the present invention will now be described in detail with reference to the accompanying drawings , wherein the same reference numerals will be used to identify the same or similar elements throughout the several views . referring to the drawings , a preferred embodiment of the invention will be described below . first , referring to fig1 , an engine 1 to which lubrication structure according to an embodiment of the present invention is applied will be described . the engine 1 is used for a motorcycle , particularly for a motorcycle for off - road driving sports . in the following description , an arrow f shown in fig1 points to the front of the motorcycle . the engine 1 includes a cylinder head cover 2 , a cylinder head 3 , a cylinder block 4 and a crankcase 5 . a cylinder chamber 6 that extends vertically and cylindrically is formed in the cylinder block 4 . a piston 7 is arranged in the cylinder chamber 6 so that the piston can be vertically slid and is connected to a crankshaft 9 rotatably held in the crankcase 5 via a connecting rod 8 . the connecting rod 8 is connected to the crankshaft 9 by a crankpin 28 . an intake port 13 and an exhaust port 14 communicate with a combustion chamber 10 formed by the cylinder chamber 6 , the cylinder head 3 and the piston 7 via an inlet 11 and an outlet 12 respectively formed in the cylinder head 3 . the respective one ends of an intake poppet valve 15 and an exhaust poppet valve 16 are attached to respective valve stems , are supported by respective retainers , and the respective other ends are pressed in directions in which the inlet 11 and the outlet 12 are ordinarily closed by valve springs 17 , 18 supported by the cylinder head 3 . furthermore , a camshaft 19 for opening and closing the intake valve 15 and the exhaust valve 16 is rotatably supported by the cylinder head 3 and a timing chain 22 is wound on a cam driven sprocket 20 arranged on the camshaft 19 and a cam driving sprocket 21 arranged on the crankshaft 9 . therefore , when the camshaft 19 is rotated in accordance with the rotation of the crankshaft 9 and a cam 23 formed on the camshaft 19 directly pushes down the intake valve 15 or pushes down the exhaust valve 16 via a rocker arm 24 , the inlet 11 and the outlet 12 are opened or closed . a throttle valve 25 and an injector 26 are attached to the intake port 13 . the quantity of air cleaned by an air cleaner ( not shown ) that flows in is regulated by the throttle valve 25 . the air is mixed with fuel injected from the injector 26 . the air - fuel mixture is supplied to the combustion chamber 10 via the inlet 11 from the intake port 13 . after the air - fuel mixture is compressed by the piston 7 , it is ignited by an ignition plug ( not shown ) and is combusted to generate energy for rotating the crankshaft 9 via the piston 7 . afterward , the ignited air - fuel mixture is exhausted outside via the exhaust port 14 from the outlet 12 as exhaust gas . the inside of the crankcase 5 is separated into a crank chamber 51 storing the crankshaft 9 and a transmission chamber 52 storing a transmission mechanism 27 by a wall 50 . the crank chamber 51 is encircled by the front of the crankcase 5 and the wall 50 . the upside of the crank chamber 51 is cylindrically open . a lower end of the cylinder block 4 is attached to the crank chamber . the crank chamber 51 and the cylinder chamber 6 are in communication with each other . a first oil reservoir 53 is formed next to the crank chamber 51 on the downside of the rear side of the crank chamber 51 . the first oil reservoir 53 is in communication with the crank chamber 51 via an opening 54 formed between the upside of the front side and the downside of the crank chamber 51 . a second oil reservoir 56 is formed via a reed valve 55 on the downside of the rear side of the first oil reservoir 53 . furthermore , a third oil reservoir ( an oil pan ) 57 in communication with the second oil reservoir 56 is formed on the downside of the second oil reservoir 56 ( at the bottom of the crank case 5 ). the reed valve 55 diagonally extends from the downside in front to the rear upside in a space that extends vertically , divides the space into the first oil reservoir 53 and the second oil reservoir 56 , and extends from the bottom of the first oil reservoir 53 to the upside of the second oil reservoir 56 . the reed valve 55 is configured by the body 55 a of the reed valve which is flat and the substantial center of which is open . a valve element 55 b like a tongue is attached to the body 55 a and covers the opening . in this embodiment , the valve element 55 b is attached to the side of the second oil reservoir 56 , a lower end located on the front side is fixed , and the side of an upper end located on the rear side is open downward . a front side wall w 1 surrounding the crank chamber 51 of the crankcase 5 and a lower side wall w 2 surrounding the first oil reservoir 53 are continuously formed in the shape of an arc . the valve element 55 b of the reed valve 55 is arranged on an extended line of the wall w 2 extended backward . therefore , lubricating oil that lubricates the crankshaft 9 and other things accumulates in the first oil reservoir 53 via a lower part of the crank chamber 51 and the opening 54 . when pressure in the crank chamber 51 is increased by the vertical motion of the piston 7 , the valve element 55 b of the reed valve 55 is opened , the lubricating oil is pushed out into the second oil reservoir 56 , and further , flows into the third oil reservoir 57 . conversely , as the valve element 55 b of the reed valve 55 closes even if pressure in the crank chamber 51 decreases , the lubricating oil never flows into the first oil reservoir 53 from the second oil reservoir 56 . in the engine 1 , the crankshaft 9 is rotated counterclockwise in fig1 ( a crankpin 28 fastened to the crankshaft 9 moves the connecting rod 8 from the upside to the downside through the front side and turns the connecting rod to the upside through the rear side ). as shown in fig1 , the front side wall w 1 is formed when pressure in the crank chamber 51 is increased by the piston 7 , that is , along a path that the crankpin 28 is moved from the upside to the downside through the front side and further , the lower side wall w 2 is formed along an extended line of the rotational locus of the crankpin 38 ( in a direction of a tangent at a lower end of the circumferential rotational locus in a side view ). therefore , the opening 54 is formed on the extended line of the rotational locus , the lubricating oil in the crank chamber 51 flows along the wall w 2 from the wall w 1 and flows into the first oil reservoir 53 . at this time , the reed valve 55 is diagonally arranged as described above and the valve element 55 b is located on an extended line in a direction in which the lubricating oil flows from the crank chamber 51 into the first oil reservoir 53 ( on the extended line of the walls w 1 and w 2 )( that is , the opening 54 and the valve element 55 b of the reed valve 55 are arranged side by side in the direction of the tangent of the rotational locus ). therefore , the lubricating oil can also smoothly flow into the second oil reservoir 56 by pushing down the valve element 55 b of the reed valve 55 . further , as the third oil reservoir 57 is formed immediately under the second oil reservoir 56 , the lubricating oil in the second oil reservoir 56 also flows into the third oil reservoir 57 soon . as the lubricating oil hardly accumulates in space before and after the reed valve 55 ( in the first oil reservoir 53 and the second oil reservoir 56 ) when the first to third oil reservoirs 53 , 56 , 57 and the reed valve 55 are arranged as described above , the lubricating oil in the crank chamber 51 is promptly exhausted and the agitation loss of the lubricating oil by the crankshaft 9 and other things can be reduced . as the reed valve 55 is diagonally arranged , the increase of the entire height of the engine 1 is inhibited , the engine can be compacted , and the oil pan ( the third oil reservoir 57 ) can be arranged under the reed valve 55 in a state in which minimum road clearance is secured . a balance shaft 80 extended substantially in parallel with the crankshaft 9 is rotatably supported by the crankcase 5 in front of the crankshaft 9 . a balancer driven gear 82 engaged with a balancer driving gear 81 arranged on the crankshaft 9 is attached to the side of a right end of the balance shaft 80 and a balancer 83 is formed at the left end . therefore , as the balance shaft 80 is rotated via the balancer driving gear 81 and the balancer driven gear 82 when the crankshaft 9 is rotated , the balancer 83 is rotated and the vibration of the piston 7 is negated . an oil pump shaft 85 is rotatably arranged substantially in parallel with the balance shaft 80 on the side of the front end of the crankcase 5 and on the downside of the balance shaft 80 . an oil pump driven gear 86 engaged with an oil pump driving gear 84 arranged at the right end of the balance shaft 80 is arranged at the left end of the oil pump shaft 85 and an oil pump 62 is arranged at the right end of the oil pump shaft 85 . therefore , when the crankshaft 9 is rotated and the balance shaft 80 is rotated , the oil pump shaft 85 is rotated via the oil pump driving gear 84 and the oil pump driven gear 85 and the oil pump 62 is operated . the lubricating oil that accumulates in the third oil reservoir 53 is pumped up via an oil passage 61 formed in the crankcase 5 by the oil pump 62 after the lubricating oil is cleaned by a strainer 87 and is utilized for lubricating the inside of the engine 1 . as shown in fig1 , the second oil reservoir 56 and the third oil reservoir 57 are protruded on the lower side of the crankcase 5 in the side view . a drain hose 59 pierced longitudinally and connecting the outside and the third oil reservoir ( the oil pan ) 57 is formed at the lower end of a side wall 58 on the rear side forming the second oil reservoir 56 and the third oil reservoir 57 in the crankcase 5 . a drain bolt 60 is ordinarily screwed on the drain hose 59 , the drain hose is closed , and the lubricating oil accumulating in the third oil reservoir 57 can be exhausted outside by detaching the drain bolt 60 from the drain hose 59 . the drain bolt 60 is protruded in space under the transmission chamber 52 . as described above , when the drain hose 59 is formed on the side wall 58 on the rear side , a part protruded on the lower side of the engine 1 is not required to be provided , minimum road clearance can be secured , and the engine 1 can be compacted . in addition , as the space under the transmission chamber 52 can be utilized by attaching the drain bolt 60 to the rear side of the crankcase 5 ( to a face on the rear side of the side wall 58 ), work for attaching or detaching the drain bolt 60 to / from the drain hose 59 can be also facilitated . the invention being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as a departure from the spirit and scope of the invention , and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims .