361 w/ a bad connecting rod bearing - why?

[taddat]

I stopped by the local dealer yesterday for some parts. In talking to the owner, I asked if he had any junk parts saws to get rid of and he let me pick through the pile. I found a 361 on the shelf. He couldn't remember what was wrong with it but he guessed it was seized. When I got it home, the p/c were in great shape but the connecting rod had about a 1/4 inch of slop on the crank. The saw looks to be in good shape (not all beat up) and I'm guessing by the condition of the rest of the saw, it has low hours on it.

What would or could cause this type of failure?

I was hoping for just a bad p/c but for $25, I got a lot of good parts.

thanks in advance - just trying to learn as I go....

 

[buzz sawyer]

I stopped by the local dealer yesterday for some parts. In talking to the owner, I asked if he had any junk parts saws to get rid of and he let me pick through the pile. I found a 361 on the shelf. He couldn't remember what was wrong with it but he guessed it was seized. When I got it home, the p/c were in great shape but the connecting rod had about a 1/4 inch of slop on the crank. The saw looks to be in good shape (not all beat up) and I'm guessing by the condition of the rest of the saw, it has low hours on it.

What would or could cause this type of failure?

I was hoping for just a bad p/c but for $25, I got a lot of good parts.

thanks in advance - just trying to learn as I go....

You got a good deal.
As for the damage, I'm no expert but since the P&C is good, I'll take a guess and say the previous owner didn't keep the RPMs up and there was lack of lube to the rod. You need a certain amount of turbulence to get the mix where it needs to go and the rod seems to be the hardest to reach area. I could be totally wrong.

 

[andrethegiant70]

I'm pretty well convinced that lugging a saw puts a lot more load on the bottom end. I think it's why the 371/372 has been known to kill big end bearings.. you can really lug them and they'll keep chugging. From what I understand the 361 is a fairly peaky saw, so I don't think you'll find a lot of big end bearing failures. However, that doesn't mean you can't get someone who has a bad habit of doing it anyways. Good find, swap out that crank!

 

[rmh3481]

RPM limited ignition. When the limiter hits, the forces on the connecting rod change and generally the bearing has to absorb the stress. There may have been other factors but this is what I seem to find.

 

[SkippyKtm]

I'm pretty well convinced that lugging a saw puts a lot more load on the bottom end. I think it's why the 371/372 has been known to kill big end bearings.. you can really lug them and they'll keep chugging. From what I understand the 361 is a fairly peaky saw, so I don't think you'll find a lot of big end bearing failures. However, that doesn't mean you can't get someone who has a bad habit of doing it anyways. Good find, swap out that crank!

My experience has been just the opposite, that over-revving the saw will cause a big end rod bearing failure. Having overly lean tuning will also exacerbate the issue due to allowing the engine speeds to be even higher.
I know of some loggers up north who go through cranks like popcorn due to over revving their limbing saws by not letting off the throttle between limbs, they just hold open the throttle for the entire duration. I disassembled one of their smoked ms250's and they had burned the big end of the conn rod blue.

 

[Andyshine77]

RPM limited ignition. When the limiter hits, the forces on the connecting rod change and generally the bearing has to absorb the stress. There may have been other factors but this is what I seem to find.

The 361's have unlimited coils.

It sounds like lack of lube to me. This one reason why I run 32:1 in ported saw, it helps keep the seals and bearings nice and wet.:msp_sleep:

 

[taddat]

The 361's have unlimited coils.

It sounds like lack of lube to me. This one reason why I run 32:1 in ported saw, it helps keep the seals and bearings nice and wet.:msp_sleep:

My first guess was lube related but the more I thought about it, I figured the piston would be the weak link to no lube. And if I find a crank to put in it, I want to rule out any other culprits.

 

[Grande Dog]

Howdy,
Skippy hit the nail on the head. Other things that will take out rod bearigs are excessive carbon in between the top of the piston and the cylinder. The bearings can also get damaged removing the clutch.
Regards
Gregg

 

[bigwhiteash]

rod bearing damage due to clutch removal?

..not wanting to derail the thread, but is there a safe way to use a piston stop or the 'rope trick' when removing the clutch? Both of these methods put a load on the rod/bearings. Since Bailey's sells a piston stop, I thought Grande Dog could address this...please? Thanks! J