The crankshaft seals on a Husqvarna 51 chainsaw keep the engine sealed with the proper pressure. There are two crankshaft seals: one on the drive side with the clutch and the other on the ignition side with the flywheel. Both seals should be replaced if damaged or cracked, and every time you remove the crankshaft. This is a complex task--if done improperly it can ruin your engine; it's better left to a qualified professional if you don't have the proper tools and small engine knowledge.
Unscrew the bar tensioner screw with the screwdriver. Loosen the bar nuts with the wrench. Remove the bar cover, bar nuts, bar and chain. Set them aside in a safe place.
Unscrew the four mounting screws holding the starter cover in place over the crankshaft. Remove the spark plug with the socket wrench. Insert the piston stop tool into the empty spark plug hole. Turn the flywheel by hand until the piston comes into contact with the stop tool.
Remove the clutch cover and attaching clip from the clutch assembly. Position clutch removal tool over the clutch and unscrew the clutch's centre bolt. Most chainsaws are reverse thread; to loosen, turn the bolt clockwise.
Remove the clutch assembly from the crankshaft. Disconnect the oil pump and remove the four mounting screws. Remove the oil pump assembly from the crankshaft.
Position the flywheel removal tool over the flywheel. Loosen the centre bolt with the wrench. Put the tool down over the flywheel and twist it counterclockwise 1-1/2 turns, until the flywheel is slightly above the crankshaft.
Hold the engine up by the flywheel. Tap on the flywheel with the hammer to loosen it from the crankshaft. Pry out the seals on both sides using the screwdriver. Remove the seals from the engine.
Replace the ignition side seal. Coat a thin layer of grease to the crankshaft seal's lip. Slide the seal over the crankshaft's end. Drive the seal, with the seal installation tool, onto the crankshaft until is even with the crankcase. Use the same method to install the drive side seal.
Reassemble parts in reverse order, making sure that all parts are properly connected before operating the chainsaw.
Improper reassembly of the oil pump, clutch and flywheel can cause permanent damage to your engine.