MS291 cracked piston

[knoxman]

My brother's MS291 lost power and wouldn't idle; upon inspection it seems to have a cracked piece out of the bottom exhaust side of the piston.

See pics. https://photos.app.goo.gl/J3Pj9dtCPq3ExKqE6

Questions:
1. what would cause this piston failure? piston and cylinder seem smooth; crank and connecting bearing seem smooth; was this likely just random piston failure >> install a new piston kit?

2. why can't I find the broken piece? I guess it ran long enough to grind it up and blow most of it out the exhaust. nothing but a few fine metal filings in the crank case

3. where best to find a new piston to ship to Canada? only compatible one I can find is from Lil Red Barn (round intake boot)

Thanks.
Ryan

 

[singinwoodwackr]

With that much damage there will be a lot of transfer to remove before any new piston & rings go in. Straight gassed?

 

[StihlTeck]

This is what we call 360 scoring. This can only happen when there is severe lack of lubrication (straight gas, not mixed) The crank and bearings would not have enough time to heat up to cause damage before the piston/cyl loose compression, that's why they are fine. The missing chunk is a mystery though...it could be in your muffler. This needs a short block replacement. Do not just put a piston in. You'll be cracking open that engine again in no time.

 

[knoxman]

Thanks for the advice.

Tank is half full of good stihl oil mix; maybe crank seals or dull chain syndrom set it running hot/lean...

I can't find a rebuild kit for this saw, and it's not worth an OEM kit. So perhaps hone the cylinder and install new piston is the best I can do.

 

[knoxman]

I see honing is not recommended. I won't do it.
I'll cleanup the cylinder, replace crank seals and piston.

 

[singinwoodwackr]

Get a bottle of pool acid, Muriatic, and cue-tips. It takes quite some time and patience to get all the aluminum transfer. If you don't get it all the new piston and rings will be ruined upon startup.
Good luck. Only work on the transfer that is on the nicasil coated, cylinder walls. Uncoated AL will dissolve so try to keep the acid off those areas. Don't worry if you do spill...just rinse with water, dry and start over.
Use a dental pic to help locate the transfer that you don't think is there...it's there

 

[huskihl]

It might have fresh fuel in it now but it was straight gassed at one time and they kept on using it until it wore the piston thin and allowed to crack

 

[stihltech]

They ALWAYS have good fuel in them when they come to the shop after being straight gassed! I would also question air filter maintenance. Appears a bit thin. And I am sure that was clean when it came in also.

 

[knoxman]

New piston arrived and I got some pics of cleaned up jug: https://photos.app.goo.gl/J3Pj9dtCPq3ExKqE6

I've spent quite some time with the 35% pool acid (Muriatic, HCl); it turned green on cue tips; does that indicate aluminum? or carbon deposits? It seems I'm left with light scoring of cylinder but I hope it will serve as a basic, reliable homeowner saw (10hrs /year) again when I'm done.

I think I'm at the point where I'm ready to put it back together; or do I need more work on the cylinder?

For reassembly, I have motoseal for the crank gasket; new crank seals; and plan to use 10w30 as assembly oil. Anyone know torque spec for cylinder bolts?

It's my first try - any advice is appreciated. Thanks!

 

[singinwoodwackr]

New piston arrived and I got some pics of cleaned up jug: https://photos.app.goo.gl/J3Pj9dtCPq3ExKqE6

I've spent quite some time with the 35% pool acid (Muriatic, HCl); it turned green on cue tips; does that indicate aluminum? or carbon deposits? It seems I'm left with light scoring of cylinder but I hope it will serve as a basic, reliable homeowner saw (10hrs /year) again when I'm done.

I think I'm at the point where I'm ready to put it back together; or do I need more work on the cylinder?

For reassembly, I have motoseal for the crank gasket; new crank seals; and plan to use 10w30 as assembly oil. Anyone know torque spec for cylinder bolts?

It's my first try - any advice is appreciated. Thanks!

Yup, Al, and it will bubble a bit. When you stop getting that you should be good. If you leave some the new ring will catch it and you’ll get to start over. The carbon should only be in the exhaust port and combustion chamber. I use a fine wire bit on my Dremel for that. Just stay away from the wall coating. Or, let it go if the buildup isn’t bad.
after acid, flush well with water to neutralize. Then. Sand with 220 and then 320 wet/dry. Go side/side, not up/down.
last, clean well with brake cleaner to get all residues out of the transfer ports as well as the cyl walls.

pics look pretty good.

 

[huskihl]

The green on the Q-tips is the muriatic acid reacting with the chrome plating. The bubbles show the presence of aluminum. I usually sand a little and then reapply some acid. And repeat a few times

 

[singinwoodwackr]

Use mix oil rather than 30w during assembly

 

[singinwoodwackr]

The green on the Q-tips is the muriatic acid reacting with the chrome plating. The bubbles show the presence of aluminum. I usually sand a little and then reapply some acid. And repeat a few times

Green-chrome...duh, basic chemistry, good call

 

[knoxman]

Thanks for the feedback; so there wasn't much aluminum on the cylinder walls; it was coming back green from the start; very little bubbling.
but a lot of black streaks came off which I take to be carbon smears.

I found this for MS290 cylinder bolts; would these specs do me on the MS291:

"You'll need to use an inch/pound torque wrench."
"11 newton meter = 8.3 ft/lb = 97.36 in/lb or pound-force inch"

Re-torque after one heat cycle.

 

[cookies]

After rinsing the acid rub the cylinder with soapy green scotchbrite to remove glaze and then rewash with soap and a soft sponge, use 2 cycle oil on a thin paint brush as assembly lube. About 10 pounds on the cylinder to pan bolts, let it sit over night in a warm room to allow the sealant to harden up. If you use aftermarket crank seals you need to very closely inspect the seals lips for any irregularities, do not use if any are found, put a little oil on the seal lip before sliding them on. Use a oem piston if possible, its not worth risking a do over to save 25 bucks. Learn to adjust the carb so the saw runs a little rich at max rpm but cleans up and runs perfect once the chain touches wood, run 32:1 or a minimum of 40:1 mix in strato saws, they operate extra lean and need the added oil for protection, Using a combi can greatly reduces the chances of strait gassing from identical fuel cans.

 

[SteveSr]

For reassembly, I have motoseal for the crank gasket;

I wouldn't recommend Motoseal as it is not rated for gap filling. Use Dirko HT instead.

I would also check the tarnsfer cover s for any gaps/looseness as they are glued on and will come loose causing an air leak if the saw overheats. If these have come loose it is game over for the cylinder (and saw).

I would also clean the cylinder with hot soapy water to make sure that you get any left over grit out of the cylinder and transfer passages.

 

[HarleyT]

Here, if you don't already have it.

 

[knoxman]

Thanks for all the tips, tricks, and links. Got cylinder sanded, cleaned, washed, dried, sealed and assembled with mixing oil.

Pics: https://photos.app.goo.gl/J3Pj9dtCPq3ExKqE6

Vac and pressure tests are good (sealing intake/exhaust with old tractor inner-tube).

Hope to test run tomorrow and check compression after a few tanks of fuel (32:1). and check torque on cylinder bolts after one heat cycle

 

[Husky77]

Everything looks very very dry, I would say straight gassed. Just out on intrest what oil was being used prior to this?

 

[knoxman]

@Husky77: should have been stihl mixing oil; but my brother admitted he lent saw to a friend who may have straight gassed it. he is properly ashamed of himself and was warned not to lend a chainsaw ever again.

So in conclusion, I've put put about 3/4" tank of 32:1 mix through the saw for break-in. Blocked some 16" ash. I have both (limited) jets backed out as rich as they go. probably leave it that way. It runs well and still revs reasonably high, and pulls the 20" chain well, cuts efficiently.

In summary, the MS291 is a poor design: slow oiler; no oiler adjustment; limited carb adjustment; strato carb (too lean); glued on cylinder transfer port covers; elevated risks of overheating and self-destruction.

Time to trade out this saw for good oldie like an 034 or 044. Hope to sell the MS291 to someone whose sensibilities inspired its design.