This Post relates to piston failure in Chainsaws. In my shop I see this all the time and most of the time if it is just a scored piston you my able to save the jug. One of the worst scenarios I have seen was a Sthil saw that the snap ring had broken off and the piston pin had eaten the jug up without a doubt this was the worst top end failure I have ever seen. I hope to shed so light on this subject. When a piston gets "scored," some of the aluminum it is comprised of transfers to the cylinder wall. This reduces the diameter of the piston. In a piston port engine (virtually all saws now) The skirt of the piston also functions as your intake "valve." The port is uncovered at the appropriate time, and fuel charge goes into the bottom end. If the clearance between the piston and the wall is large enough, some of the fuel charge will spit back through after the port is closed. Also, exessive piston to cylinder clearance causes piston slap, which is the piston rocking in the bore. Piston slap, in turn causes more wear on the skirt. In the end, the very thin piston skirt will break, and trash the entire top and bottom end. Running a scored piston is never desirable. Sometimes you must for one reason or another. Saving money is not usually a good reason. This is false economy. Once that piston breaks, it will trash a whole bunch of parts, other than just the $35 piston. With that said....... If the ring grooves are free of scoring, and it is not totally wasted, here is a basic test. Take the rings off the piston, and put it in the bore you intend to run it in. Make sure it is dry (no oil) for the test. Put your thumb over the spark plug hole, and turn the cylinder over. If the piston slowly drops out in maybe 5 or more seconds, it is probably tight enough. If it flops right out, it is way too loose. I have seen folks here say to sand down the scoring. This only increases tolerances. If you must use a scored one, just knock off the high spots, and then do the previously mentioned test. Some may disagree but these are my findings based on years of experience. The tolerances on a 2 stroke engine are crucial for it to run concurrently and idle correctly remember there should be a negative pressure in crank case for the intake and exhaust to work properly. Sthil dealers like myself cannot honor a saw warranty without a vacuum test being done now. Just taking the muffler off and looking at the piston days are gone, as many of us have done in the pass. Blow by of the rings will upset the performance of your saw and basically cause major problems. The 660 Sthil if famous for this unless you were lucky enough to buy one straight from Germany, it’s predecessor the 661 Sthil is just as bad. Hope I didn’t ruffle anyone feathers. Have a nice day... Stumper
Stumper, Thanks for digging this up. Some of the content can be found by scrolling down to "Saw Dr's" post here: Scored Piston & Ring Questions
That sucks @ stumper I have a 661. But it's been giving me more trouble than what it's worth right now with not starting. It's just over a year old and on and off I've had trouble with it not wanting to start. Three months after I bought it it won't start at all took it back to the dealer where I bought it from and they replaced the fuel mixing solenoid. Worked good for 6 months most of the time then that same thing happened again they replaced the same part again . That was just 2 months ago and now I am having problems with it not starting again. It does have the original coil "4700" in it instead of the updated 4701. Could this have anything to do with it? I am getting a WJ-69 carb for it to try to put an end to this M-tronics nightmare!
Good question...Carb solenoid and ignition modules seem to be the big issues for the 661. Ignition modules had a recall when they came out. Have you ever tried to crack the fill cap loose to relieve vacuum. Plainly put we work on them constantly. Should have bought an 880 and forget about the m-tronic 661. I just wrote a Post an mentioned the 661 in it. It overheats ,and causes the ignition modules to send false readings to the carb solenoid which make it lean out even more causing more heat. In my opinion Sthil really make a bad move from the 660 to the 661 m-tronic. Sthil does not have a new module yet, but it like they just want acknowledge the problem, it is in the module to start with. So the rest of the auto tunes it makes to the engine are false signals. Making it a bastard to keep running. Have a nice day! Stumper
My son which helps at the shop had some additional information for you but if your saw is under warranty (which) really doesn’t matter a this point. It has to do with cylinder and coil which we have found can sometimes be the fix. What number is stamped on it? The latest is 1144 C. The A’s were a problem. The B’s and C’s are okay. What is your number on your coil? If it not 4701A you will have a problem. You also need a straight boot connecting to your plug. These are some the less costly fixes. I glad he added some insight into the mix, so we my be able to help you. Have a nice day, “blacksmith”... Stumper
Yes I've tried everything. I always crack the fuel cap to release the vacuum, I keep my saw very clean after every time I use it I blow off the whole saw with compressed air and blow out the air filter as well after every day of use. I even calibrate it every so often like they say to do!
I didn't get a chance to see if it was an A, B or C. But the coil is a 4700. That is one thing that I looked at the other day when I was cleaning it up! I do however mix my fuel a little heavy due to people saying that it does tend to run a little lean.
“blacksmith” I will be in a telephone conference with Rich Dougan today and I will see if he has anything new on the 661 that I don’t have. Sometimes the West Coast gets information before us because all there tress are big therefore they use bigger saws. Have a nice day! Stumper
Good afternoon “blacksmith” hope you are doing fine! Question do you know how to reset your sáw to factory specifications? If not I will be glad to show you. Myself and Rich have come to the same conclusion, if your cylinder is not stamped with 1144C, and has a coil stamped with 4701A, and the coil wire is a straight boot. If you do have all this then it a bastard to start and run correctly, then more than likely you need the up dated module. But keep in mind the A’s were a problem with the cylinder and when saw get hot it will do funky stuff. If you can see what is stamped on your cylinder. Here are your resets: (1) With the chain brake engaged… a. Hold throttle and set start lever to Start position (Δ) b. Start saw and let idle in Start position (Δ) for 90 seconds c. Blip throttle to bring the machine to the Run position (I) d. Immediately move lever to Stop position (O) e. This will reset the carb to original factory setting. (2)With the chain brake engaged… a. Hold throttle and set start lever to Start position (Δ) b. Start saw then Blip throttle to bring the machine to the Run position (I) c. Let idle in Run position (I) for 90 seconds d. Immediately move lever to Stop position (O) e. This will calibrate the idling characteristics of the saw. (3) With the chain brake engaged… a. Hold throttle and set start lever to Start position (Δ) b. Start saw then Blip throttle to bring the machine to the Run position (I) c. Disengage chain brake and make five uniform cuts through a minimum 12-inch-diameter log. d. This will calibrate the full throttle characteristics of the saw. I hope this my hel you, maybe you already know this but I added them anyway. Have a nice day! Stumper
What is stamped on my coil is 4700. Is the newest and updated coil 4701? And actually I do calibrate the saw after every couple of jobs and I totally clean my saw after every job so it's not a dirt issue. The problem with it not starting is random sometimes I take it out of the truck in the morning won't start but other times no problem. Other times I will be running it for most of the day one time shut it off and it won't start again. But I was making hot starts on and off all day! Other days not one issue. I may try throughout a particular day to try to start it and it won't. Put it away for the night take it out the next day and it fires on the 1st at or 2nd pull! The times that it won't start is random! I can't seem to pinpoint any particular combination of things that keep it from starting. And it doesn't seem to have anything to do with the outside temperature weather it's very hot outside or very cold. I guess when it doesn't want to work and is tired it says I amnot going to start hahaha! I did call the dealer that I bought it from told them that I am still having issues and they told me to bring it back in and they are going to replace all of the elecrtonics with the updated parts.
There was a 4700A, 4700B, 4700C, 4700D, 4700E, and a 4701A. The E and 4701A are pretty much the same. The others are leaner and turn more rpm unloaded.
Thanks mdavlee I didn't realize that there were so many coils! I'll have to pull off my cover and take a picture and post it.
Here is a picture of my coil. Just in case you can't read the info it says, 4700 D SMT 1504 What does the SMT 1504 stand for?
This random problem makes me think of this post by Moparmyway ...I'm 'Bout Ready to Toss My 661 in the Trash!!!
This is the second or third time that youre talking about the coil wire being a straight boot. How on this green earth are you even saying this ? Do you know what you are actually saying ? You are going to help guys destroy their saws ............. not help them to get them running better. I hesitate to even tell you the correct way, because you're going to screw it up anyway.
