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084 Cylinder kit FAIL

torch

torch

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A bit of history:

Last fall, I was having idling issues with my 084 when tipped on one side. Traced it to slight leakage from a crank seal, so I replaced both. Seemed to be OK after that. A few weeks ago, I was using it to take off a 4' stump from a clump of 10 maple trunks. I have been using Stihl Moto Mix in that saw and cracked open a brand new can that morning. Saw died at idle on the same side again, wouldn't restart initially. Restarted after sitting a couple of minutes, but died when idled on it's side again. Wouldn't restart thereafter. Used MotoMix from the same can in an MS261CM and also had sudden idling issues with that saw. I've been wondering if there was a problem with the fuel.

On investigation, compression was down to 60 psi. Scoring on the cylinder walls visible through the spark plug hole and on the piston through the exhaust port. Ordered a "Cross Performance" cylinder kit, based on some good reviews here. Kit arrived yesterday.

Before dissassembly I did a vacuum test via the impulse line. Initially I had a slow bleed down, which I traced to the decompression valve via a pressure test. A light daub of transmission assembly grease (really sticky grease) on the valve seat cured that and thereafter it held 10inHg for 10 minutes, including 8 or 10 pulls of the starter cord.

This morning I installed the kit. Looks like decent stuff. Maybe a touch more casting flash on the cylinder than OEM, but certainly not like some Chinese cylinders I've seen. Advertised as Nikasil plated; very light honing marks visible. Piston looked very nice. Came with Caber rings as well as an exhaust gasket, piston pin, bearing and clips. Had to supply my own base gasket though.

I wiped the cylinder wall with 2-stroke oil before assembly. Made sure the arrow was pointing to the exhaust port. 120psi compression on assembly, before any break-in runs. Took 2 jerry cans to the gas station; put 1L in one can to flush the hose of regular and filled the second can with 10 liters of ethanol-free premium fuel after dumping in 200mL of Amsoil Saber Professional 2-stroke oil. Dumped out the remaining MotoMix and refilled with the 50:1 premium.

Saw started right up and idled perfectly, 360° of rotation. I fattened the H slightly so it was just 4-stroking at WOT. Alternately let it idle and blipped the throttle for a couple of minutes, before trying it in wood -- a 30" maple. I made 1 cut at a time, allowing it to idle for a minute or two between cuts. There was an occasional 4 stroke burble in the wood. After a total of 4 cuts, I shut it down and let it cool for a few hours.

Once cooled down, I went at it again. First cut I didn't touch the mix. Between cuts I leaned it out slightly -- 1/8 of a turn or less. Second cut there was no 4 stroking, so I left the mix alone thereafter. It still idled perfectly, 360° rotation. Third cut seemed same as the first two, but it died and would not restart instead of idling. Pulls over easily without touching the decomp. Compression down to 50 psi. This is what I found:






Any theories as to what went wrong here???
 
torch

torch

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The black lines seem to be casting flash. They look more ominous due to the shadow from the camera flash.

I just did a cursory check of the bearings. They seem completely intact and roll smoothly. However, the crank web obstructs complete inspection. I would have to split the crankcase to inspect thoroughly.
 
drf255

drf255

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I’m not sure what you mean by “no 4 stroking”. Does that mean you were 4 stroking out of the wood and it cleaned up in the cut?

The piston is classic for lean condition scoring. They typically score first on the PTO side of the exhaust.

Even if you passed vac test, your fuel system can be leaning out. Could be the tank vent or anything else that would limit fuel supply.

The cross cylinders are notorious for bad ex port chamfers. Looks like your ex roof got chipped off to me.
 
torch

torch

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I’m not sure what you mean by “no 4 stroking”. Does that mean you were 4 stroking out of the wood and it cleaned up in the cut?
Yes.

The piston is classic for lean condition scoring. They typically score first on the PTO side of the exhaust.
Even if you passed vac test, your fuel system can be leaning out. Could be the tank vent or anything else that would limit fuel supply.
Well that is certainly where the scoring occurred. Although looking at the piston top, I would think the leanest area is actually on the intake side. Looks like very little combustion on that side, anyway:



I'll check the tank vent. But the saw didn't feel or sound like it was leaning out. The plug doesn't look lean either:



The cross cylinders are notorious for bad ex port chamfers. Looks like your ex roof got chipped off to me.
Do you think that might be what caused the damage? Ring chipped of a piece of the coating?

There is a fair bit of casting flash around the ports:





At first glance, the edges seem nicely chamfered:



But I can catch a fingernail on many of the chamfered edges, including the area above the exhaust port.
 
hotshot

hotshot

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Need 325 ml in that jerry can next time, as cutting 48” stumps is harder than milling on a big Stihl saw.

Hate to see that happen with a brand new aftermarket P&C kit.

Plus one on the detailed exhaust port roof chamfering, piston ring end gap check, squish check, wrist pin & big rod end needle bearing cages & even main ball race examinations, and then P & V testing again AFTER final assembly. Suggest a deeper pull down of 14” Hg vacuum, equal to about 7 psig. Should hold there all day long.

No side exhaust port chamfering, only the roof top, but it looks more like a piece of shrapnel got loose and made a quick trip in & out of there, to make those three ding marks.
 
torch

torch

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Need 325 ml in that jerry can next time, as cutting 48” stumps is harder than milling on a big Stihl saw.
But in this case, I was just breaking in the saw by bucking some wood -- maybe 30" or so. Not really leaning on it, just letting the weight of the saw drop the bar through the log.
 
Mad Professor

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No. It is my understanding that the coated cylinders cannot be refinished.

I meant clean up the cylinder, not replating which can be done, but is $$$$$$.

Do you still have OEM cylinder? Post a picture.


I’m not sure what you mean by “no 4 stroking”. Does that mean you were 4 stroking out of the wood and it cleaned up in the cut?

The piston is classic for lean condition scoring. They typically score first on the PTO side of the exhaust.

Even if you passed vac test, your fuel system can be leaning out. Could be the tank vent or anything else that would limit fuel supply.

The cross cylinders are notorious for bad ex port chamfers. Looks like your ex roof got chipped off to me.

I've never seen a cylinder with that deep of gouge, due to catching a ring, unless the ring breaks.

But then again, I don't use Chinesium
 
Mad Professor

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Good chance that cylinder would clean up. You won't know until you try. Lots of ways to do that, but be careful, you can ruin it in the process.

For starters I'd take some #220 wet/dry paper and dish detergent work it around with my fingers. Give it rinse and see what it looks like? Any grooves or stuff you can catch a finger nail on? Post some more pictures after.

There are many ways to get aluminum transfer off. But don't harm the nikasil coating.

If it cleans up get an OEM piston or Meteor piston.

It's better than chinesium.

P.S. you still need to see what made the divot in the cylinder, for sure, before going on.

Original piston is scored on same area as the Chi-Com one. Same issue? Resolve that too
 
blsnelling

blsnelling

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Check your tank vent. If there's a grub screw in the hose, GET RID OF IT and install the little orange "umbrella" in the tank.

I scored a brand new OEM piston in a clean OEM cylinder. It leaned out in the cut because the grub screw caused a vacuum in the tank and leaned out the saw in a longer cut.
 
pioneerguy600

pioneerguy600

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I have found junk in the upper transfers and intake that can and did come loose on AM cylinders like those little ball like casting or plating flash deposits. I always grind all ports clean and add a touch of bevel to the exhaust port roof, sometimes they need some arch added to prevent ring snag. That piston looks like lean seizure though and the gouging was secondary damage, already a few good suggestions why the lean condition is happening but tuning rich and more oil would be what I run in my big work saws.
 
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