressed to a range that satisfies the required standard of engine performance (for example, fuel efficiency) for all cylinder heads formed by casting.

  As described above, in the present invention, first, the floating amount F at the time of pouring the combustion chamber side end 2a of the port core 1, the protrusion 4 provided on the skirting board portion 3 of the port core 1, and the casting mold 10 are provided. Based on the relationship with the minimum clearance C formed between the first and second clearances, the permissible clearance is acquired, and the minimum clearance C is measured for the port core 1, and then the previously acquired permissible clearance and the port core 1 are measured. On the basis of the measured value of the minimum clearance C for each, whether or not the port core 1 is used for casting of the cylinder head is individually determined. Thereby, only the port core 1 in which the floating amount F at the time of pouring falls within an allowable range can be selected, and finally the port core 1 usable for casting of the cylinder head can be obtained. Therefore, the position accuracy (dimension guarantee accuracy) of all intake / exhaust ports molded by casting can be controlled by keeping the molding position (displacement) of the intake / exhaust ports within an allowable range without any special measures for the casting mold 10. It becomes possible to improve. Moreover, since the quality of using the said port core 1 for casting can be determined in the manufacture stage of the port core 1, the ratio of the rough material (casting) which requires rework after casting and disposal is greatly increased. It is possible to reduce the yield and improve the yield.

  In particular, in the case of using the casting mold 10 in which the slide mold 13 is attached to the lower mold 11 that is a fixed mold as in this embodiment, the slide mold that forms the minimum clearance C with the port core 1. Since the position accuracy of 13 can be ensured, the minimum clearance C can be managed with only the actual measurement dimensions of the port core 1 without considering the mold dimensional accuracy. That is, when the slide mold is attached to the movable upper mold as in Patent Document 1, the position where the minimum clearance C is formed is not stable, and the minimum clearance C is determined by the individual measurement dimensions of the port core 1. Therefore, it is difficult to manage the floating amount F. On the other hand, when the cylinder head is cast using the casting mold 10 in which the slide mold 13 is attached to the lower mold 11 serving as the fixed mold as in this embodiment, the dimensions of the casting mold 10 are determined. Since the accuracy level is substantially negligible compared to the dimensional accuracy level of the port core 1, the variation in the height clearance of the port core 1 can be regarded as the variation in the minimum clearance C. Therefore, for example, the minimum clearance C is obtained as a reliable value for each port core only by measuring the height dimension of the port core 1 using a reproduction jig 20 as shown in FIGS. Therefore, it is possible to easily and accurately perform the above-described pass / fail judgment.

DESCRIPTION OF SYMBOLS 1 Port core 2 Port shaping | molding part 2a Combustion chamber side edge part 3 Base board part 4 Protrusion part 10 Casting die 11 Lower mold 12 Upper mold 13 Slide mold 14 Mounting surface 15 Cavity 16 Lower surface 17 Combustion chamber shaping | molding part 18 Communication hole 20 Reproduction jig 21 Positioning surface 22 Clearance forming surface 23 Clearance forming portion 24 Measuring portion C Minimum clearance F Lifting amount M Molten metal

Claims (2)

In a casting method of a cylinder head, a port core for forming an intake / exhaust port is placed in a casting mold and molten metal is poured, thereby casting the cylinder head and forming the intake / exhaust port on the cylinder head. ,
An allowable clearance is obtained based on the relationship between the minimum clearance formed between the upper part of the port core and the casting mold and the amount of floating at the end of the combustion chamber side of the port core during pouring. And a process of
Measuring the minimum clearance for the port core using a reproduction jig capable of reproducing the minimum clearance;
And a step of using a port core whose measured value of the minimum clearance is included in the allowable clearance for casting the cylinder head. In the method of manufacturing a port core for casting a cylinder head and forming an intake / exhaust port in the cylinder head by pouring by pouring in a casting mold,
An allowable clearance is obtained based on the relationship between the minimum clearance formed between the upper part of the port core and the casting mold and the amount of floating at the end of the combustion chamber side of the port core during pouring. And a process of
Measuring the minimum clearance for all of the manufactured port cores using a reproduction jig capable of reproducing the minimum clearance;
A port used for casting a cylinder head, comprising: individually determining whether or not the port core is used for casting the cylinder head based on the measured value of the minimum clearance and the allowable clearance. A method of manufacturing the core.

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