Internal combustion engine having crankcase with stress-relieving wall structure

An internal combustion engine, comprising:

• a crankcase partitioned into an upper section and a lower section;
• a crankshaft rotatably supported between the upper and lower sections of the crankcase;
• and
• a fastening bolt that fastens the upper and lower sections together, wherein
• the crankcase has a bolt hole formed therein in an area of the crankcase proximate the crankshaft, into which the fastening bolt is inserted and tightened,
• the upper section has an opening formed therein at an upper end of the bolt hole, the upper end of the bolt hole communicates with the opening, and
• when the fastening bolt is fastened, an end of the fastening bolt protrudes into the opening;
• wherein the opening is formed in an arbitrary anomalous shape, a V-shaped rib is formed along an edge of the opening, and the upper end of the bolt hole is disposed in the center of the V-shaped rib.

An internal combustion engine, comprising:

• a crankcase partitioned into an upper section and a lower section;
• a crankshaft rotatably supported between the upper and lower sections of the crankcase;
• and
• fastening bolts that fasten the upper and lower sections together, wherein
• the crankcase has a first bolt hole formed in the crankcase between the crankshaft and a front side of the crankcase, into which a first fastening bolt is inserted and tightened,
• the crankcase has a second bolt hole formed in the crankcase between the crankshaft and a rear side of the crankcase, into which a second fastening bolt is inserted and tightened,
• the upper section has a first opening formed therein at an upper end of the first bolt hole, and the upper end of the first bolt hole communicates with the first opening,
• the upper section has a second opening formed therein at an upper end of the second bolt hole, and the upper end of the second bolt hole communicates with the second opening, and
• when the first and second fastening bolts are fastened in their respective bolt holes, an end of the fastening bolts protrude into the respective opening;
• and
• wherein the first opening extends laterally and outwardly on sides of the first bolt hole and expands towards an upper face of the upper section.

An internal combustion engine, comprising:

• a crankcase partitioned into an upper section and a lower section;
• said crankcase having a bolt hole formed therein;
• a fastening bolt disposed in said bolt hole;
• said fastening bolt fastening the upper and lower sections of the crankcase together;
• a protrusion formed in the upper section;
• said protrusion having a portion of the bolt hole formed therein;
• and
• said upper section of the crankcase having an anomalous shaped opening formed therein;
• wherein
• said opening communicates with an upper end of the bolt hole via said portion of the bolt hole formed in the protrusion.