For the type of single cylinder internal combustion engine described above and which for instance is used for chain saws, bush cutters, trimmers and so on it is previously known to manufacture the crankcase halves each at a time and then to join the two halves to one another before the crankcase is machined to achieve a plane surface i. e. a crankcase plane on which the cylinder is intended to be placed. However, this method has some disadvantages since it is cumbersome because you are forced to assemble and dismantle the two halves (the lastmentioned in order to clean the halves from chips and dirt collected during the machining operation). You are also forced during the continued machining operation to keep the crankcase halves in order such that they can later be combined for assembly.

In order to simplify the process today instead a method is used where the two crankcase halves are machined separately in order to form the surfaces in the parting plane between the two halves as well as the surfaces forming the crankcase plane. The parting plane is placed such that it is positioned perpendicular to the crankshaft whereas the cylinder foot plane is arranged parallel to the crankshaft. By using fine tolerances and close fitting clearances the different crankcase halves are directly exchangeable to one another which means that one crankcase half can be combined with any of the other crankcase halves in order to achieve a sufficient good fit between the cylinder foot plane and the crankcase plane.

It has however tured out that even if the last mentioned method most frequently is sufficient it happens that the perpendicularity to the crankshaft will not become sufficiently good to build in the crank bearings. Further the parts of the surfaces for each crankcase half forming the crankcase plane could be inclined outwardly which means problems with sucked in cylinder foot gaskets, built in tensions in the crankcase and the cylinder which might give endurance cracks and that the perpendicularity of the cylinder does not become sufficiently good which means that the crank bearings become badly mounted resulting in a crankshaft bearing breakdown.

The purpose of this invention is to achieve an arrangement where the cylinder foot plane always will rest with its reference points at the edges and hence eliminate the drawbacks that have been mentioned above. This is achieved by means of a device having the characteristics mentioned in the claims.

An embodiment of the inventionwill now be described with reference to the accompanying drawing on which Fig. 1 shows a vertical section through a part of an internal combustion engine where the invention is intended to be used whereas Fig. 2 is a schematic, exaggerated drawing of the localisation of the surfaces being involved.

As appears from the figure the engine comprises two separately machined crankcase halves 10 and 11 that are intended to cooperate at a parting plane 12. The parting plane extends perpendicular to a crankshaft 13 supported by the crankcase halves.

The crankshaft is supported by bearings 14 in the crancase halves. The crankshaft in a conventional manner supports a piston rod 15 that is supported excentrically. The crankcase halves together constitute a crankcase plane 16 that is parallel with the axis direction of the crankshaft and that is formed by a flat minor surface 17,18 on each of the crankcase half. These flat surfaces are shaped such that they from a tolerance point of view are somewhat inclined (angle a) with respect to said axis line and with respect to one another such that a slight V-shaped profile is created having its lowest point positioned at the parting plane. The inclination os is such that the plane of each surface from a tolerance point of view is between 0 and 0,2 mm, preferably 0,05-0,1 mm, inclined from an absolutely plane position P.

On the crankcase plane 16 a cylinder 19 is intended to be placed the cylinder being provided with a cylinder foot plane 20 that is machined such that is completely flat. When the cylinder 19 is placed on the V-shaped crankcase plane diametrically opposite parts of the cylinder foot plane will first be brought to abut the crankcase plane after which the cylinder can be fixed in such a manner that the disadvantages mentioned in the introductory part are eliminated.

It is of course possible within the scope of the invention to achieve the same result by instead forming the cylinder foot plane such that it becomes slightly V-shaped which however from a machining point of view is more complicated.

It should be mentioned that the invention is not limited to the type of separately machined crankcases described in connection with the figure shown in Fig. 1. Thus, the

same type of arrangement can be used on moulded crankcases in plastic (with or without details moulded into the construction) or in metal. With a moulded crankcase plane and with the tool parting against this plane there also is the advantage that the crankcase plane can be angled or formed inwardly in all directions with respect to the crankshaft such that the outer edges of the crankcase plane are the highest which means that the periphery of the cylinder foot plane will always rest on the crankcase plane.