MS200T AM Piston-Cylinder Clearance too tight?

[Axotopia]

I am trying to diagnose why the intake boot on my 90% Farmertec MS200T melted. My Initial though was that the AM boot is not to spec, but now I am wondering if the tight fit between the piston and cylinder is causing excessive heat.

As I was disassembling the cylinder to delete the base gasket to move squish from .052" to .033", I took measurements of the P&C (attached image below). I am wondering if the clearance is too tight on the Farmertec P&C.

The saw had ran 20 odd tanks of 32:1-40:1 with Stihl Ultra and various combinations of gas, but mostly 93 octane with ethanol. 7 tanks were on the bench primarily because of trying to solve the AM carburetor misbehavior.

Can someone tell me if the dimensions of P&C is typical? Thanks.

---------------------------------------------------------
Cylinder 39.99 mm
Piston 39.75mm above top ring, 39.88 below bottom ring, 39.77mm @ skirt


Intake boot melted

 

[alexcagle]

I'd suspect a bad intake boot rubber compound, or more likely a muffler leak that is pulsing a jet of hot exhaust gases in the boot's direction.
The clearances are somewhat typical, and the squish is only an issue when you get below .020".
Is the piston a used unit? I say that, because it looks like it. If it is, then I would consider that your carb issue may be masked by excessive skirt wear.
Does it spray fuel out of the carburetor during acceleration with the filter off?

 

[alexcagle]

Also remember that the fuel air mixture cools the intake side as it's being drawn in, so that points more to muffler heat.

 

[cuinrearview]

That "rubber" just looks cheesy. I wonder, would an OEM boot solve all of your problems? Because I don't think your tolerances caused that.

 

[alexcagle]

LOL!

 

[Giles]

I am trying to diagnose why the intake boot on my 90% Farmertec MS200T melted. My Initial though was that the AM boot is not to spec, but now I am wondering if the tight fit between the piston and cylinder is causing excessive heat.

As I was disassembling the cylinder to delete the base gasket to move squish from .052" to .033", I took measurements of the P&C (attached image below). I am wondering if the clearance is too tight on the Farmertec P&C.

The saw had ran 20 odd tanks of 32:1-40:1 with Stihl Ultra and various combinations of gas, but mostly 93 octane with ethanol. 7 tanks were on the bench primarily because of trying to solve the AM carburetor misbehavior.

Can someone tell me if the dimensions of P&C is typical? Thanks.

---------------------------------------------------------
Cylinder 39.99 mm
Piston 39.75mm above top ring, 39.88 below bottom ring, 39.77mm @ skirt
View attachment 676795

Intake boot melted
View attachment 676793

I build a lot of 200t and I have seen melted intake boots in about three saws.
I use some aftermarket parts but no longer use the aftermarket intake boot, for this reason.
Actually seems to be more plastic then rubber.
I only use genuine OEM-- Meteor--or Caber on internal parts.
From my experience, intake boot is the only aftermarket part I have had to fail, and you definitely don't need this!

 

[Axotopia]

I'd suspect a bad intake boot rubber compound, or more likely a muffler leak that is pulsing a jet of hot exhaust gases in the boot's direction.
The clearances are somewhat typical, and the squish is only an issue when you get below .020".
Is the piston a used unit? I say that, because it looks like it. If it is, then I would consider that your carb issue may be masked by excessive skirt wear.
Does it spray fuel out of the carburetor during acceleration with the filter off?

Yes, the carb does spray fuel. Good question and it may be one possible cause of the meltdown. It may not be the skirt wearing down but that combustion air may be blowing back into the carb. I placed the cylinder back on with the base gasket and it looks like the upper piston ring is below the intake port. This may also explain why the inside of the boot is also slightly melted and flowing into the port. At 15000ish rpm on WOT, I am sure that like bit of blowback adds up very quickly under load.

I am lapping the the cylinder down and deleting gasket so that it brings the cylinder down towards the crank and hopefully allow the upper piston ring to seal more combustion chamber. Lets see if this theory hold.

FYI, the cylinder and piston are all procured new. This MS200T is an experiment to assemble and run everything stock as received to see if it will start and if so, how long it will last. So far it starts, and the boot failure is TBD until I do the next test of improving the sealing of the combustion chamber. The scoring on the piston may be from junk in the crankcase and my field disassembly trying to diagnose the Farmertec carb issues I was experiencing. This chainsaw gets disassembled on an average of once every two weeks since I received the parts in June, so I am probably introduce more junk into the system than normal. I am probably more the cause of the worn out look of the piston and cylinder.

 

[smokey7]

If that pic of the piston below ine intake port is actually bdc. Then you have major issues.

 

[67L36Driver]

This:

Not good kee-mo-sabe.

 

[Thommo]

That cylinder looks like someone has ported the intake and raised the roof too high

 

[smokey7]

Ive never seen a cylinder have tge piston uncover the intake above the piston. If it was raised it was raised a half inch.

 

[Axotopia]

Piston is at BDC. No porting, just some 350 grit sandpaper to ease out the edge a little on the inside. Port is stock size and location.
I took some thickness off the cylinder base and a gasketgdelete, now squish is .028, down from .052 shown in the picture posted yesterday. Now the top edge of the intake port is at half way thickness of the top piston rings. Figure taking the extra .008 off may help a little more but possibly introduce other issues.
This MS200T was an education build for me to learn about chainsaws, guess I should be careful what I ask for.

 

[Thommo]

I would definitely take more off the base until that top ring is covered. If the squish is too tight you might get away with skimming a bit from the top of the piston but it would be better to cut the top of the cylinder. If you have no way of cutting the squish band on the cylinder, you could glue some emery tape to the top of piston and twist it around to sand the squish band. I think that intake is the cause of your running issues and melted boot

 

[huskihl]

Ive never seen a cylinder have tge piston uncover the intake above the piston. If it was raised it was raised a half inch.

I don't think that's the top of the piston. It's the top ring

 

[drf255]

Pretty sure that’s the top ring at BDC he is showing. It’s just dipping below the intake roof.

Stihls generally have bottom rings into the intake port at BDC. Usually not the full top ring, sometimes one can just see it. You can just run a top ring, the bottom one is superfluous.

Bet that’s a design flaw, but not the cause of your boot melting. If it was an issue, that P&C woulda gone boom already.

The clearance of the slug sounds loose if my ROT calculations are correct. Most OEM clearance around .002-3 inch ar the bottom of the skirt at one side, so half that for each side. So you have .22mm or .009 inch clearance. That piston is likely slapping and rubbing the skirt on the intake floor.

Even where you only have .11mm clearance, you’re still at .0043”. That piston will rock in the bore and that’s not great.

Nothing beats OEM, please repeat, nothing. I realize the cost of OEM replacement parts may require AM parts use to some.

 

[Axotopia]

Pretty sure that’s the top ring at BDC he is showing. It’s just dipping below the intake roof.
Stihls generally have bottom rings into the intake port at BDC. Usually not the full top ring, sometimes one can just see it. You can just run a top ring, the bottom one is superfluous.
Bet that’s a design flaw, but not the cause of your boot melting. If it was an issue, that P&C woulda gone boom already.
The clearance of the slug sounds loose if my ROT calculations are correct. Most OEM clearance around .002-3 inch ar the bottom of the skirt at one side, so half that for each side. So you have .22mm or .009 inch clearance. That piston is likely slapping and rubbing the skirt on the intake floor.
Even where you only have .11mm clearance, you’re still at .0043”. That piston will rock in the bore and that’s not great.
Nothing beats OEM, please repeat, nothing. I realize the cost of OEM replacement parts may require AM parts use to some.

Makes sense! Rookie dumb observation on the rings. I took apart my OEM 046 and looks like the top piston ring is about mid thickness at top of intake port @ BDC, very similar to my MS200T at this point after sanding the base of the cylinder; looked at my OEM 018C and it's even sloppier (picture attached) and the 018C is running fine for the past 16 years.

Agree with the comment that it is likely not the piston ring; the transfer ports would have equalized the pressure by the time the piston hit BCD. The heat from a loose bottom skirt dancing rock and roll and hitting things up sound right if the Farmertec skirt is so far off spec from OEM. .002-3" vs .0045" is a good 150-225% off spec.

Guess it may be time to bite the bullet and buy a decent piston to test the theory. As mentioned, this is a test saw to see what running AM parts stock is like, now I need to find out if its the piston, intake boot ... or both that goes on the Black List.

My 90%Farmertec MS200T may need to be renamed 85%Farmertec if the piston is culprit.

Stihl 018C Piston rings BDC - intake port


90%Farmertec MS200T Farmertec Cylinder - Transfer and Intake port

 

[HarleyT]

I think the problem is with this black dog turd you have stuck on your engine!!!

 

[HarleyT]

But that would be an insult to dogs!!!!

 

[smokey7]

Ive never worked on 200s or 018 017 170 180s bus its generally not ideal to have rings in the intake port. Many of the saws ive seen do not have rings in the intake atall. Maybe its ok if the locating pins have the ring ends rlfar enough away from the port edge to make it safe. I still think there is exhaust or some type of extra heat source causing the boot to melt like that. Even the worst quality am boot shouldnt melt like thwt. Also that boot looks more like shiny plastic then a rubber boot. Piston to cylinder clearance is loose but i dont feel like its loose enough to cause any other issues then some extra spitback upon wfo. The cssting plating and ports actually look pretty nice compared to many aftermarket offerings ive seen. I believe that even at the clearance it has it still would offer a useful service life for a homeowner hobiest. When i re read thru the thread i see you mentioned the saw tached out at 15,000 rpm. Holy cow i dont think ive ever seen or heard of a 200 spinning that fast.

 

[Axotopia]

Ive never worked on 200s or 018 017 170 180s bus its generally not ideal to have rings in the intake port. Many of the saws ive seen do not have rings in the intake atall. Maybe its ok if the locating pins have the ring ends rlfar enough away from the port edge to make it safe. I still think there is exhaust or some type of extra heat source causing the boot to melt like that. Even the worst quality am boot shouldnt melt like thwt. Also that boot looks more like shiny plastic then a rubber boot. Piston to cylinder clearance is loose but i dont feel like its loose enough to cause any other issues then some extra spitback upon wfo. The cssting plating and ports actually look pretty nice compared to many aftermarket offerings ive seen. I believe that even at the clearance it has it still would offer a useful service life for a homeowner hobiest. When i re read thru the thread i see you mentioned the saw tached out at 15,000 rpm. Holy cow i dont think ive ever seen or heard of a 200 spinning that fast.

Sorry for the confusion, but I did not tach the saw. the information is from the Stihl spec for the MS200T, which is supposed to run at 15000rpm. But now that you mentioned it, I wonder if my saw is tuned wrong and running WOT at higher than spec recommendation, and causing extra heat. I don't have a tachometer, but the saw is definitely noticeably running at WOT at a much higher pitch than my other saws. I had been having carburetor issues with the AM carbs, ended up rebuilding a used OEM carb; no telling if my tuning is way off.
Other possibility is that saw is running lean because of my carb setting.
Maybe next steps should be tuning the 'high' setting down on the carb and widening the exhaust port, run richer and modding the muffler opening to help with purging the combustion chamber. If it still melts the boot, then try an OEM boot. Last step if the OEM boot self-destruct ... , put in an OEM piston.