The Crankcase

Building a crankcase for the 4 stroke model engine

The crankcase is the largest and most complex part of the engine. It has the most machine work in it. Below is a picture of the crankcase with the crankshaft installed.

picture of crankcase

In the image above A points to the section of the crankcase where the valve lifters go. The thickness of the wall needs to be at least 0.250 inch thick. My casting had only about an eighth of an inch, not nearly enough. So I milled a flat spot an added some plate aluminum to increase the thickness enough for the lifters to operate.

B indicates the thrust sleeve that keeps the prop nut from pulling the crankshaft thru the ball bearings. I forgot to put it in once when I assembled the engine, and the prop would not tighten up. The crank would not rotate smoothley. It is a wonder that the bearings were not destroyed.

In the image above C points to the joint where the bearing seat is pressed into the front of the crankcase. If you can take a little extra effort and incorporate this into the crankcase casting it would be much better. I had planned on making the bearing seat into the front cover. After making the crankcase I began to make the front cover and discovered the problem of machining the clearance for the timing gear. So instead of making a new crankcase I made a seperate part to hold the crankshaft bearing and pressed it into the end of the crankcase.

If you look on the left side of the picture you can see one of the reasons why this engine is so heavy (about 5 pounds). You can see that the metal is about half an inch thick. What needs to be done to eliminate this is to design a core (in the casting stage of construction) for the inside of the crankcase. The core that I used was just straight through the middle. It had no other shape to it. The walls of the crankcase are about a quarter inch thick minimum.

crankcase and wood pattern
Here is a picture of the crankcase with the wood pattern used for making the sand mold. The ends of the crankcase have a 'core print' added onto it. This leaves an impression in the sand that will hold the solid sand core.