When a Stihl chainsaw develops an oil leak or begins to smoke when used, chances are the crankcase needs to be repaired. Replacement crankcases are available for sale from a variety of sources as an entire unit. However, if the problem can be resolved by simply replacing the gasket, repairing the crankcase is more economical. Once the chainsaw is broken down and all parts removed so the crankcase can be accessed, it must be removed before the crankcase can be split open to perform the repairs.
Remove the two crankcase mounting bolts using a socket wrench.
Push two Stihl crankcase dowels into the holes on the face of the ignition side of the crankcase cover. Crankcase dowels are specialised tools made by Stihl for the purpose of splitting the case.
Place one hand on the top of the chainsaw frame to hold it steady. Strike the crankcase dowels with a soft hammer, alternate the dowels being hammered, until the crankcase separates.
Pry the two halves apart using a flathead screwdriver.
Judge which is better in terms of time: repairing the crankcase or installing a replacement unit. Crankcase repairs can take quite some time, meaning the chainsaw is not available for use. The average homeowner may be able to take the time to make repairs, but arborists may not have the luxury of being down one chainsaw for a day or two.
Do not use a regular hammer to drive the crankcase dowels, this could cause damage to the crank pin and rod inside the case.