Engine crankcase (small) for the "Baikal" chainsaw. This is the housing in which the crankshaft runs; it holds the bearings and oil seals and keeps the crank chamber airtight, without which a 2-stroke engine cannot create the necessary vacuum.
Replacement is needed if the housing has cracks or chips, the fastener threads are stripped, the bearing seats are worn, or air leaks in at the joint. When repairing, also check the crankshaft oil seals and replace the seals.
