Crank/rod bearings... what's good & what's no so good

[whitedogone]

How do you tell when a crank /rod bearing is no longer fit for service? How much play is "normal" I understand there is lateral movement which is fine. But what amount of "lean" is acceptable? And how much radial play is OK? Then the vertical play? Is there an accepted way to quantify these values? Or is it just experience on what is "normal" ? WDO

 

[Lakeside53]

Radial -bascially none.

"Lean".. Tough to quanitify.... I've seen so many it's easy to figure what's good and bad, but in the middle.. heck.. if in doubt, use it.

 

[blsnelling]

My Yamaha Banshee had a spec on how much lean acceptable. That's how big end clearance was measured. I've always understood that there should be no radial play. If there were any, I'd think it would soon beat the crank to death. An early 066 crank should be cheap to come by should you need one. Probably several laying in members shops.

 

[Martinm210]

What about checking a crank for any bending of the rod. I have a spare crank, but my new MS660 project appears to have a shattered piston skirt on the exhaust side that appears to be due to debris in the compression chamber. I don't know how they managed, but there are wood chips stuck to the squish area. It tore the coating off the cylinder and shattered the piston skirt.

I was figuring they were both toast anyway being a fleebay saw, but now I'm wondering about that crank. The bearings all look to be in really good shape from what I can tell, overall a low hour looking saw, just obviously a hard low hours (was a gardening rental saw it appears).

Should I just be taking a square to the rod relative to the case or any thoughts?

 

[LAndrews]

I got pretty spoiled by Honda CR125 cranks, replacement cost $125. I'd replace them annually as cheap insurance.

Having said that, I've never lost a crank that wasn't obviously loose. It's not a big loose, but it's obvious. The trick is to have a good crank available for comparison. Some dealers are cool and will let you feel up their junk (ew!)

A teeny bit of sliding side to side is normal. Rocking side to side isn't, and movement up/down is a good sign the big-end bearing is weak. Crank often takes the piston and jug with it when it starts ejecting bearing pieces...better to err on the safe side.

Real trick is to find someone who can rebuild the crank right the first time.

 

[Jacob J.]

Bent rods in modern saws are pretty rare. The piston is relatively soft in relation to the linear strength of the modern connecting rods. So in that instance, it's usually the piston that gives and the rod is ok. The bent rods I've seen were from owner abuse and carelessness, most often from using an incorrect method to remove the crank from the case.

 

[whitedogone]

I got pretty spoiled by Honda CR125 cranks, replacement cost $125. I'd replace them annually as cheap insurance.

Having said that, I've never lost a crank that wasn't obviously loose. It's not a big loose, but it's obvious. The trick is to have a good crank available for comparison. Some dealers are cool and will let you feel up their junk (ew!)

A teeny bit of sliding side to side is normal. Rocking side to side isn't, and movement up/down is a good sign the big-end bearing is weak. Crank often takes the piston and jug with it when it starts ejecting bearing pieces...better to err on the safe side.

Real trick is to find someone who can rebuild the crank right the first time.

I didn't know they were servicable. WDO

 

[whitedogone]

My Yamaha Banshee had a spec on how much lean acceptable. That's how big end clearance was measured. I've always understood that there should be no radial play. If there were any, I'd think it would soon beat the crank to death. An early 066 crank should be cheap to come by should you need one. Probably several laying in members shops.

It's looking like I might be needing one.

 

[blsnelling]

A teeny bit of sliding side to side is normal. Rocking side to side isn't, and movement up/down is a good sign the big-end bearing is weak.

Every crank I have ever seen in a 2-stroke has side-to-side rocking play and is totally normal and acceptible. Like I said before, the amount it leaned was the spec measure to see if it was good. Also, there is quite a bit of room to slide the rod side-to-side. I'm not sure what engines you have worked on, but some your statements do not hold true for chainsaw engines. I will agree that any "up and down" or radial play is bad.

It's looking like I might be needing one.

They're cheap enough on the 'bay. I know, the parts costs are piling up though

 

[whitedogone]

Every crank I have ever seen in a 2-stroke has side-to-side rocking play and is totally normal and acceptible. Like I said before, the amount it leaned was the spec measure to see if it was good. Also, there is quite a bit of room to slide the rod side-to-side. I'm not sure what engines you have worked on, but some your statements do not hold true for chainsaw engines. I will agree that any "up and down" or radial play is bad.


They're cheap enough on the 'bay. I know, the parts costs are piling up though

That's for sure. A little over 4 bills if the crank is going to go as well. I never was one to do things half ass. I can get a used OEM in "Good" condition or a new AM for about the same $. Which way should I go??? WDO

 

[blsnelling]

I can get a used OEM in "Good" condition or a new AM for about the same $. Which way should I go??? WDO

Nothing wrong with a good used crank. I prefer OEM when I can.