New saw compression

[Marshy]

Call it lean, call it hot... whatever. The bottom line is that the piston got too hot and expanded to remove all of the oil clearance and begin to seize. There may have been other complicating factors as well such as:
1. As mentioned previously the ring gap may have gone to zero creating debris or a high spot.
2. The saw was brand new and not broken in so the saw was already tight and didn't take much overheating to start a seizure.

As for the location of the seizure it is usually on the hottest part of the piston (most thermal expansion) which is on the exhaust port side. However on a new unbroken-in saw the rules may change to favor the tightest part of the cylinder as where the seizure begins. In this case it appears to be on the "4-corners".

So yes, I still think it was lean. If this saw was well broken-in I doubt that the carb setting was lean enough to cause this kind of damage as the clearances would likely have opened up enough to prevent the seizure.

No, it does not appear to be 4-corner, Its also not a lean seizure. Take a look at the piston wash, plenty of fuel in the F/A ratio.

 

[Karla]

I had one that did not close 2 out of 3 times, and the piston showed the same results & similar scoring on intake side, So I don't know if its a coincidence or a similar occurence. Anyhow, I could not find anything really wrong with the engine, and all I did was replace the piston & ring, and its running.

 

[Deleted member 135597]

What's ring end gap and piston to cylinder clearance for this engine? I can't find it in the maintenance manual

 

[SteveSr]

What's ring end gap and piston to cylinder clearance for this engine? I can't find it in the maintenance manual

I don't believe that Stihl specifies this for their engines. I have not seen it in any of their manuals that I have used. I think that they expect their techs to be parts changers and not engine machinists. They just sell pistons and cylinders in matched sets... No thinking required!

 

[Deleted member 135597]

With the original parts I'm about .011 end gap and .002 piston to cylinder clearance

 

[Full Chisel]

I had one that did not close 2 out of 3 times, and the piston showed the same results & similar scoring on intake side, So I don't know if its a coincidence or a similar occurence. Anyhow, I could not find anything really wrong with the engine, and all I did was replace the piston & ring, and its running.

Interesting. I suppose a leaky decomp could create a "hot spot" that could cause focused scoring on the piston.

 

[Trx250r180]

The strato valve is not adjusted right ............Jeesh .

The husky blue decomp valves fit the stihls exactly,I have had good luck with them ,but they are spendy for oem ones ,The stock decomp valve in my 461 failed pretty fast also .

 

[Karla]

I recently Ordered some Brand New Aftermarket decomp valves for $1.62 each with free S&H which also state they fit Husqvarna, Jonsered, & Stihl, Not sure If they will fit Model 461. Anyhow, I buy them like Spark Plugs in Quantities, never had any problem with them before either, maybe only complaint is they close at a much lower compression value, which is Not all that bad since each time you pull the cord, you just have to push the decompress valve. Also, I was trying to Upload an Excell file which will allow you to calculate the closing, opening force, and how much compression it will take from the engine to close the decompression valve but unfortunately I could not get the extension "Xlsw" to upload to this site. I tried to convert it to an "XLS" extension but then the program would not work correctly. However If you take some measurements, I can put them into the program, and calculate the compression for you.

 

[Karla]

Oh, another thought is that alot of Individuals have stated "That they eliminate the decompression valve entirely" and Just plug the decompression plug hole with the correct parts list plug, again Not sure If there is a Husqvarna plug part number for a Model 461. Also, don't know how difficult it will be to pull start a Model 461 without a decompression valve.

 

[Deleted member 135597]

Oh, another thought is that alot of Individuals have stated "That they eliminate the decompression valve entirely" and Just plug the decompression plug hole with the correct parts list plug, again Not sure If there is a Husqvarna plug part number for a Model 461. Also, don't know how difficult it will be to pull start a Model 461 without a decompression valve.

when I ordered the piston and rings, I also ordered the plug to eliminate the decompression valve. All the parts should be at the dealer tomorrow. I got a new OEM unlimited coil coming also. I should be up and running for the weekend which is good because I have a lot of work to get done. The unlimited coil is going to eliminate or comfirm if my carb settings were too lean.

 

[Karla]

Sounds good, and like you have it in control, and are doing the right stuff. If you need any additional help please give a shout, there are lots of great help here, also I have Not worked on a Model 461, but I have worked on a lot of other stihl and Husqvarna models, and to some degree they are all very similar. Good luck with your re-assembly, also be careful Not to rush too much, I have made most of my mistakes when I get in to much of a hurry. Lol

 

[Ryan'smilling]

Just as a reference point, I prefer to start my 460 without the decomp. Using the button makes it easier to pull the rope, but you have to pull it more times. It's not too hard to pull over without the button.

 

[Gypo Logger]

I've also noticed if you make about 1/2 doz girlie drop starts on choke, then one manly drop start, the saw will start no prob.

 

[Full Chisel]

Just as a reference point, I prefer to start my 460 without the decomp. Using the button makes it easier to pull the rope, but you have to pull it more times. It's not too hard to pull over without the button.

That's been my experience with them as well.

 

[Khntr85]

I know my 461 isn't hard to start without compression button pushed... however it does have a high stock compression, and any half-as$ attempt at pulling it and you will have a problem....

By the way I have thought about plugging my decomp, does anyone know off hand thebpart number for an OEM plug by chance....

 

[Little Al]

Wanted a baseline for compression readings for my new ms461r. What's an average reading for a saw that's not fully broken in yet? I'm getting about 105 after about 5 pulls. Seems low to me?

Have the saw in space holding it by the recoil handle . how does the saw descend on the recoil rope if i goes down in a series of jumps slowly then you have no problems, if it descends smoothly/& quicker you could have a problem If as said it starts & runs OK just carry on & see if it gets higher Check your compression Pour a teaspoon of 2T oil in through the plug hole & re check the compression if it reads higher it will probably be higher when bedded/run in bottom line, if it's a good runner carry on without worry

 

[drf255]

I don't understand why the OP would spend big bucks at a dealer on a new saw and not take it back when something goes wrong with it under warranty. That damage is likely Stihl's fault. Not every single unit that comes from the factory is going to be 100% perfect...there are going to be a few turds in the sandbox.

Thought the same the entire time, then he revealed that he trimmed the limiters. There goes your warranty.

Looks like foreign debris to me, not lean.

I woulda ordered limiter caps or anything to make it back to factory and brought that sucker right back.

Shouldn't a limited coil cause a rich condition in and of itself? You should be getting a misfire with unburned charge coating and cooling the cylider when it happens. Isn't that in part why they created them?

 

[Deleted member 135597]

I know my 461 isn't hard to start without compression button pushed... however it does have a high stock compression, and any half-as$ attempt at pulling it and you will have a problem....

By the way I have thought about plugging my decomp, does anyone know off hand thebpart number for an OEM plug by chance....

Plug p/n 1122 025 2200

 

[Deleted member 135597]

Thought the same the entire time, then he revealed that he trimmed the limiters. There goes your warranty.

Looks like foreign debris to me, not lean.

I woulda ordered limiter caps or anything to make it back to factory and brought that sucker right back.

Shouldn't a limited coil cause a rich condition in and of itself? You should be getting a misfire with unburned charge coating and cooling the cylider when it happens. Isn't that in part why they created them?

My warranty "isn't gone" at any moment I can take this saw back for warranty work. I chose to fix it myself for multiple reasons. Foremost being I thought I was to blame and personally I take responsibility for my actions. Second, for about 75 bucks I got a huge education that I would have not recieved. Third, my saw would be on a shelf right now with a repair tag on it. I got **** to do and need this saw running.

 

[drf255]

I don't think the issue was your fault. Not a bit.

A failure that early points to a factory defect. That's why warranties exist.

It's not a matter of honesty.

I'd be worried it would happen again, possibly be worse, and you'd be up ***** creek.

The last guy to touch the Saw is "it". That would be you, and not Stihl, now.