Is my bottom end shot ?

[Jacob J.]

Just for some insight into what goes on with the con rod bearing here is a good write up that explains it way better than I could at a technical shop school........

Excessive RPM. Big end bearings can fail in minutes if subjected to excessive RPM. The reason is: During regular operation the rollers (in the big end bearing) roll on the outside surface of the crankshaft journal and on the inside surface of the connecting rod. A light film of mix oil coats the bearings with every rotation the crankshaft makes. This bath of oil (and fuel) lubricates the bearing, which removes friction and keeps the bearing running cool. When the engine is subjected to excessive RPM (like limbing with the throttle wide open), the rollers roll as fast as they can and then start "skating." This skating or sliding activity scrapes away the cushion of oil on the bearing surfaces. The bearing gets hot instantly. If the engine speed does not come down to where the rollers can roll again, it fails quickly. Even if the speed returns to normal, the bearing sustains some damage.

Keep the RPM's down , I usually go 500 below saw spec to allow for fluctuations from conditions that can change the setting .

But what is the threshold for excessive RPM in a modern saw? There was a crowd here years ago that swore up and down that running a saw over 14k would result in "slinging the oil off the crank and bearings and burning up the saw immediately." However, some of the saw builders here routinely run their work saws at 16-17k and the saws are having long service lives. As oil products and metal components in the engines continue to improve, I would think the allowable RPM threshold would continue to climb.

 

[nitehawk55]

Excessive RPM is a grey area , just because the manufacturer says 14.5K is the limit doesn't mean the saw can run beyond that without failing but they have to set a limit for warranty and safety reasons as well as the longevity of the saw .

You can run a saw up to 16-17K but just like any high performance machine you're pushing the limits . I think modern synthetic oils do help but there has got to be a point where they stop working too .

 

[blsnelling]

I prefer to exceed the RPMs by about 2000

 

[blsnelling]

You can run a saw up to 16-17K but just like any high performance machine you're pushing the limits .

How do we know that? Real world evidence shows otherwise.

 

[nitehawk55]

Common sense should tell you that Brad , you of all people should understand that pushing a saw or any engine has it's risks .

 

[blsnelling]

Common sense should tell you that Brad , you of all people should understand that pushing a saw or any engine has it's risks .

I have never lost a bottom end due to RPMs.

 

[nitehawk55]

Well maybe you've been lucky , I saw lots cooked in the Tech Dept.

These RPM caps for saws isn't a number the engineers pull out of the air , saws are tested on dyno's and out in the feild under various conditions before they are released to consumers. All this information determines what is the best or maximum recommended RPM the saw should run at for longevity . (that's last a long time with proper use)

What are you trying to make a point of , saw owners should lean the saws out so they are screaming at 17K with no worry of it blowing just because you say it's ok ??

That's all I'm saying , just trying to give a little education on what happens when things go wrong and the cause . Use it or not , it's not my problem what happens to yours or anyones saw .

 

[blsnelling]

I haven't seen it as an issue on any chainsaw forum.

 

[blsnelling]

You might have noticed, I'm not a by the book kind of guy

 

[Chris-PA]

I'm of two minds on this one - that rod is awfully blue, but I'm not familiar with Stihls. If it were mine I'd be trying to look at the big end rollers for signs of discoloration.

These RPM caps for saws isn't a number the engineers pull out of the air , saws are tested on dyno's and out in the feild under various conditions before they are released to consumers. All this information determines what is the best or maximum recommended RPM the saw should run at for longevity .

The "max rpm" settings are not for that purpose at all, they are simply the way the factory tells you to set the fuel mixture. You adjust it with the H screw alone - it is purely the fuel mixture adjustment.

The factory works out the correct mixture at load at working rpm, and then sees what the terminal rpm will be when the mixture becomes so rich it won't rev any higher. Then if you repeat that max rpm setting, your mixture at under load should be pretty close to theirs. Unless the saw is modified and then that relationship no longer holds.

 

[nitehawk55]

Rules are made to be broken Brad , it's always been like that from the invent of cars or anything with an engine....faster is better

Chris , those RPM settings were part of the engineering of the saw and many hours of testing in controlled conditions and out in the real world , and yes they are calculated by many factors (in cut , out of cut...etc ) to determine what is best , this is part of the field testing under hot , cold , elevations and other various conditions .
Saw Mods do change all that but I'm talking stock saws .
The worse people I found to try and present a problem to were the German engineers , their reply was almost always "you/we don't have a problem".....got pretty interesting sometimes

To the OP , sorry this got off track a bit (normal) , know that the bearing has got hot and that blueing is NOT normal . The case hardening of that bearing has been compromised which means it is not as strong as it was and it may or may not fail on you at some point depending on how you run your saw . I would set it a bit rich , use a stout mix and hope for the best .

Good luck and keep us posted

 

[burnses]

I think its called flame hardening......

 

[nitehawk55]

I think its called flame hardening......

Well the con rod is all the same color when new and I know for a fact when they turn blue it's because the bearing got hot .
Stihl Technical Dept/warehouse . Stihl Canada 17 years , not a dealership the actual head office for all of Canada .

That doesn't make me an expert but I do know a couple things

 

[Chris-PA]

Chris , those RPM settings were part of the engineering of the saw and many hours of testing in controlled conditions and out in the real world , and yes they are calculated by many factors (in cut , out of cut...etc ) to determine what is best , this is part of the field testing under hot , cold , elevations and other various conditions .
Saw Mods do change all that but I'm talking stock saws .

There certainly are max rpm numbers for the reciprocating assembly and the engineers surely know what those are - but there is no reason for them to publish those numbers, nor are they the numbers used to set max rpm. When you set a stock saw at some max rpm you are not making an adjustment to keep the bottom end together, you are merely setting the fuel mixture.

 

[nitehawk55]

OK Chris , I'll take your word for it....thanks .

 

[Tim87aus]

16600 that's when mine went bang haha

Sent from my SM-G900I using Tapatalk

 

[spudulike]

My take would be that a rebuild would be damn expensive, if it were my only saw and a new one was out of the question then I may run it and rebuild it during a quiet period. If the saw isn't a primary saw and if it goes pop and you would just grab another one then just run it for as long as you can. Run 40:1 part or full synthetic and chances are it will last but at the first sign of tapping or abnormal noises, STOP it before the bottom end detonates and takes out the top end and rebuild it.

The saw may just have had some bad mix through it during a large cut causing the bluing, will it last....maybe but it is really the inconvenience it will cause and loss of earnings that will give you an answer to if you rebuild or not...that and how much you worry