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Stihl 029/MS290 to Stihl 039/MS390 Project

Tools Needed:
* 8mm socket (muffler removal)
* 3" extension
* 1/4" ratchet
* 3/8" ratchet (used to remove clutch)
* 19mm deep socket (clutch removal)
* T27 torx tool
* piston stop 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 (speeds up disassembly and reassembly)
* torque wrench (inch-pounds)

I recently won auctions on two different Stihl 029 chainsaws off of eBay that were both missing some parts but when put together could make a running chainsaw. The Stihl 029 chainsaws were the beginnings of the Stihl 'Farm Boss' MS290, Stihl's best selling chainsaw ever. My goal on this project is to replace the piston and cylinder with an aftermarket one that fits a Stihl 039/MS390 and have it run a 25" bar/chain which is what a MS390 is advertised to do. Everything on the 029/MS290 and the 039/MS390 were the same except the different size piston and cylinders on each chainsaw so basically I'm taking a 46mm piston and cylinder off the chainsaw and replacing it with a 49mm one.

So the Stihl 029 chainsaws arrive in the mail a few days later so I took some pictures of them as I received them. Dang previous owner shipped one of them in a Husqvarna chainsaw box and it had with about a half tank of old fuel in it and nearly a full tank of what appeared to be engine oil in the bar oil tank as it wasn't nearly as tacky as the Tractor Supply or Stihl bar oil I use, I promptly emptied both the fuel and bar oil from the machine that was going to be the chainsaw used for the project. The recoil is missing and I removed the chain side cover and I stuffed a shop towel in the exhaust port as the muffler is missing as well then proceeded to spray the saw down with Simple Green engine degreaser and let it sit for 5-10 minutes before hosing it off then using an air compressor to dry them.

The other Stihl 029 chainsaw I got off of eBay isn't in as good of shape as the one I'm using in the project but the second one has a muffler and recoil so it'll be used for parts for the project. Both chainsaws are missing the air filter cover so that'll have to be ordered. I replaced the recoil rope and pull handle as it was broke and the pull handle was missing from the parts machine. The project chainsaw was advertised as having a scored piston/cylinder but checking the piston from the exhaust side it doesn't seem like the piston was too bad but it's still going to get the replacement 039/MS390 piston and cylinder regardless but I'll hang onto the old cylinder as it could be cleaned up with some muriatic acid and honed to be reused. I hooked up a spark tester to check for fire and the ignition coil tested to be okay. The fuel line and the impulse hose will be getting replaced before the saw is ever ran as they're usually one of the first things to go bad and cause issues due to the ethanol in the gas these days.

I removed the carburetor and disassembled it for cleaning. After running the carburetor through an Ultra Sonic cleaner I reassembled the carburetor and put it in a zip-lock bag for the time being until it is ready to go back onto the chainsaw.

I removed the top handle and engine cover and carefully pushed the rubber intake boot through the engine housing towards the engine. The clutch drum was missing from both chainsaws so it'll have to be ordered & replaced along with the drive sprocket that was also missing from both machines. I removed the clutch assembly(reverse thread!) from the chain side and the flywheel(standard thread!) from the recoil side of the machine. Four torx screws that hold the engine to the engine housing were removed and the engine was removed from the housing.

Both of the crankshaft oil seals were leaking so they'll be replaced. A list of needed parts was compiled and ordered from the local Stihl dealer. Once the parts were received from the Stihl dealer the cylinder was split from the bottom pan exposing the crankshaft, connecting rod, and piston from the cylinder. The circlips were removed for the old piston and the wrist pin was slid out.

The new larger 49mm piston was installed on the connecting rod. The old crankshaft oil seals were removed and new ones were installed at this time as well while the crankshaft and piston was still out of the engine. The old Dirko sealant was scraped off of the bottom pan and the cylinder and new Dirko sealant was put on. I honed the cylinder even though it was new just to be on the safe side. I lightly coated the piston, crankshaft, and cylinder walls with engine oil and the crankshaft was placed back onto the bottom pan and the cylinder slid down over the piston and seated onto the oil pan.

The engine was carefully placed back into the engine housing so that the cylinder didn't separate from the bottom pan and loctite was applied to the threads of the four torx bolts then they were reinstalled from the bottom of the engine housing and torqued to specification.

I proceeded to reinstall the clutch assembly with new clutch drum and chain drive sprocket. The top engine housing was reinstalled along with the top handle. The flywheel followed by the recoil assembly was put on and I spun the engine over several times to make sure everything inside the engine was okay and spun over freely. I guided the intake boot back through it's opening in the engine, installed the impulse house, new fuel line, and the carburetor. The new air filter and spark plug were next to be installed and finally the air filter cover. Finally the new 25" Stihl ES-Rollomatic bar was installed and then the Stihl aggressive chain. The bar oil tank was filled and the gas tank was filled with non-ethanol 89 octane mixed with Stihl Ultra synthetic 2-stroke mix. A few pulls of the recoil with the spark plug out of the chainsaw to again make sure everything was okay with the engine then the spark plug was reinstalled. Within a few pulls of the rope while the carburetor was in the choke position the engine started and I quickly turned the choke off and in the run position. I checked the RPMs with a tachometer and made the necessary adjustments to the carburetor to get the RPMs within specifications. Below are pics of the newly finished saw with a 25" bar/chain & my MS280 with 20" bar/chain.

Stihl 029/MS290 to Stihl 039/MS390 Project.pdf
View attachment 433362


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