This invention relates to a crankshaft assembly composed of a pair of crankshafts each having a crank arm formed with an opening for a crankpin to be force fitted and secured therein and a crank web disposed in a position diametrically opposed to that of the crank arm and an intermediate crank arm formed at opposite end portions thereof with the crankpins each force fitted and secured in one of the openings at the crank arms, and more particularly it is concerned with a crankshaft assembly of the type described which is suitable for use with a two-cylinder, two-cycle internal combustion engine wherein fuel-air mixture is supplied from a carburetor to a crankcase and pressure is applied thereto.
In a crankshaft assembly of the type described of the prior art, the assembling of the parts is performed as follows. A crankpin formed at one end of the intermediate crank arm is force fitted and secured in an opening formed in the crank arm of one crankshaft to provide a sub-assembly, and a crankpin formed at an opposite end of the intermediate crank arm of the sub-assembly is force fitted and secured in an opening formed in the crank arm of the other crankshaft, to provide a complete assembly.
In producing the crankshaft assembly of the aforesaid type, it has hitherto been usual practice to insert, when the operation of assembling the parts is performed, a plate-like jig in a clearance between the crank web of the crankshaft and the intermediate crank arm of the sub-assembly so that the jig will bear pressure which is applied to the other crankpin to force fit same in the opening formed in the crank arm of the other crankshaft, to thereby prevent excessive pressure from being applied to the crankshaft of the sub-assembly. In the internal combustion engine of the type described hereinabove, it is necessary to minimize the dead space in the crankcase to improve engine efficiency by increasing the primary compression pressure of the fuel-air mixture. To this end, the clearance between the crankshaft and the intermediate crank arm is made as small as possible. This makes it imperative to use a jig of a small thickness for insertion in the clearance of a small size. Thus, the crankshaft assembly of the type described of the prior art has had the problem that the jig itself might undergo deformation when a crankpin force fitting operation is performed, resulting in the crankshaft and intermediate crank arm being out of alignment with each other and adversely affecting the crankshaft sub-assembly.