JP3565298B2 - Crankcase structure of 4-cycle internal combustion engine - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a crankcase structure of a four-stroke internal combustion engine that employs a cam chain method for driving a valve and includes a belt-type continuously variable transmission.
[0002]
[Prior art]
In this type of four-stroke internal combustion engine, the crankshaft passes through the partition wall of the crankcase that separates the belt-type continuously variable transmission chamber from the cam chain chamber in which lubricating oil is used. A seal member is fitted into the through hole to prevent oil from leaking into the belt-type continuously variable transmission room.
[0003]
Japanese Patent Publication No. 5-47437 also discloses such a configuration. In this example, a large-diameter circular hole is formed in a partition wall that separates a cam chain chamber and a belt-type continuously variable transmission chamber, thereby forming a hollow disk. A lid member is fixed by screws so as to close the circular hole, and a crankshaft passes through a central through hole of the lid member.
The seal member closes a through hole slightly larger than the diameter of the crankshaft of the lid member.
[0004]
Therefore, during maintenance of the internal combustion engine, if the lid member is removed, a large opening is opened in the partition wall, and the cam chain wound around the drive sprocket can be removed from the opening.
[0005]
[Problems to be solved by the invention]
However, a lid member separate from the partition wall must be prepared in this way, and a mounting structure such as a screw for mounting the lid member must be formed, which complicates the mechanism and increases the number of parts. Was invited.
[0006]
In the case of a conventional internal combustion engine that does not use such a lid member, the only way to remove the cam chain for maintenance is to split the left and right combined crankcase into left and right, which is extremely troublesome work. I was
[0007]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to seal a gap between a cam chain chamber and a belt-type continuously variable transmission chamber in a watertight manner with a small number of parts, and to reduce a cam chain during maintenance. Another object of the present invention is to provide a crankcase structure of a four-cycle internal combustion engine that facilitates attachment and detachment.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a four-cycle internal combustion engine in which a crankshaft penetrates a partition wall of a crankcase that separates a cam chain chamber and a belt-type continuously variable transmission chamber, and a seal member closes the through hole. In the engine, an annular seal member having an outer diameter larger than the maximum outer diameter in a state where the cam chain is wound around the cam chain drive sprocket fitted on the crank shaft passes through the through hole. Between the crankshaft The closed four-stroke internal combustion engine has a crankcase structure.
[0009]
When removing the cam chain from the cam chain drive sprocket for maintenance, remove the annular seal member that has blocked the through hole in the partition wall of the crankcase and remove the cam chain from the cam chain drive sprocket. Since a through hole larger than the diameter is formed, the cam chain can be easily removed from the cam chain drive sprocket by using the through hole, and the number of parts makes it easier to attach and detach and improve maintainability. Can be.
[0010]
When the annular seal member is mounted so as to close the through hole, the annular seal member has a movable side pulley half of a belt-type continuously variable transmission having an outer peripheral edge thereof. Outer edge of Close to the cam plate sliding boss provided on the And the crankshaft side By adopting the crankcase structure of the four-stroke internal combustion engine according to the first aspect of the present invention, the layout can be maintained with high maintainability without the seal member interfering with the belt drive pulley.
[0011]
3. A four-stroke internal combustion engine having a crankcase structure according to claim 1 or 2, wherein a step is formed on an outer peripheral edge of a side surface of the annular seal member on the side of the belt-type continuously variable transmission chamber. The removed seal member can be easily removed using the step, and the maintenance work can be further facilitated.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment according to the present invention will be described with reference to FIGS.
FIG. 1 is a side view of a scooter type motorcycle 1 provided with a unit swing type internal combustion engine according to the present invention.
A vehicle body front portion 2 and a vehicle body rear portion 3 are connected via a low floor portion 4, and a vehicle body frame forming a skeleton of the vehicle body generally includes a down tube 6 and a main pipe 7.
[0013]
That is, a down tube 6 extends downward from the steering head 5 of the front portion 2 of the vehicle body, and the down tube 6 is bent horizontally at the lower end and extends rearward below the floor portion 4, and at the rear end, a pair of left and right main pipes 7 is provided. The main pipe 7 rises obliquely rearward from the connection portion, bends horizontally at a predetermined height, and extends rearward.
[0014]
A fuel tank and a storage box are supported by the main pipe 7, and a seat 8 is disposed above the fuel tank and the storage box.
On the other hand, in the front part 2 of the vehicle body, a handle 11 is provided above and supported by the steering head 5, a front fork 12 extends below and a front wheel 13 is supported at its lower end.
[0015]
A bracket 15 projects from the lower end of the rising portion of the main pipe 7, and a swing unit 17 is swingably connected to and supported by the bracket 15 via a link member 16.
[0016]
At the front of the swing unit 17, a single-cylinder four-cycle internal combustion engine 30 is mounted at a front portion thereof with the cylinder block 32 largely inclined forward to a substantially horizontal state, and corresponds to a crankcase of the unit swing case 31. An end of a hanger bracket 18 protruding forward from a lower end of the portion to be connected is connected to the link member 16 via a pivot shaft 19.
[0017]
A belt-type continuously variable transmission 35 is formed from the internal combustion engine 30 to the rear, and a rear wheel 21 is supported by a reduction mechanism 38 provided at a rear portion thereof.
A rear cushion 22 is interposed between the upper end of the speed reduction mechanism 38 and the upper bent portion of the main pipe 7.
[0018]
Above the swing unit 17, a carburetor 24 connected to an intake pipe 23 extending from an upper part of a cylinder head 33 of the internal combustion engine 30 and an air cleaner 25 connected to the carburetor 24 are arranged.
On the other hand, a main stand 26 is pivotally attached to a hanger bracket 18 protruding from a lower portion of the unit swing case 31, and a kick arm 28 is attached to a kick shaft 27 protruding from a transmission case cover 36 of a belt-type continuously variable transmission 35. And a kick pedal 29 is provided at the tip of the kick arm 28.
[0019]
The front part 2 of the vehicle body is covered from the front and rear by a front cover 9a and a rear cover 9b from the left and right sides by a front lower cover 9c, and a central part of the handle 11 is covered by a handle cover 9d.
The floor portion 4 is covered with a side cover 9e, and the rear portion 3 of the vehicle body is covered with a body cover 10a and a tail side cover 10b from left and right sides.
[0020]
FIG. 2 is a cross-sectional view of the swing unit 17 cut and developed generally along the line II-II in FIG.
The unit swing case 31 is configured by uniting a left and right split left unit case 31L and a right unit case 31R. The right unit case 31R forms a half of a crankcase portion, and the left unit case 31L includes The front and rear parts are long and include a front crankcase part 31a, a central transmission case part 31b, and a rear reduction gear case part 31c.
[0021]
The left open surface of the left unit case 31L is covered by a transmission case cover 36, in which a belt-type continuously variable transmission 35 is housed, and the right open surface of the rear reducer case portion 31c is covered by a reducer case 37. The speed reduction mechanism 38 is housed inside.
[0022]
A crankshaft 40 is rotatably supported by left and right main bearings 41, 41 in a so-called crankcase of the crankcase portion 31a and the right unit case 31R. An AC generator 60 is provided in the section, and a cam chain drive sprocket 55 and a belt drive pulley 76 of the belt type continuously variable transmission 35 are provided in the left extension.
[0023]
In the internal combustion engine 30, a connecting rod 43 connects a piston 42 that reciprocates in a cylinder liner 44 of a cylinder block 32 and a crankpin 40 a of a crankshaft 40.
The four-stroke internal combustion engine 30 employs an OHC type valve system. A valve mechanism 50 is provided in the cylinder head cover 34, and a cam chain 51 that transmits drive to the valve mechanism 50 includes a camshaft 53. A cam chain chamber 52 for this purpose is provided in communication with the crankcase portion 31a, the cylinder block 32, and the cylinder head 33.
[0024]
That is, the cam chain 51 passes through the cam chain chamber 52 between the driven sprocket 54 fitted on the left end of the cam shaft 53 oriented in the horizontal direction and the driving sprocket 55 fitted on the crankshaft 40. It has been bridged.
An ignition plug 45 is fitted into the cylinder head 33 from the opposite side (right side) of the cam chain chamber 52 toward the combustion chamber.
[0025]
As shown in FIG. 3, the valve operating mechanism 50 in the cylinder head cover 34 is tilted forward to a state where the cylinder is nearly horizontal, so that the intake valve 56 and the exhaust valve 57 are arranged above and below the camshaft 53. Rocker arms 59, 59 are pivotally attached to the upper and lower rocker shafts 58, 58, respectively, and the rocker arms 59, 59 swing by the rotation of the cam of the camshaft 53 to take in air at a predetermined timing. The opening and closing operation of the valve 56 and the exhaust valve 57 is performed.
[0026]
The AC generator 60 provided on the right side surface of the right unit case 31R has a bowl-shaped outer rotor 62 fixed to an end of the crankshaft 30 projecting from the central cylindrical portion 31d of the right unit case 31R via an ACG boss 61, A stator 64 around which a stator coil 65 is wound is fixed to the central cylindrical portion 31d inside a magnet 63 disposed on the inner peripheral surface in the circumferential direction.
[0027]
A forced air cooling fan 66 is attached to the right side surface of the outer rotor 62, and a fan cover 67 covers the side of the forced air cooling fan 66. The fan cover 67 is connected to the fan cover 67 so as to cover the periphery of the cylinder block 32 and the cylinder head 33. 68 are provided.
[0028]
On the other hand, in the crankcase portion 31a of the unit swing case 31, the cam chain chamber 52 is formed just by being separated from the crank chamber by the main bearing 41, and the left side wall of the cam chain chamber 52 further has a left belt type. A partition wall 71 for partitioning the continuously variable transmission chamber 70 and the cam chain chamber 52. The partition wall 71 is formed with a large-diameter flat cylindrical circular through hole 71a through which the crankshaft 40 penetrates. The annular seal member 72 is press-fitted into 71 a, and the crankshaft 40 penetrates a hollow portion of the annular seal member 72.
[0029]
In the annular seal member 72, the rubber member 73 connects the outer ring 73a and the inner ring 73b with the outer wall 73c to form a U-shaped cross section to form an annular space inside, and the outer ring 73a and the outer wall 73c are thin. It has a structure in which a steel plate 74a is embedded, and a pressing ring 74b is fitted around the outer periphery of the inner ring 73b.
[0030]
The drive sprocket 55 is fitted on the crankshaft 40 between the seal member 72 and the main bearing 41. When the cam chain 51 is wound around the drive sprocket 55, the drive sprocket 55 is larger than its maximum outer diameter. The outer diameter of the outer ring 73a of the annular seal member 72 and the inner diameter of the through hole 71a are large.
Therefore, if the annular seal member 72 is removed from the through hole 71a, the large-diameter through hole 71a is opened, so that the cam chain 51 can be removed from the drive sprocket 55 or wound around using the opening. .
[0031]
The belt-type continuously variable transmission chamber 70 is water-tightly separated from the cam chain chamber 52 by the seal member 72, thereby preventing oil from leaking into the belt-type continuously variable transmission chamber 70.
A belt drive pulley 76 is rotatably provided on the crankshaft 40 extending through the seal member 72.
[0032]
The belt driving pulley 76 includes a fixed pulley half 77 and a movable pulley half 78. The fixed pulley half 77 is fixed to the left end of the crankshaft 40 via a boss 79, and is movable to the right thereof. The side pulley half 78 is spline-fitted to the crankshaft 40, and the movable pulley half 78 rotates together with the crankshaft 40 and slides in the axial direction to approach / separate from the fixed pulley half 77. The V-belt 75 is sandwiched between the two pulley halves 77 and 78 and is wound.
[0033]
A cam plate 80 is provided at a fixed position close to the annular seal member 72 on the right side of the movable pulley half 78, and a slide piece 80 a provided on the outer peripheral end thereof is provided on the outer peripheral end of the movable pulley half 78. It is slidably engaged with a cam plate sliding boss 78a formed in the axial direction.
The side surface of the movable pulley half 78 on the cam plate 80 side is tapered toward the cam plate 80, and a dry weight roller 81 is accommodated between the cam plates 80 inside the tapered surface.
[0034]
Therefore, when the rotation speed of the crankshaft 40 increases, the dry weight roller 81, which rotates between the movable pulley half 78 and the cam plate 80, moves in the centrifugal direction due to centrifugal force, and the movable pulley half 78 moves The pulley is pressed by the weight roller 81 and moves leftward to approach the fixed pulley half 77, and the V-belt 75 sandwiched between the pulley half 77, 78 is moved in the centrifugal direction to increase the winding diameter. It is configured as follows.
[0035]
A fixed pulley half 87 is fitted to an inner sleeve 89 of the rear belt driven pulley 86 corresponding to the belt driving pulley 76, which is rotatably supported relative to a speed reducer input shaft 92 of the speed reduction mechanism 38. A movable pulley half 88 is fitted to an outer sleeve 90 slidably supported in the axial direction by an inner sleeve 89 on the left side of the fixed pulley half 87, and is constituted by the two pulley halves 87, 88. ing.
The V-belt 75 is held between the two pulley halves 87 and 88.
[0036]
A centrifugal clutch 91 is provided on the left side of the speed reducer input shaft 92 and the inner sleeve 89, and the power transmitted to the inner sleeve 89 via the V-belt 75 is connected to the centrifugal clutch 91 when its rotation speed increases. And transmitted to the reduction gear input shaft 92.
The speed reduction mechanism 38 transmits the power transmitted to the speed reducer input shaft 92 to the output shaft 94 via the intermediate shaft 93 at a reduced speed by meshing gears. The output shaft 94 is an axle of the rear wheel 21 and a rear wheel. 21 is rotated.
[0037]
A drum brake 95 is provided on a hub portion of the rear wheel 21, and a drum brake arm 97 to which a brake cable is connected is fitted to a drum brake shaft 96, and when the brake cable is pulled, the drum brake arm 97 is moved. The drum brake 95 is operated by swinging and rotating the drum brake shaft 96.
[0038]
The transmission case cover 36 that covers the belt-type continuously variable transmission chamber 70 from the left side covers the belt drive pulley 76 at the front and the centrifugal clutch 91 at the rear, and the kick shaft 27 is rotatable slightly forward from the center. The drive helical gear 100 is fitted to the inside end of the kick shaft 27 and is urged by a return spring 101.
[0039]
A sliding shaft 102 is supported on the front inner surface of the transmission case cover 36 coaxially with the crankshaft 40 so as to rotate and slide in the axial direction. A driven helical gear 103 is formed on the sliding shaft 102. In addition to being engaged with the drive helical gear 100, a ratchet wheel 104 is fixed to the right end, and the whole is urged leftward by a friction spring 105.
On the other hand, a boss 79 on the side of the crankshaft 40 is formed with a ratchet 79a facing the ratchet wheel 105, and both can be brought into contact with and separated from each other by sliding the sliding shaft 102.
[0040]
Accordingly, when the kick pedal 29 is depressed and the kick shaft 27 rotates via the kick arm 28 against the return spring 101, the drive helical gear 100 rotates integrally with the kick shaft 27, and the driven helical gear 103 meshing with the drive helical gear 103 rotates. While rotating integrally with the sliding shaft 102, it slides rightward against the friction spring 105, and the ratchet wheel 105 meshes with the ratchet 79 a of the boss 79 to forcibly rotate the crankshaft 40, causing the internal combustion engine 30 to rotate. Can be started.
[0041]
On the other hand, in this internal combustion engine, a starter motor 110 is disposed above the crankcase portion 31a of the left unit case 31L. As shown in FIG. 4, the starter motor 110 is surrounded by magnets 111 of the outer stator. A pinion 113 around which the coil 112 of the inner rotor is wound protrudes leftward, and a gear 113 a is formed at an end of the pinion 113.
[0042]
A dive gear mechanism 115 is provided between the starter motor 110 and the belt drive pulley 76 with the shaft directed horizontally in the left-right direction, sandwiched between the crankcase portion 31a and the transmission case cover 36. The pinion gear 113a of the starter motor 110 meshes with the input gear 116 on the right side of the gear mechanism 115, and the left-side dive gear 117 protrudes to the left. Mesh with.
[0043]
Therefore, when the starter motor 110 is driven, the rotation of the pinion 113 is transmitted to the input gear 116 of the dive gear mechanism 115, and the dive gear 117 protrudes to the left to bite into the starter ring gear 77 a of the fixed pulley half 77, and the fixed pulley The internal combustion engine 30 can be started by rotating the half body 77 together with the crankshaft 40.
When the starter motor 110 stops, the dive gear 117 is disengaged from the retraction starter gear 77a.
[0044]
The fixed pulley half 77 formed with the starter gear 77a receiving the engagement of the dive gear 117 has a rubber cooling fan 118 on the side (left side) opposite to the side holding the V-belt 75 by rubber baking. The belt-type continuously variable transmission 35 is cooled by the rotation of the cooling fan 118.
[0045]
Referring to FIGS. 1 and 2, the transmission case cover 36 has a cylindrical suction duct 36a protrudingly formed at a front portion thereof, and a cylindrical discharge duct 36b is formed obliquely along a side wall at a substantially central position in the front-rear direction. A rear end of a bellows hose 14 is connected to the suction duct 36a, and the bellows hose 14 extends forward to a discharge tube 6b formed to project from a bent portion of the down tube 6. The front end is connected.
[0046]
At the height of the down tube 6 close to the steering head 5, a suction tube 6a is formed so as to protrude, and the inside of the down tube 6 communicates the suction tube 6a and the discharge tube 6b.
[0047]
Accordingly, when the cooling fan 118 rotates by rotation of the crankshaft 40 in the belt-type continuously variable transmission chamber 70, air is sucked from the suction tube 6a of the down tube 6, passes through the down tube 6, and flows from the discharge tube 6b to the bellows hose 14b. Then, it is sucked into the belt-type continuously variable transmission chamber 70 from the suction duct 36a through the bellows hose 14, cooled down the belt-type continuously variable transmission 35, and discharged from the discharge duct 36b to the outside.
[0048]
The four-cycle internal combustion engine 30 has the above-described structure, and the belt-type continuously variable transmission chamber 70 is water-tightly partitioned by a large-diameter annular seal member 72 to prevent oil leakage. At the time of maintenance, if the seal member 72 is removed, the large-diameter through hole 71a is opened, so that the cam chain 51 can be easily removed from the drive sprocket 55, and can be wound easily. The unit swing case 31 is disassembled left and right, and no special cover member is required, the number of parts is small, the structure is simple, and the maintenance work is simplified.
[0049]
Although the annular seal member 72 has a large diameter, the annular seal member 72 has a size such that the outer peripheral edge is close to the cam plate sliding boss 78a into which the slide piece 80a on the outer periphery of the adjacent cam plate 80 is slidably fitted. In addition, it is possible to provide a layout that maintains high maintainability without the seal member 72 interfering with the belt driving pulley 76.
[0050]
Next, a modified example of the seal member will be described with reference to FIG.
The structure of the internal combustion engine other than the seal member 120 is the same as that of the above embodiment, and the same reference numerals are used.
[0051]
The main seal member 120 press-fitted into the flat cylindrical through-hole 71a of the partition wall 71 formed in the crankcase portion 31a of the left unit case 31L has an outer ring 121a and an inner ring substantially similar to the seal member 72. A steel plate 122a is embedded in an outer ring 121a and an outer wall 121c of a rubber member 121 composed of a rubber member 121b and an outer wall 121c, and a pressing ring 122b is fitted on an outer periphery of the inner ring 121b. A step 123 is formed at the bent portion of the.
[0052]
When the sealing member 120 is press-fitted into the through hole 71a, the outer wall 121c is positioned outside the step 122a so as to protrude outward from the opening of the through hole 71a, and the end face of the flat cylindrical portion of the through hole 71a and the step 123 are hollow from the left. The disc-shaped retaining plate 124 is pressed down, and the retaining plate 124 is fixed to the screw boss portion 125 of the crankcase portion 31a by a screw 126.
[0053]
The stopper plate 124 has its hollow circular opening reduced in diameter by the step 122a of the seal member 120 and fitted to the outer periphery of the projected outer wall 121c to hold the step 123 to prevent the seal member 120 from coming off.
[0054]
Therefore, if the screw 126 is unscrewed and the retaining plate 124 is removed, the outer wall 121c of the seal member 120 slightly projects from the through hole 71a and is exposed. 120 can be easily removed, and maintenance work can be further facilitated.
[0055]
【The invention's effect】
The present invention provides a state in which the cam chain is wound around the cam chain driving sprocket when removing the cam chain from the cam chain driving sprocket during maintenance by removing the annular seal member that closed the through hole of the partition wall of the crankcase. The cam chain is easily removed from the cam chain drive sprocket by using the through hole because the through hole is larger than the maximum outer diameter of the hole. Can be improved.
[0056]
When the annular seal member is mounted by closing the through hole, the outer peripheral edge of the annular seal member is close to the cam plate sliding boss provided on the movable pulley half of the belt-type continuously variable transmission. With the crankcase structure located at the position, the layout can be maintained with high maintainability without the seal member interfering with the belt driving pulley.
[0057]
By forming a step on the outer peripheral edge of the side surface of the annular seal member on the side of the belt-type continuously variable transmission chamber, the seal member press-fitted into the through-hole can be easily removed using the step and maintenance work can be further facilitated. be able to.
[Brief description of the drawings]
FIG. 1 is an overall side view of a scooter type motorcycle according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of the internal combustion engine of the scooter-type motorcycle, taken along line II-II in FIG.
FIG. 3 is a sectional view taken along line III-III in FIG. 2;
FIG. 4 is a sectional view taken along line IV-IV in FIG. 3;
FIG. 5 is a sectional view of a main part showing a crankcase structure according to another embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Scooter type motorcycle, 2 ... Body front part, 3 ... Car body rear part, 4 ... Floor part, 5 ... Steering head, 6 ... Down tube, 7 ... Main pipe, 8 ... Seat, 9a ... Front cover, 9b ... Rear cover 9c: front lower cover, 9d: handle cover, 9e: side cover, 10a: body cover, 10b: tail side cover,
11 handle, 12 front fork, 13 front wheel, 14 bellows hose, 15 bracket, 16 link member, 17 swing unit, 18 hanger bracket, 19 pivot shaft, 21 rear wheel, 22 rear Cushion, 23 ... intake pipe, 24 ... vaporizer, 25 ... air cleaner, 26 ... main stand, 27 ... kick shaft, 28 ... kick arm, 29 ... kick pedal,
Reference Signs List 30 internal combustion engine, 31 unit swing case, 32 cylinder block, 33 cylinder head cover, 34 cylinder head cover, 35 belt-type continuously variable transmission, 36 transmission cover case, 37 reduction gear case, 38 reduction mechanism,
40 ... crankshaft, 41 ... main bearing, 42 ... piston, 43 ... connecting rod, 44 ... cylinder liner, 45 ... spark plug,
Reference numeral 50: valve operating mechanism, 51: cam chain, 52: cam chain chamber, 53: cam shaft, 54: driven sprocket, 55: driving sprocket, 56: intake valve, 57: exhaust valve, 58: rocker shaft, 59: rocker arm,
60 AC generator, 61 ACG boss, 62 outer rotor, 63 magnet, 64 stator, 65 stator core, 66 fan, 67 fan cover, 68 fan shroud,
Reference numeral 70: Belt type continuously variable transmission chamber, 71: Partition wall, 72: Seal member, 73: Rubber member, 74a: Steel plate, 74b: Pressing ring, 75: V belt, 76: Belt drive pulley, 77: Fixed pulley half , 78: movable pulley half, 79: boss, 80: cam plate, 81: dry weight roller, 86: belt driven pulley, 87: fixed pulley half, 88: movable pulley half, 89: inner sleeve , 90: outer sleeve, 91: centrifugal clutch, 92: reduction gear input shaft, 93: intermediate shaft, 94: output shaft, 95: drum brake, 96: drum brake shaft, 97: drum brake arm,
100: Drive helical gear, 101: Return spring, 102: Sliding shaft, 103: Driven helical gear, 104: Ratchet wheel, 105: Friction spring,
110 ... starter motor, 111 ... magnet, 112 ... coil, 113 ... pinion, 115 ... diving gear mechanism, 116 ... input gear, 117 ... diving gear, 118 ... cooling fan,
Reference numeral 120: seal member, 121: rubber member, 122a: steel plate, 122b: holding ring, 123: step, 124: retaining plate, 125: screw boss portion, 126: screw.

Claims (3)

In a four-cycle internal combustion engine in which a crankshaft penetrates a partition wall of a crankcase that partitions a cam chain chamber and a belt-type continuously variable transmission chamber, and a seal member closes the through hole,
An annular seal member having an outer diameter larger than the maximum outer diameter when the cam chain is wound around the cam chain drive sprocket fitted on the crankshaft has closed the through hole between the camshaft and the crankshaft. A crankcase structure for a four-stroke internal combustion engine, characterized in that: When the annular seal member is mounted so as to close the through hole, the annular seal member has a cam plate sliding whose outer peripheral edge is provided at an outer peripheral end of a movable pulley half of a belt-type continuously variable transmission. 2. The crankcase structure for a four-stroke internal combustion engine according to claim 1, wherein the crankcase is located at a position close to the boss and on the crankshaft side . The crankcase structure for a four-stroke internal combustion engine according to claim 1 or 2, wherein a step is formed on an outer peripheral edge of a side surface of the annular seal member on the side of the belt-type continuously variable transmission chamber.

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