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farmall cub motor knocking

E

Eli B

New User
Motor knocks at idle. Pull the dipstick out and knocking stops. Pulled pistons, second and third pistons have been scuffed on the sides and upper rod bearings have slight wear. Just put motor back together after rebuild. Knock seems to high in cylinders.
 
by upper rod bearings are you refering to the, wrist pin bushings? If so and you had really any play you can feel then its too much. Very tight tolerances on them.

Andrew
 
No the bearing half on the top of rod has wear, not the wrist pins. The pistons move side to side as they should, but other than that there is no excessive piston movement on the pins. Thanks for replying
 
Was there a ridge in the cylinders? Did you have the crankshaft measured for wear? The Cubs are known for losing their oil pressure how does your pressure look? Hal
 
Engine was bored over and has new pistons, so no ridges. Oil pressure is as should be. Not sure if crankshaft was tested during rebuild. Thanks for replying.
 
I had the same problem. The aluminum pistons we get from China expand quite a bit; too much in fact. I had mine bored to .040 over, with .004/.005 clearance. Immediately after the rebuild the engine siexed right up. If I ran it above idle, it would overheat and size up. I put in STP, thicker oil, etc. Finally, I pulled the pistons, saw the scuffing, and honed the cylinders .007 over. Not a lick of trouble since. I have heard that this is a common problem with current aluminum pistons on cubs....we're not the only ones.
 

If you have noticeable rod bearing wear and scuffed pistons this soon after a rebuild you have major problems. I think you better have a mechanic look at it.
 
One thing that's throwing me here - you pull the dipstick and the knocking stops?

Is that a sure thing? Like - ALWAYS stops, and always starts again when you put it back in?
 
Yes pulling the dipstick out to a certain point stops the knocking, any farther in either direction and it starts again, stick is not hitting anything. Thanks to everyone who has given ideas or solutions.
 
Wish I was a little more familiar with the cub engine, because I'm having a real hard time thinking of ANYTHING that would be effected by pulling the dipstick halfway out.

I'd say it's a coincidence, but you sound pretty sure of yourself.

It almost HAS to be hitting something, no?

I know the cubs use the dipstick as a breather - maybe it's a pressure thing?... but that doesn't really make sense either.

... loose oil pump and the dipstick moves it a bit???

Just grasping at a straws

would love to hear what the solution is though!
 
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