Rebuilding a Stihl 025 - advice welcome

[ericm979]

With new or overbored cylinders on two strokes I always clean up the ports. There's often sharp edges which can damage the rings as they bulge slightly into the port. With iron liners you can put a significant chamfer on the ports to ease the rings back but with nikasil I just make sure they're not sharp and maybe put a small gentle chamfer on the wider ports (i.e. exhaust). You don't want to go through the plating. Small stones are safer than a dremel.

The piston to cylinder clearance and the ring gap should be measured. I've gotten mis marked parts from major Japanese motorcycle manufacturers that would have damaged the engine had I run them. Chinese clone saw parts are probably not up to the same standards (though some of the cylinders look pretty good). Sharp edges on the piston skirt should be chamfered as well so oil goes between the cylinder and skirt rather than being scraped off. It's a good idea to measure the squish band too though with the way the 025 is constructed you can only file the piston down if it's too small or buy another piston and hope it's taller if it's too large. Around .025 to .030 is a safe target that's still effective for squish.

There's a lot of good fuel resistant case sealants. I like Yamabond #4. It even kept my vintage Triumph made during their labor troubles from leaking, and those always leak.

 

[N2DFire]

Well here's the update thus far.

Finally managed to get a day off to tear into the saw.

First of all the guy in the YouTube video I watched has obviously done this a couple of times because it took Dad & I a lot longer than a half hour. LOL

The replacement cylinder assembly was fairly decent (esp given the price) and after a little wiggling here & there we managed to get it inside the case. Compression / Vacuum test were good so on to final assembly.

Being a generic / clone piece it did not sit exactly like the older one. In hind sight I suspect the thickness of the two mating surfaces may have been thicker (taller??) that the OEM one which caused it to sit just slightly higher in the saw body.

This in turn caused the clutch bell to rub ever so slightly at the top. If we weren't already cold, tired, and frustrated at this point I probably would have taken it out and measured it against the old one and done some filing / adjusting as needed.
As it is, this saw will now probably be run less than two to three days a year at best so we loosened the mounting bolts, "adjusted" things slightly and tightened it back down. Then reset the coil gap on the flywheel side.

After everything was all reassembled we filled the oil tank and put fresh pre-mixed fuel in it and after about a dozen pulls to get things primed - she fired right up. Subsequent "warm starts" were first pull on half choke. Quick blips to full throttle felt strong but the low end idle was rough.

At that point we called it a day. Future plans are to let it run at low throttle (as close to idle as I can keep it and still run) till it gets good and warm and then take a shot at adjusting the low end a little bit more.

After that I plan to take it and the new Husky out and work them both a little to break them in and see how things go.

I really appreciate all the help and advice from everyone here. Thanks!!