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Stihl MS280 Chainsaw Piston Replacement

Tools Needed:
* 8mm socket
* 3" extension (I used 1/4" tools during the repair)
* 1/4" ratchet
* T27 torx tool
* gasket scraper
* o-ring pick(used to remove and install piston wrist pin circlip)
* Stihl Dirko sealant
* flashlight
* cylinder hone and hone oil
* drill (for cylinder hone)
* small impact driver(really speeds up disassembly and reassembly)
* torque wrench(inch-pounds)

Recently I came across a MS280 chainsaw with a 20” bar that wouldn't start even after confirming spark from the ignition coil, swapping in a new spark plug, and checking for good fuel I pulled the muffler and carburetor off and was able to see the piston and cylinder walls and found scoring on the exhaust side on the cylinder wall. I got the chainsaw for free so I figured with it being a $500 chainsaw when it was new then it was worth fixing. The MS280 was rated as a ‘medium use’ chainsaw but included features that the professional chainsaws had such as the heavy-duty anti-vibration system & a heavy-duty air filter for longer cleaning intervals. This model of chainsaw is Stihl’s first chainsaw to have a control system that electronically optimizes the fuel mixture throughout the RPM range to ensure top engine performance; this also helps if cutting in different altitudes.

With the amount of visible scoring seen through the exhaust port after the muffler was removed it was evident that the previous owner had more than likely ran straight gas through the machine.

I began the tear down by first removing the chain side cover followed by the bar and chain to allow maneuvering the chainsaw easier. Then off came the top engine cover, rear air cleaner cover, air filter and intake, carburetor, recoil assembly, the handle and lastly the four screws that hold the cylinder to the bottom pan. On this model of chainsaw the bottom pan of the engine is made in the engine housing and isn't available separately.

Once the cylinder screws were removed the cylinder comes loose and slides up off of the piston. Be careful not to damage the rubber boot(right side on engine) by using a screwdriver or other sharp tool to slide it through the intake housing. I was able to just use my fingers to slide it through.

Upon inspection the cylinder wasn't terrible and could be reused after some muriatic acid is used to remove metal transfer left by the piston. Muriatic acid can bought from Lowe’s and will remove the metal transfer left from the piston due to the cylinder walls being plated with Nikasil whereas the piston is a softer metal. In the pictures the little small white dots are from the cloth I used to wipe the 2-stroke mixture from the cylinder walls.

The piston will have to be replaced, as there isn’t any way it could be reused.

I ordered an aftermarket piston made by VEC after reading some reviews and being somewhat limited to options; either Stihl, VEC, or a Golf brand(?). I ordered a 46mm cylinder hone and hone oil online as well.

*Muriatic acid is dangerous, use in a well-vented area and wear rubber gloves!*
I used a Q-tip to apply the muriatic acid to the cylinder walls and waited for the bubbling to stop. There was one spot that was pitted on the exhaust side of the cylinder wall that isn’t large enough to cause a problem with the new piston but I still made sure the muriatic acid didn't make contact with it since the hole was in the Nikasil and would be bad if it made its way to the softer metal behind the Nikasil. After the muriatic acid was done I applied the hone oil to the cylinder and the cylinder hone and proceeded to hone the cylinder. I double-checked for any rough/bad spots and everything appeared to be okay.

I then cleaned the Dirko sealant off of the bottom of the cylinder and swapped the old piston from the connecting rod in favor of the new piston making sure the arrow on top of the piston faces the muffler. Then I applied a bead of new Dirko sealant on the bottom of the cylinder and a coat of engine oil to the piston and cylinder walls then carefully lowered the cylinder onto the piston making sure the piston rings didn't move from where they have to be during assembly. There are marks where the piston rings have to be on the piston so it would be hard to mess that up. The bottom of the cylinder is tapered to compress the piston rings so a piston ring compressor tool isn't necessary. Sorry I didn’t have any pictures of the cylinder reassembly but with the Dirko sealant on the bottom of the cylinder and lining the piston up so the piston rings were in the correct spot so they’d be compressed correctly I didn’t have any free hands available.

Next the four cylinder screws were torqued to specification after a Google search to find out how Newton meters(Stihl only lists their torque specification in Newton meters) converted to inch-pounds. The top engine cover, handle, carburetor and intake, and muffler were all reinstalled. While doing the reassembly I went ahead and replaced the impulse hose and fuel line due to the ethanol in fuel these days causing so many issues on small engines, after the rebuild the chainsaw will be fed a healthy diet of non-ethanol 89 octane mixed with Stihl's Ultra synthetic 2-stroke mix. The old fuel line was hard rubber from age and ethanol whereas the new fuel line was a softer rubber(a Stihl brand fuel line was installed). A few pulls of the pull rope with the spark plug out of the cylinder to help lubricate the piston rings against the cylinder walls and to make sure everything was moving smoothly and then I reinstalled the spark plug.

I then set the engine to the choke position to start the chainsaw and quickly flipped it off the choke position to the run position to let the engine idle for a minute to check for any possible issues. I then used a tachometer to check the low and high idle and adjusted as needed so the RPMs were within Stihl's specification.

Overall this repair could be completed in a few hours for the average wrench-turner and isn’t too terribly hard to do with some basic tools. The other tools I ordered specifically for this project was the 46mm cylinder hone & the honing oil but I can use both tools in future projects. Disassembly and reassembly was quick but you want to spend the most time using the muriatic acid & cylinder hone to make sure there isn’t any scoring left on the cylinder walls or it won’t take long for the engine to have problems again.

View attachment Stihl MS280 Chainsaw Piston Replacement.pdf
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