These instructions were originally written by Roger Schroeder
The crankcase is detailed on the drawing accompanying this part. Study the drawing carefully before starting and make sure you understand which surfaces are round and which are flat. Note that the venturi is a separate part that fits in a cavity on the front of the crankcase. Epoxy resin glue is used to keep it in place.
This is a ball bearing crankcase. In a ball bearing case, the shaft must fit into three separate, but concentric diameters and still turn freely. These diameters are the front bearing, the seal around the shaft inlet port and the rear bearing. One ball bearing seat and the seal can be machined in a single lathe set-up. The second ball bearing seat can only be machined by removing the case from the first set-up, turning it around and positioning it in a second set-up. The centering of the second set-up must be about plus/minus 0.0 if the shaft is to turn free. The following instructions for machining the case will show you how to perform the necessary operations easily.
Find a one inch square piece of Aluminium stock if you possibly can for the crankcase. The size is perfect and the square faces are a help in the set-ups. Mark the position of the crankshaft centerline (C/L) on one end of the aluminium stock. Chuck the stock in a 4-jaw chuck with the faces of the stock parallel to the lathe bed and the marked shaft C/L at the center of the lathe spindle rotation. Face the end of the stock, then drill and ream a .250" diameter hole through the stock. This forms the seal around the shaft inlet port.
Now turn the cavity for the front ball bearing. You want a light push fit of the bearing into the case cavity. Remember, the bearing cavity is a blind hole and it will be very difficult to remove a bearing that is too tightly fitted. In the event you do get a bearing stuck in the cavity during fitting, warm the Aluminium. The Aluminium will expand faster than the steel bearing and thus free the bearing.
After turning the bearing cavity, immediately turn the front of the crankcase to the .700" diameter shown on the drawing. Do not allow the case to shift in the chuck until this diameter is completed because it must be concentric with the .250" diameter shaft bore and bearing cavity. Don't forget to turn the groove shown on the front of the .700 diameter This groove will guide the ignition timer assembly. It is not needed for the diesel version of the engine, however.
Remove the case from the lathe and set it aside. Chuck a piece of steel bar stock, approximately 1" in diameter and 1" long in the lathe. Bore this steel bar (you may wish to finish the bore by grinding) until the .700" diameter on the front of the crankcase is a press fit in the bore. Insert the case into the steel bore so that the rear of the case can be worked. Assuming that you have machined the .700 diameter on the case concentric with the .250 diameter and the bearing cavity, and assuming your lathe is in reasonable condition, you now have the two case bores chucked and centered to plus/minus 0.0" and you are ready to machine the rear bearing cavity.
Face the rear of the Aluminium crankcase stock until the case is at the 1.387" overall length shown. The length should have been marked before the case was chucked. Bore a .780" dia to a depth of .635" as shown. Carefully machine the rear bearing cavity. The instructions pertaining to the fit of the front bearing also apply to the rear bearing. Cut the threads for the rear case cover and you are finished with this set-up.
The venturi bore should be cut next. A milling attachment will be handy for holding the case and boring in the lathe. The 10° angle isn't really necessary, but it looks neat! If you do follow the drawings and use the 10° angle, the bore starts at a point .335" back from the front face of the case. Use a center drill to start the bore and then drill 3/16" dia through. Get a good start with the center drill or the 10° angle will cause the 3/16" dia drill to wander, which will be total disaster! If you have a 1/4" end mill, use it to start the inlet port. This will prevent wandering of the following drill and ream operations to form a .250" bore that is 5/32" deep.
Remove the case from the last set-up and carefully mark the position of the cylinder on the top of the case. Put the case in a 4-jaw chuck and position until the marked cylinder is concentric with the lathe spindle. The crankshaft bore must be positioned perpendicular to the lathe spindle. In other words, the bore you are about to make to hold the cylinder in position must be at exactly 90° to the shaft bore.
After you are satisfied with the set-up, drill or bore a .500" diameter hole through to the crankshaft cavity. Bore to .595" dia, 7/32" deep as shown on the drawing and then cut threads (40 threads/in.) into this cavity to match the threads on the cylinder base. Turn the top of the case to a 3/4" diameter for a distance of .130". Face the top of the case where the cylinder flange seats, check all dimensions and you are through with the crankcase interior. You should file away any burrs and/or sharp edges resulting from the breakthrough of the .500" dia into the crankcase cavity.
The exterior of the crankcase will still be a square block at this point. You may leave the crankcase as is, or proceed to mill off the square corners as shown in the drawing. If you decide to mill the corners, proceed slowly and take light cuts. It would be a shame to ruin a good crankcase at this late stage. I took the risk to get rid of a lot of un-needed weight and have a better appearing engine.
We are now over the hard parts of the Schroeder .09 and things will move faster now.