Engine and engine assembly method

Abstract

To provide an engine which enables a piston to be inserted into a cylinder bore from the crank case side to improve assembly workability of the piston.SOLUTION: An engine includes: a cylinder block having a cylinder bore in which a piston is housed; and a crank case integrated with the cylinder block and having a crank bearing. The crank bearing is arranged so as to overlap with an extension of the cylinder bore. The crank case has piston slide spaces each of which is formed by cutting out the crank bearing from the cylinder bore side and may house the piston in a state that a center axis of the piston corresponds with a center axis of the cylinder bore.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an engine and a method for assembling an engine. [Background technology]

[0002] Conventionally, there is known a multi-cylinder opposed engine configured such that pistons are inserted into cylinder bores of a cylinder block separated from the cylinder head from the cylinder head side, and then the cylinder head is attached to the cylinder block (see, for example, Patent Document 1).

[0003] Conventionally, an engine in which the cylinder head and the cylinder block are integrated is also known (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 52-46567 [Patent Document 2] Japanese Patent Application Publication No. 7-317594 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of an engine in which the cylinder head and cylinder block are integrated, the pistons must be inserted into the cylinder bores of the cylinder head from the crankcase side, which is located on the opposite side of the cylinder block from the cylinder head.

[0006] However, the crank bearing that rotatably supports the crankshaft journal is positioned in the crankcase so that it overlaps the extension of the cylinder bore, which causes the crank bearing to get in the way when inserting the piston into the cylinder bore from the crankcase side, resulting in a problem of poor piston assembly workability.

[0007] An object of the present invention is to provide an engine and an engine assembly method that enable pistons to be inserted into cylinder bores from the crankcase side, thereby improving the ease of piston assembly. [Means for solving the problem]

[0008] (1) The engine according to the present invention is an engine (e.g., engine 1 described below) that includes a cylinder block (e.g., cylinder block 22 described below) having a cylinder bore (e.g., cylinder bore 221 described below) in which a piston (e.g., piston 3 described below) is housed, and a crankcase (e.g., crankcase 23 described below) that is integrated with the cylinder block and has a crank bearing (e.g., crank bearing 232 described below), the crank bearing being arranged to overlap an extension of the cylinder bore, and the crankcase has a piston slide space (e.g., piston slide space 233 described below) that can house the piston, with the center axis of the piston (e.g., center axis J2 described below) coinciding with the center axis of the cylinder bore (e.g., center axis J1 described below) by cutting out the crank bearing from the cylinder bore side.

[0009] (2) In the engine described in (1) above, the cylinder bore may have a circumferential groove (e.g., circumferential groove 223 described later) on the periphery of the end portion on the piston slide space side into which a cylindrical jig (e.g., cylindrical jig 33 described later) attached to the outer periphery of the piston when the piston is inserted into the cylinder bore can be fitted.

[0010] (3) The engine described in (1) or (2) above may be a multi-cylinder opposed engine in which the crankcases of engine block halves (e.g., engine block halves 2 and 2A described below) in which the cylinder block and the crankcases are integrated are connected to each other and the pistons are arranged opposite each other.

[0011] (4) In the engine described in any one of (1) to (3) above, a cylinder head (e.g., cylinder head 21 described later) may be provided on the opposite side of the cylinder block from the crankcase, and the cylinder head may be integrated with the cylinder block.

[0012] (5) A method for assembling an engine according to the present invention includes assembling an engine including a cylinder block (e.g., a cylinder block 22 described later) having a cylinder bore (e.g., a cylinder bore 221 described later) in which a piston (e.g., a piston 3 described later) is housed, and a crankcase (e.g., a crankcase 23 described later) integrated with the cylinder block and having a crank bearing (e.g., a crank bearing 232 described later), wherein the crank bearing is arranged to overlap an extension of the cylinder bore, and the crankcase has a piston slide space (e.g., a piston slide space 233 described later) capable of housing the piston with a central axis of the piston (e.g., a central axis J2 described later) coinciding with a central axis of the cylinder bore (e.g., a central axis J1 described later) by cutting out the crank bearing from the cylinder bore side, A method for assembling an engine (e.g., engine 1 described later) in which the cylinder bore has a circumferential groove (e.g., circumferential groove 223 described later) at an end on the piston slide space side, and the piston is inserted into the cylinder bore of the cylinder block from the crankcase side, includes the steps of: attaching a cylindrical jig (e.g., cylindrical jig 33 described later) that can be fitted into the circumferential groove to the outer periphery of the piston; accommodating the piston with the cylindrical jig fitted into the piston slide space from the crankcase side; fitting the cylindrical jig into the circumferential groove so that the central axis of the piston coincides with the central axis of the cylinder bore within the piston slide space; and inserting the piston into the cylinder bore while sliding it inside the cylindrical jig, with the cylindrical jig fitted into the circumferential groove. [Effects of the Invention]

[0013] According to the above (1), the crankcase has a piston slide space capable of accommodating the piston with its central axis aligned with the central axis of the cylinder bore, and this piston slide space can be used to insert the piston into the cylinder bore from the crankcase side, thereby providing an engine with improved piston assembly workability.

[0014] According to (2) above, by fitting a cylindrical jig attached to the outer periphery of the piston into a circumferential groove in the cylinder bore, the central axis of the piston and the central axis of the cylinder bore can be easily aligned, which improves the ease of inserting the piston into the cylinder bore from the crankcase side, thereby providing an engine with further improved piston assembly workability.

[0015] According to the above (3), it is possible to provide a multi-cylinder opposed engine with improved piston assembly workability.

[0016] According to the above (4), the cylinder head is integrated with the cylinder block, so that the number of parts of the engine block can be reduced, and the cost of the engine can be reduced.

[0017] According to (5) above, by fitting the tip of the cylindrical jig into the circumferential groove of the cylinder bore within the piston slide space of the crankcase, the central axis of the piston and the central axis of the cylinder bore can be easily aligned, and in that state, only the piston can be inserted into the cylinder bore, which improves the ease of inserting the piston into the cylinder bore from the crankcase side and improves the ease of assembling the piston. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a perspective view of an engine according to an embodiment of the present invention. [Figure 2] FIG. 1 is a perspective view showing an engine block half of an engine according to an embodiment of the present invention. [Figure 3] FIG. 3 is a side cross-sectional view of a half engine block shown in FIG. 2. [Figure 4] FIG. 4 is an enlarged view of part A in FIG. [Figure 5] FIG. 2 is a bottom view showing a state in which a piston is inserted into a cylinder bore of the engine according to the embodiment. [Figure 6] 4 is a side cross-sectional view showing a state in which a piston is inserted into a cylinder bore of the engine according to the embodiment. FIG. [Figure 7] FIG. 2 is a perspective view showing how engine block halves of the engine according to the present embodiment are connected to each other. [Figure 8] FIG. 10 is a perspective view showing a state in which a cylindrical jig is attached to a piston of an engine according to another embodiment. [Figure 9] FIG. 1 is a perspective view of a piston to which a cylindrical jig is attached. [Figure 10] FIG. 10 is a side cross-sectional view of a half engine block of an engine according to another embodiment. [Figure 11] FIG. 10 is a side cross-sectional view showing a state in which a piston with a cylindrical jig attached thereto is inserted into a cylinder bore of an engine according to another embodiment. [Figure 12] FIG. 10 is a perspective view showing a state in which a cylindrical jig is removed from an engine according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view of an engine according to this embodiment. The engine 1 is a multi-cylinder, opposed-cylinder engine configured by integrally assembling engine block halves 2, 2, each having two cylinder bores 221, each accommodating a piston 3, on the crankcase 23 side. The engine 1 of this embodiment has four cylinders. However, the number of cylinders in the engine is not limited to four, and the cylinder arrangement is not limited to an opposed-cylinder arrangement. Note that the two engine block halves 2, 2 of the engine 1 have substantially the same structure, and therefore the following description of the internal structure will refer to only one of the engine block half 2.

[0020] 2 and 3, the engine block half 2 is an integrally molded product having a structure in which a cylinder head 21, a cylinder block 22, and a crankcase 23 are integrated together. Such an engine block half 2 can be manufactured by an additive manufacturing method using, for example, an aluminum-based or other metal material (powdered metal, metal wire, etc.).

[0021] In additive manufacturing using a 3D printer, for example, when powdered metal is used as the metallic material, the engine block half 2 is three-dimensionally additively manufactured by repeatedly irradiating the powdered metal spread on a base plate with a laser or electron beam as a heat source to melt and solidify the portion to be manufactured, and then moving the base plate and spreading new powdered metal. Using additive manufacturing, even if the engine block half 2 has a complex shape including the cylinder head 21, cylinder block 22, and crankcase 23, the entire engine block half 2 can be easily molded as a single unit using a 3D printer. However, the engine block half 2 of the engine 1 is not limited to being additively manufactured by additive manufacturing.

[0022] 3, the engine block half 2 has two cylinder bores 221, 221 arranged in parallel. The cylinder head 21 has an intake port 211 and an exhaust port 212 that communicate with the cylinder bores 221, 221, respectively. Therefore, as will be described in detail later, it is not possible to insert a piston 3 into each of the cylinder bores 221, 221 from the cylinder head 21 side.

[0023] The cylinder block 22 has a water jacket 24 integrally formed on the outside of a cylinder liner 222 that forms a cylinder bore 221. The water jacket 24 is formed from the cylinder block 22 to the cylinder head 21. The water jacket 24 cools the areas around the intake port 211 and exhaust port 212 of the cylinder head 21 and the areas around the cylinder bore 221 of the cylinder block 22 by coolant flowing in from a coolant inlet 241. The cooled coolant is discharged from a coolant outlet 242 to the outside (for example, a radiator, etc.).

[0024] The crankcase 23 is disposed on the opposite side of the cylinder block 22 from the cylinder head 21. A crankshaft accommodating portion 231 is formed inside the crankcase 23. The crankshaft accommodating portion 231 is a space for accommodating the crankshaft 5 (see FIG. 7) to which one end of a connecting rod 4 (see FIGS. 7 and 9) connected to the piston 3 is attached. The crankshaft accommodating portion 231 is commonly connected to the two cylinder bores 221, 221 of the cylinder block 22.

[0025] The crankcase 23 has a plurality of crank bearings 232 at positions close to the cylinder bores 221, 221 of the cylinder block 22. The crank bearings 232 rotatably support the journal 51 (see FIG. 7) of the crankshaft 5 (see FIG. 7) housed in the crankshaft housing portion 231. In this embodiment, three crank bearings 232 are provided in the crankcase 23 of one engine block half 2.

[0026] As shown in Figures 3, 4, and 5, the three crankshaft bearings 232 are arranged along the arrangement direction X of the parallel-arranged cylinder bores 221 in the engine block half 2, and are positioned on either side of the cylinder bore 221 when the cylinder bore 221 is viewed from the crankcase 23. Of these, the two crankshaft bearings 232, 232 located on the left end and the adjacent left side in the drawings, are arranged to overlap on extensions of the cylinder bores 221. In other words, if the cylindrical cylinder bores 221 are extended toward the crankcase 23, the cylinder bores 221 will interfere with the crankshaft bearings 232. Therefore, when inserting the piston 3 into the cylinder bore 221 from the crankcase 23, the crankshaft bearings 232 act as an obstacle, making it impossible to position the piston 3 coaxially below the cylinder bore 221 in the crankcase 23.

[0027] 3 and 4, the crankcase 23 of this embodiment has piston slide spaces 233, 233 capable of accommodating the pistons 3 below the cylinder bores 221, respectively, within the crankcase 23. The piston slide spaces 233, 233 are formed continuously with the lower ends of the cylinder bores 221, 221 by providing notched portions 232a, 232a cut out from the cylinder bores 221, 221 sides in the crank bearings 232, which are arranged so as to overlap on extensions of the cylinder bores 221, 221.

[0028] 4, the height h1 of the piston slide space 233 is equal to or greater than the axial height h2 (see FIG. 6) of the cylindrical portion 31 of the piston 3, which will be described later. The piston slide space 233 is formed so that the entire cylindrical portion 31 of the piston 3 can be accommodated in the piston slide space 233 with the central axis J2 of the piston 3 coinciding with the central axis J1 of the cylinder bore 221.

[0029] Next, a method for assembling the engine 1 by inserting the piston 3 into the cylinder bore 221 from the crankcase 23 side will be described with reference to FIGS. 5, 6 and 7. FIG.

[0030] As shown in Fig. 6, the piston 3 has a cylindrical portion 31 that forms the portion with the largest outer diameter of the piston 3. The cylindrical portion 31 has a plurality of piston rings 32 housed in a plurality of grooves formed on the outer circumferential surface. Note that Figs. 5 and 6 omit illustration of the connecting rod 4 (see Fig. 7) that connects the piston 3 and the crankshaft 5 (see Fig. 7).

[0031] 5 and 6, the piston 3 is inserted from the crankshaft accommodating portion 231 side of the crankcase 23 toward the cylinder bore 221 (P1). At this time, the piston 3 is inserted to a position that avoids the overlapping crank bearing 232. As a result, the central axis J2 of the piston 3 is disposed at a position far from the crank bearing 232 with respect to the central axis J1 of the cylinder bore 221.

[0032] Since the piston slide space 233 can accommodate the entire cylindrical portion 31 of the piston 3, after the piston 3 is inserted into the piston slide space 233, the piston 3 is slid laterally toward the overlapping crank bearing 232 so that the central axis J2 coincides with the central axis J1 of the cylinder bore 221 (P2).

[0033] Thereafter, the piston 3 is pushed upward toward the cylinder bore 221 (P3) and is accommodated within the cylinder bore 221.

[0034] After the pistons 3, 3 are inserted into the two cylinder bores 221, 221 of one engine block half 2, as shown in Figure 7, the remaining two pistons 3, 3 connected to the crankshaft 5 by connecting rods 4 are similarly inserted into the two cylinder bores 221, 221 of the other engine block half 2. The two engine block halves 2, 2 are then assembled together by connecting the crankcases 23, 23 to each other.

[0035] Incidentally, when inserting the piston 3 into the cylinder bore 221 from the crankcase 23 side as described above, it is necessary to align the central axis J2 of the piston 3 with the central axis J1 of the cylinder bore 221 within the piston slide space 233. However, because it is difficult to directly see the tip of the cylindrical portion 31 of the piston 3, the task of aligning the central axes J1 and J2 requires the skill of an operator.

[0036] 8 to 12, another embodiment of the engine 1 will be described, in which the central axis J2 of the piston 3 can be easily aligned with the central axis J1 of the cylinder bore 221. In Figs. 8 to 12, parts with the same reference numerals as those in the above embodiment indicate parts with the same configuration. Detailed descriptions of these parts will be cited from the above embodiment, and may be omitted in the following description.

[0037] First, before inserting the piston 3 into the cylinder bore 221, as shown in FIGS. 8 and 9 , a cylindrical jig 33 is attached to the outer periphery of the cylindrical portion 31 of the piston 3, on which the piston ring 32 is attached. The cylindrical jig 33 is composed of two jig halves 331, 331 each formed in a semicircular arc shape corresponding to half of the outer periphery of the cylindrical portion 31, i.e., a range of 180°. The jig halves 331 have mounting portions 332, 332 that protrude radially outward at both ends. The two jig halves 331, 331 are combined in a circular shape to sandwich the cylindrical portion 31, and are attached to the entire circumference of the cylindrical portion 31 by butting the mounting portions 332, 332 against each other. The outer diameter of the cylindrical jig 33 attached to the cylindrical portion 31 is larger than the inner diameter of the cylinder bore 221. The butted mounting portions 332, 332 are fixed to each other by a fastener (not shown), such as a clip.

[0038] Piston ring 32, which is provided on the outer periphery of cylindrical portion 31 of piston 3, protrudes slightly from the outer periphery of cylindrical portion 31 in order to slide on the inner wall surface of cylinder bore 221. A cylindrical jig 33 attached to the outer periphery of cylindrical portion 31 compresses piston ring 32 radially inward so that the outer diameter of piston ring 32 approximately matches the outer diameter of cylindrical portion 31.

[0039] As shown in Figure 8, each jig half 331, 331 of the cylindrical jig 33 has a step 333, 333 at the tip end on the side facing the direction of advancement when inserted into the cylinder bore 221. By combining the jig halves 331, 331 in a circular shape, the step 333 is provided around the entire circumference of the cylindrical jig 33. The step 333 is formed in a tapered shape that gradually reduces in diameter toward the tip. When the cylindrical jig 33 is attached to the cylindrical portion 31 of the piston 3, the step 333 protrudes above the upper end surface 31a of the cylindrical portion 31 and is arranged coaxially with the cylindrical portion 31.

[0040] 10 shows an engine block half 2A according to another embodiment. Circumferential grooves 223, 223 into which a cylindrical jig 33 can be fitted are formed in advance in the cylinder bores 221, 221 of the engine block half 2A. The circumferential groove 223 is located at the lower end of the cylinder bore 221 adjacent to the piston slide space 233 of the crankcase 23, and is provided around the entire inner periphery of the cylinder bore 221. The diameter of the circumferential groove 223 increases toward the piston slide space 233, and the circumferential groove 223 has a tapered shape that matches the tapered shape of the step 333 of the cylindrical jig 33.

[0041] 11, the piston 3 with the cylindrical jig 33 attached thereto is accommodated in the piston slide space 233 from the crankcase 23 side. At this time, the attachment portions 332, 332 of the cylindrical jig 33 are arranged in a direction perpendicular to the arrangement direction X of the cylinder bores 221, 221. Therefore, the attachment portions 332, 332 of the cylindrical jig 33 do not interfere with the crank bearing 232 of the crankcase 23. The piston slide space 233 is large enough to accommodate the entire cylindrical portion 31 of the piston 3 with the cylindrical jig 33 attached thereto.

[0042] Thereafter, in the same manner as above, the piston 3 slides laterally within the piston slide space 233. During this sliding movement, the piston 3 moves while the step 333 of the cylindrical jig 33 is pressed against the periphery of the lower end of the cylinder bore 221. Therefore, when the position of the step 333 and the position of the circumferential groove 223 match, the cylindrical jig 33 is fitted into the circumferential groove 223 as the step 333 is guided by the tapered shape of the circumferential groove 223. As a result, the central axis J2 (not shown in FIG. 11) of the piston 3 coincides with the central axis J1 (not shown in FIG. 11) of the cylinder bore 221.

[0043] Thereafter, with the cylindrical jig 33 fitted into the circumferential groove 223, the piston 3 is pushed upward toward the cylinder bore 221. Because the outer diameter of the cylindrical jig 33 is larger than the inner diameter of the cylinder bore 221, when the piston 3 is pushed up, the cylindrical jig 33 abuts against the lower end of the cylinder bore 221, and only the cylindrical portion 31 of the piston 3 slides inside the cylindrical jig 33 and is inserted into the cylinder bore 221. As a result, the piston 3 is inserted into the cylinder bore 221 with the central axis J2 reliably aligned with the central axis J1 of the cylinder bore 221. The central axes J1, J2 naturally align when the step portion 333 of the cylindrical jig 33 attached to the piston 3 fits into the circumferential groove 223 of the cylinder bore 221, so no skill on the part of an operator is required.

[0044] 12, after all of the pistons 3 have been inserted into the cylinder bores 221 and the two engine block halves 2A, 2A have been assembled together, the cylindrical jig 33 is removed from the openings 234, 234 that open to the sides of the crankcases 23, 23. The cylindrical jig 33 may also be removed from the openings 234 of each engine block half 2A to the outside before the two engine block halves 2A, 2A are assembled together.

[0045] The engine 1 according to each of the above embodiments has the following advantages. Specifically, the engine 1 includes a cylinder block 22 having a cylinder bore 221 in which a piston 3 is housed, and a crankcase 23 integrated with the cylinder block 22 and having a crank bearing 232. The crank bearing 232 is disposed so as to overlap an extension of the cylinder bore 221. The crankcase 23 has a piston slide space 233 that can house the piston 3 with the central axis J2 of the piston 3 aligned with the central axis J1 of the cylinder bore 221, formed by cutting out the crank bearing 232 from the cylinder bore 221 side. Thus, the crankcase 23 has the piston slide space 233 that can house the piston 3 with the central axis J2 of the piston 3 aligned with the central axis J1 of the cylinder bore 221. Therefore, the piston slide space 233 can be used to insert the piston 3 into the cylinder bore 221 from the crankcase 23 side. This improves assembly workability of the piston 3.

[0046] The engine 1 shown in the above embodiment has a circumferential groove 223 around the end periphery of the cylinder bore 221 on the piston slide space 233 side, into which a cylindrical jig 33 attached to the outer periphery of the piston 3 can be fitted when inserting the piston 3 into the cylinder bore 221. By fitting the cylindrical jig 33 attached to the outer periphery of the piston 3 into the circumferential groove 223 of the cylinder bore 221, the central axis J2 of the piston 3 and the central axis J1 of the cylinder bore 221 can be easily aligned, which improves the ease of inserting the piston 3 into the cylinder bore 221 from the crankcase 23 side, making it possible to provide an engine 1 with further improved ease of assembly of the piston 3.

[0047] The engine 1 shown in the above embodiment is a multi-cylinder opposed engine in which the crankcases 23, 23 of the engine block halves 2, 2A, each having an integrated cylinder block 22 and crankcase 23, are connected to each other and the pistons 3 are arranged opposite each other. This makes it possible to provide a multi-cylinder opposed engine with improved assembly workability for the pistons 3.

[0048] The engine 1 shown in the above embodiment has a cylinder head 21 on the opposite side of the cylinder block 22 from the crankcase 23, and the cylinder head 21 is integrated with the cylinder block 22. By integrating the cylinder head 21 with the cylinder block 22, the number of parts in the engine block halves 2, 2A can be reduced, and the cost of the engine 1 can be reduced.

[0049] The method of assembling the engine 1 shown in the above embodiment includes a cylinder block 22 having a cylinder bore 221 in which the piston 3 is housed, and a crankcase 23 integrated with the cylinder block 22 and having a crank bearing 232, the crank bearing 232 being arranged so as to overlap on an extension of the cylinder bore 221, and the crankcase 23 has a piston slide space 233 capable of accommodating the piston 3 with the central axis J2 of the piston 3 coinciding with the central axis J1 of the cylinder bore 221 by cutting out the crank bearing 232 from the cylinder bore 221 side, and has a circumferential groove 223 at the end of the cylinder bore 221 on the piston slide space 233 side, and the piston 3 is inserted into the cylinder bore 221 of the cylinder block 22 from the crankcase 23 side. The method includes the steps of: attaching a cylindrical jig 33 that can be fitted into the circumferential groove 223 to the outer periphery of the piston 3; accommodating the piston 3 with the fitted cylindrical jig 33 in the piston slide space 233 from the crankcase 23 side; fitting the cylindrical jig 33 into the circumferential groove 223 to align the central axis J2 of the piston 3 with the central axis J1 of the cylinder bore 221 within the piston slide space 233; and inserting the piston 3 into the cylinder bore 221 while sliding it inside the cylindrical jig 33 with the cylindrical jig 33 fitted into the circumferential groove 223. According to this, by fitting the tip of the cylindrical jig 33 into the circumferential groove 223 of the cylinder bore 221 within the piston slide space 233 of the crankcase 23, the central axis J2 of the piston 3 and the central axis J1 of the cylinder bore 221 can be easily aligned, and in this state, only the piston 3 can be inserted into the cylinder bore 221. This improves the ease of inserting the piston 3 into the cylinder bore 221 from the crankcase 23 side, and improves the ease of assembling the piston 3. [Explanation of symbols]

[0050] 1 engine 2,2A Engine block half 21 Cylinder head 22 Cylinder block 221 cylinder bore 223 Circumferential groove 23 Crankcase 232 Crank bearing 233 Piston slide space 3 pistons 33 Cylindrical jig J1 Cylinder bore center axis J2 Piston center axis

Claims

1. An engine comprising: a cylinder block having a cylinder bore in which a piston is housed; and a crankcase integrated with the cylinder block and having a crank bearing, the crank bearing is disposed so as to overlap an extension of the cylinder bore, and a surface of the crank bearing facing the cylinder bore at the overlapping portion has a notch portion cut out from the cylinder bore side, the crankcase has a piston slide space formed by the cutout portion in the crank bearing and capable of accommodating the piston with the central axis of the piston coinciding with the central axis of the cylinder bore.

2. 2. The engine according to claim 1, wherein the cylinder bore has a circumferential groove on a peripheral edge of an end portion thereof on the piston slide space side, into which a cylindrical jig attached to an outer periphery of the piston when the piston is inserted into the cylinder bore can be fitted.

3. 3. The engine according to claim 1, wherein the engine is a multi-cylinder opposed engine in which the crankcases of engine block halves, each of which is an integrated cylinder block and each of which is connected to each other, and the pistons are arranged opposite each other.

4. a cylinder head on the opposite side of the cylinder block from the crankcase, 3. The engine according to claim 1, wherein the cylinder head is integrated with the cylinder block.

5. An engine as described in claim 1 or 2, wherein the piston slide space is a space that can accommodate the piston so that the piston can slide along a path that intersects the central axis of the piston, so that the central axis of the piston changes from a state in which the central axis of the piston and the central axis of the cylinder bore are misaligned to a state in which the central axis of the piston coincides with the central axis of the cylinder bore.

6. a cylinder block having a cylinder bore in which a piston is housed; and a crankcase integrated with the cylinder block and having a crank bearing, the crank bearing is disposed so as to overlap an extension of the cylinder bore, and a surface of the crank bearing facing the cylinder bore at the overlapping portion has a notch portion cut out from the cylinder bore side, the crankcase has a piston slide space formed by the cutout portion of the crank bearing and capable of accommodating the piston with the central axis of the piston coinciding with the central axis of the cylinder bore; a cylinder bore having a circumferential groove at an end portion thereof on the piston slide space side, the piston being inserted into the cylinder bore of the cylinder block from the crankcase side, a step of attaching a cylindrical jig that can be fitted into the circumferential groove to an outer periphery of the piston; a step of accommodating the piston, to which the cylindrical jig is attached, into the piston slide space from the crankcase side; a step of fitting the cylindrical jig into the circumferential groove to align a central axis of the piston with a central axis of the cylinder bore within the piston slide space; and inserting the piston into the cylinder bore while sliding the piston inside the cylindrical jig with the cylindrical jig fitted into the circumferential groove.

7. A method for assembling an engine as described in Claim 6, wherein the piston slide space is a space that can accommodate the piston so that the piston can slide along a path that intersects the central axis of the piston, so that the central axis of the piston changes from a state in which the central axis of the piston and the central axis of the cylinder bore are misaligned to a state in which the central axis of the piston coincides with the central axis of the cylinder bore.

8. The crankcase has an opening on a side thereof, 8. The engine assembly method according to claim 6, further comprising the step of removing the cylindrical jig from the opening after inserting the piston into the cylinder bore.

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