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  #1  
Old 12th January 2020
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Default How much is too much?

I've been chasing an engine knock, which I suspect is coming from the front cylinder. I pulled the motor and removed the top end. I don't feel any up/down of the rod, but I can wiggle the rod side to side quite a bit. How much movement is too much?

Btw, this is what the piston looks like:

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Old 12th January 2020
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There is no side shake specification in the fsm for ‘77 and up, only up and down on the crank pin to rod fit. Your pistons look like they might have been fit tight previously. Maybe the noise you are hearing is piston slap? Does the other piston look the same?
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Old 12th January 2020
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I'm pretty sure what I've been hearing is piston slap. The rear piston looks perfect.
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Old 13th January 2020
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What causes piston slap anyway?
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Old 13th January 2020
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I would check cyl for egg shaping,check bore and piston size but I'm betting on bad egg shape cyl.
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Old 13th January 2020
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Only cure is cylinder boring and over sized pistons and rings. Best to bore both cylinders so they match.
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Old 13th January 2020
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I would say you have a lean condition in your front cylinder, caused by an air leak, that causes just enough overheating so that the trailing surface of the front cylinder, where your air flow is minimal, gets hot enough to negate the oil splash and cause the galling that’ we see.
The piston is hotter because of the lean condition and over expands into the fit tolerance where everything is hotter.
Your not at a point of seizing up, but pretty damn close.
How’s your wrist pin fit? Piston slap has a sort of a ring to it rather than a plain knock.

Your rear cyl is ok without the leak. Running richer and staying cooler.

In the old days...if the top of the male rod moved more than 3/32 total, side to side, they recommend doing a crank pin and bearings. I ran a motor at that max for years without any problems.

I’ve seen parts like yours cleaned up, fits checked and go back in service.
It is farm equipment after all. 😛
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Old 13th January 2020
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piston slap is from too much clearance between the piston and cylinder wall.
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You can't clean up piston slap, you might be able to clean the bore if it's not too fuxored. It certainly needs a new piston in the front hole.
If there is anything in the bore that you can catch a fingernail on it is probably past it's "use by date". Cool thing is you don't have to bore BOTH cylinders if it comes to that.
The pistons are all pretty much the same weight and the motor doesn't care if one hole is bigger. I would not bore a good cylinder just to make it match, you can't get OEM cylinders any more and sleeving is not an option in my book.
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I took the cylinders to my favorite machinist today. There's no scratches that can be felt on either cylinder, and they measure up fine. He's going to hone them, and I've ordered a set of std pistons and rings. He also suggested the front hole had a lean condition, causing this issue.
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