After you have determined you need to replace your piston and cylinder (generally by doing a compression test or seeing visible scoring marks through the exhaust port of your cylinder) here are the simple steps to do the repair yourself (this is a general guide and won’t apply to all models of chainsaws).
What you will need: Allen or Torx wrenches, flat and phillips screwdrivers, basic wrenches and sockets, needle nose pliers, a bottle of carburetor cleaner, plenty of rags.
Be sure to figure out what caused your original cylinder to fail. Replacing it with a new one without fixing the problem will just cause it to fail again. The most common causes of failure are bad gas (using straight gas or gas with ethanol), air leaks (intake manifold, crankshaft seals, etc), and over-revving.
1. Remove covers and shrouds around your chainsaw cylinder. Thoroughly clean your saw. You don’t want any dirt to get inside it.
2. Remove the spark plug wire and spark plug. Also remove decompression valve if you have one.
3. Remove the intake manifold (the tube going from the carburetor to the cylinder). Remove the muffler.
4. Remove the bolts holding the cylinder head to the crankcase
5. Remove the cylinder from the saw. You will most likely see scoring and pitting on the inside of the cylinder wall and the outside of the piston and rings.
6. Remove the piston from the crankshaft (it is held onto the crankshaft by two circlips and a wrist pin that easily slides out).
7. Remove the old cylinder gasket from off the crankcase. Get this surface extremely clean.
You are now ready to check everything and get ready for reassembly.
Make sure your piston bearing and crankshaft bearings feel smooth and tight. Here are the steps for reassembly:
1. Lubrication is key to success when assembling the new cylinder and piston. It takes a while for oil to get to everything when you first start up your saw, so if you don’t lubricate properly, your saw will most likely seize up and you will ruin the new parts. Use regular 2 cycle oil for assembly lubricant.
2. Replace the cylinder gasket with a new one.
3. Install the new piston (with the arrow on top of the piston pointing to the muffler) and piston bearing. Check ring end gap by sliding ring into cylinder, set to at least .004″ gap per inch of bore. Install the rings onto the piston, making sure the grooves slide into the pins. Lube the rings and piston.
4. Lube the cylinder wall. Squeeze the piston rings together (a ring clamp set will help make this process easier) and slide the cylinder down over the piston. Bolt it down to the crankcase, using a cross pattern to be sure to get the tension equal.
At this point a crankcase vacuum/pressure test should be performed to ensure it is airtight.
5. Install a new muffler gasket and then bolt on the muffler. Install a new intake manifold gasket (if your saw uses a gasket here) and connect the manifold.
6. Install your spark plug and connect the plug wire. Install the decompression valve if your saw has one.
7. Replace the plastic covers and anything else you had to remove to get to the cylinder.
On initial startup be sure to let the saw run at low RPM’s for several minutes to ensure proper break-in. Set the top end RPM’s to OEM specs. After doing this you are ready to go! You may need to re-adjust your carburetor slightly after replacing the cylinder.
2 Cycle Carb adjustment:
- Turn both Low and High screws all the way in and then back them both out 1/2 turn.
- Your T screw adjusts the idle, but don’t turn it yet unless the saw won’t stay running.
- Start the saw, and adjust the Low screw until the saw will idle, and keep adjusting it while you pull the throttle until the saw will accelerate without bogging down. You aren’t looking to adjust idle speed yet, just that the saw will idle without dying.
- If it won’t idle you may need to turn the T (idle) screw in until it will idle.
- Now adjust your High screw which adjusts your fuel/air mixture at high speed.
- Ideally you would want a tachometer to set your top end speed to around 12,500, but if you don’t have one just make sure your saw isn’t screaming at full throttle because that will burn it up. If the saw runs too fast back the High screw out to add a little more fuel to the mixture until it starts “4-stroking” a little bit.
- After adjusting the High screw make sure the Low screw is still where you want it (sometimes you need to readjust it at this point to make the saw idle and accelerate properly)
- Now adjust your idle speed with the T screw to where it runs without turning the chain.