In the last installment, we conducted a leak check on a chainsaw that had damage to the top end of the engine (piston, rings, and cylinder). Before replacing those components, a leak-down pressure test gave us an important clue as to why this chainsaw failed. Leaks from both the crankshaft seals and the base gasket allowed additional air to enter the saw, causing it to run too lean (too high of an air-to-fuel ratio) and hot, ultimately damaging the top end.
Replacing the crankshaft seals can be tedious but isn’t difficult. Professional repair shops will have specialized tools for pulling and replacing the seals, but with a couple of tricks, you can do this repair work at home with tools you already may have.
Removing the old crankshaft seals:
On the left side of the chainsaw, you will need to remove the recoil and flywheel. Use an inexpensive flywheel puller (not a screwdriver, which could damage the flywheel or crankshaft) to pry off the flywheel. With the flywheel removed, you’ll have access to the crankshaft seal, which is rubber with a metal lip around the outside. On the right side of the saw, you’ll need to remove the clutch and oil pump (on most models). To pull the old seal, use a small pick with a slight bend at the point. Carefully slide the pick down the crankshaft and rotate the tip upward to grab the edge of the seal. Apply pressure until the seal pops out. (Image 1) While pulling the seal, it is imperative that you pull on the metal lip of the seal and don’t score the side of the crankcase. In my experience, using the curved pick or a seal-pulling tool works well. Flat screwdrivers tend to slip and risk damaging the crankcase.
Installing the new crankshaft seals:
The new seal must be driven in squarely and seated at the proper depth to create an airtight seal. A socket that is slightly smaller than the outside diameter of the seal is ideal as a seal driver. (Image 2) Using a mallet, lightly tap the socket to drive in the seal. A light coat of oil on the outside rim of the seal will help it seat. Be sure that the seal is driven downward evenly. A seal that is not driven in squarely will leak.
Installing the piston, rings, and cylinder head:
Replacing the piston rings begins by working the ring around the new piston and into the bottom piston groove. Be careful not to overstretch the rings, as they easily can snap. The ring gap should align with a circular divot on the piston, indicating proper positioning. Once the bottom ring is installed, repeat for the top ring. Remove the clips holding the wrist pin and replace the old piston with the newly assembled piston and rings.
Using a plastic ring compressor, compress the rings so that the cylinder can slide over the piston. (Image 3) Be sure to have your new cylinder base gasket in place before doing this. Slide out the ring compressor and bolt down the cylinder head, torquing to the manufacturer’s spec. You can now repeat the leak test to confirm your saw is holding pressure. If it continues to hold steady at 7psi, you’re ready to gas up your saw and get out to the woodpile!