MS290 piston replacement project

[Patrick1903]

Hi all, a friend of mine gave me a MS290 that he took to the dealer for a hard start then no run type situation. Dealer said it would cost more than a new saw to fix, so he bought a MS291. That’s how I ended up with this saw. A decent amount of water was in the old fuel that was in the saw. I tried to pressure/vac test but couldn’t figure out where leak was coming from. I think I didn’t have intake blocked properly. I have yet to master the press/vac testing, but have had varying success. Definitely an area of improvement for me.

I’ve only done two other tops, but it seems to me that this cylinder should clean up pretty well. Just transfer, no scoring. Damage was only on exhaust side. Intake side seems really good on piston and cylinder. Is this likely just a lean situation from water and running saw too hard (with lean situation from bad fuel)? I’ve seen some rust on his other saws/bars, and he doesn’t sharpen his chain (replaces with new). I fixed a broken tensioner on his 291 with the good tensioner from this 290.

So in addition to new tensioner, I’m planning on doing piston/rings, crank seals, fuel line/filter, impulse line, carb clean/kit, maybe oil line. Any other thoughts or advice? And apologies in advance, this may be slow to get done due to lots of simultaneous projects.

Here are some pics (thumbnails, click for larger).

 

[Patrick1903]

The bearings roll smoothly and quietly, visual inspection doesn't show anything unusual/broken, but they really don't spin like the bearings on @Vintage Engine Repairs video below - 3:25 minute mark. My video uploaded, then Tom's.
View attachment IMG_5957.MOV


Tom's video, 3:25 min mark.

 

[Vintage Engine Repairs]

That’s not terrible, mine has no lube. I literally cleaned them with brake cleaner and compressed air. Give that a go.

 

[a. palmer jr.]

If it was my saw I'd probably replace the top end with a 390 top end which will give it more power. A slight muffler mod and carburetor richening will help also. While you have it apart it's a good time to check the bearings and replace the rough ones.

 

[Patrick1903]

If it was my saw I'd probably replace the top end with a 390 top end which will give it more power. A slight muffler mod and carburetor richening will help also. While you have it apart it's a good time to check the bearings and replace the rough ones.

Thanks for the input @a. palmer jr. . I was under the mistaken impression that I needed a new crank, perhaps because that’s how I see the “upgrade” kits set. Do you have a preferred source/mfg for top ends? Part of my project was going to be to save some money, but spending a little more to get more power could be a good investment. (I suppose if I really wanted to save money I wouldn’t have 9 chainsaws!). I feel like salvaging the OEM cylinder is a benefit, so it is a trade off of more power with a not-as-good cylinder. Thoughts on that equation?

Here’s one I see, Hyway, for about $85.
https://www.hlsproparts.com/Stihl-039-MS390-Nikasil-cylinder-assembly-49mm-p/h30039.htm

 

[a. palmer jr.]

Thanks for the input @a. palmer jr. . I was under the mistaken impression that I needed a new crank, perhaps because that’s how I see the “upgrade” kits set. Do you have a preferred source/mfg for top ends? Part of my project was going to be to save some money, but spending a little more to get more power could be a good investment. (I suppose if I really wanted to save money I wouldn’t have 9 chainsaws!). I feel like salvaging the OEM cylinder is a benefit, so it is a trade off of more power with a not-as-good cylinder. Thoughts on that equation?

Here’s one I see, Hyway, for about $85.
https://www.hlsproparts.com/Stihl-039-MS390-Nikasil-cylinder-assembly-49mm-p/h30039.htm

A lot of the people on this site wouldn't approve of my choice of top end kits but it kind of depends on how you use the saw. I usually go on ebay and find one for about half the price of the highway kit you listed. I'm sure it's a good unit but on a homeowner saw I'm kind of a tightwad. I've never had a failure with one of their cheapo kits but I put it together carefully, use new seals and run a mixture of 40:1 or even richer, like 35:1. I usually don't run them for extended periods during break in and don't race the engine when not cutting wood.
You probably don't need a new crank unless your rod bearing is bad. You can replace the main bearings if they are rough, it's not that hard. Use Dirko or motoseal around the bottom to seal the bearing plate..