the present invention will now be described by way of example with reference to the accompanying drawings . in the embodiment shown in fig1 an oil seal in accordance with the present invention is applied to a crankcase precompression type two - cycle internal combustion engine . a crankcase 1 of the internal combustion engine includes an upper portion 2 and a lower portion 3 which are divided by a horizontal surface including a center axis of a crankshaft 6 . the upper and lower portions are coupled to each other by means of bolts and the like ( not shown ) into a one - piece unit . ball bearings 4 and 5 are arranged on opposite sides of the crankcase 1 . the crankshaft 6 is rotatably supported through the ball bearings 4 and 5 by the crankcase 1 . crank webs 7 and 8 are fixed to the crankshaft 6 within the crankcase 1 . the crank webs 7 and 8 are coupled to a piston 11 through a crank pin 9 and a connecting rod 10 . the piston 11 is reciprocatingly disposed within a cylinder 12 . the piston 11 is reciprocated by a combustion pressure of mixture introduced into the cylinder timely , thus rotating the crankshaft 6 . a cooling fan 13 is mounted on one end portion of the crankshaft 6 to serve to apply cooling air to the internal combustion engine . at the same time , the internal combustion engine is coupled to a recoil starter 14 through the cooling fan 13 so that it may be manually started . on the other end of the crankshaft 6 , there are provided an oil pump 15 and a clutch 16 so that the saw chain ( not shown ) may be drivingly rotated and the lubricant may be supplied to the saw chain . the mixture to be introduced into the cylinder 12 is subjected to a primary compression within the crankcase 1 . therefore , the interior of the crankcase 1 is kept at a high pressure . in order to prevent the fluid kept at the high pressure from leaking from the opposite end portions of the crankcase 1 , oil seals 17 and 18 in accordance with the present invention are arranged adjacent to the axially outward faces of the ball bearings 4 and 5 , respectively . the arranging directions of the oil seals 17 and 18 are opposite to each other but the structures and the mounting methods of the oil seals are the same . therefore , the right side oil seal 17 as viewed in fig1 will now be explained . as shown in fig1 in more detail , the oil seal 17 includes an annular support member 19 made of rigid plate - like material such as metal , and an annular sealing member 20 made of suitable sealing material such as rubber and adhered to the support member 19 by , for example , baking . the annular support member 19 is composed of a planar portion 22 extending radially outwardly , a sleeve portion 23 extending axially from the radially inward edge of the planar portion 22 , and an inner edge portion 24 projecting radially inwardly from an end of the sleeve portion 23 . the annular support member 19 is made into a one - piece having a cross - section of a generally l - shape . the annular sealing member 20 is arranged on and along an outer surface of the planar portion 22 of the annular support member 19 on the sleeve portion 23 side and an outer circumferential surface of the sleeve portion 23 . an outside diameter of the radially outward edge 25 of the sealing member 20 is slightly smaller than that of the radially outward edge 26 of the planar portion 22 . an annular elastic sealing member 21 made of elastic material such as rubber is disposed in a space defined by the sleeve portion 23 of the annular support member 19 and is fixed at one end 27 to the inwardly projecting end portion 24 of the support member 19 . the sealing member 21 is provided on its inner circumferential surface integrally with an annular sealing edge 28 and an annular dust - proof lip portion 29 to be brought into sliding contact with the outer circumferential surface of the crankshaft 6 . also , a spring ring 30 is provided on the outer circumferential surface of the sealing member 21 . the sealing edge 28 is brought into contact with the outer circumferential surface of the crankshaft 6 under a suitable pressure to mainly serve to prevent the pressure fluid ( mixture ) within the crankcase 1 from leaking through the ball bearing 4 to the outside . also , the dust - proof lip portion 29 is brought into contact with the outer circumferential surface of the crankshaft 6 to mainly serve to prevent dust from entering from the outside into the crankcase 1 . fig4 is an exploded view of the oil seal according to the present invention . the figure shows the planar portion 22 of the annular support member 19 engaged within the groove 34 in the circumferential surface 33 of the lower portion 3 of the crankcase 1 . in assembling , as shown in fig1 the oil seal 17 is held without any displacement in the axial or radial direction , by the engagement of the radially outwardly extending edge 26 of the planar portion 22 of the annular support member 19 with a circumferential groove 32 formed in an end inner circumferential surface 31 of the upper portion 2 of the crankcase 1 and with a circumferential groove 34 formed in an end inner circumferential surface 33 of the lower portion 3 of the crankcase 1 . the radially outermost edge 25 of the annular sealing member 20 is brought into intimate contact with the end inner circumferential surface 31 and 33 of the crankcase 1 , thus interrupting the flow of fluid that would otherwise leak through the oil seal and the inner circumferential surfaces of the crankcase 1 . the planar portion 22 of the support member 19 is brought into direct contact with an outer end face of the ball bearing 4 at the opposite surface to the sealing member 20 , thus holding the ball bearing 4 without any axial displacement . thus , the oil seal 17 also serves as a thrust bearing plate for the ball bearing 4 . also , it is apparent that instead of the ball bearings 4 and 5 , any other type of bearings may be used . an oil seal in accordance with another embodiment of the invention will now be described with reference to fig3 . in this embodiment , the oil seal includes an annular rigid support member 35 made of an annular plate extending radially , and an annular elastic sealing member 36 made of elastic material such as rubber . the sealing member 36 is fixed , by baking , to one side surface of the support member 35 along an annular portion 37 extending radially . the sealing member 36 is provided integrally with a sleeve portion 38 extending axially from a radially inward end of the annular portion 37 and further with an inside portion 39 extending from one end of the sleeve portion 38 and bent in the axial direction inside the sleeve portion 38 . the inside portion 39 is provided with a sealing edge 40 and a dust - proof lip portion 41 that are to be brought into contact with the outer circumferential surface of the crankshaft 6 . the inside portion 30 is urged radially inwardly under pressure by a spring ring 42 mounted on an outer circumferential surface of the inside portion 39 . the annular portion 37 of the sealing member 36 is to be brought into intimate contact with inner circumferential surfaces 31 and 33 of the end portion of the crankcase 1 along its outer circumferential edge 43 , to seal the contact portion therebetween . an outer circumferential edge of the support member 35 which is to be engaged with the circumferential grooves 32 and 34 formed in the inner circumferential surfaces of the end portion of the crankcase 1 is provided with a ring 44 made of the same material as that of the sealing member 36 . the ring 44 serves to damp the engagement shock with the circumferential grooves 32 and 34 and to further enhance the sealing effect between the grooves 32 and 34 and the support member 35 . the annular portion 37 of the sealing member 36 and the ring 44 are connected to each other through a connecting portion 46 extending within a recess 45 formed in the support member 35 . with such a structure , the sealing member 36 as well as the ring 44 may be formed by a single molding die as a whole .