Sticky Piston Question

[MicroMachinery]

I have become aware that one of my pistons is slightly sticky when pushed to one side of the connecting rod; meaning, if I hang the piston upside down, and move the piston all the way to the side of the wrist pin pivot, it doesn't(unless given a VERY light touch) drop back down and even out.

Perhaps a pic better demonstrates my concern.

Now, I wouldn't say it's a hard spot. Probably better put as a dry spot? I can just barely put pressure on it and it swings down into the normal hanging position like so.

Also, it only acts this way if the connecting rod is pushed all the way to the front side of the bore in the piston.

If pushed the the rear, or centered, it doesn't do this at all. Swings freely.

 

This should be ok, right? The way I see it, is if the piston never reaches that angle, then it will never have to experience a scenario in which that friction becomes an issue. Also, if in fact it isn't just a "dry spot", won't the aluminum expand enough to make this a non-issue anyway?

I do understand, that in the most extreme of conditions(which are a mechanical physics impossibility), that this spot would be touched thousands of times in a minute. What I mean is, if I went for a 5 minute drive, at an average of 2k RPM, this spot would be touched 20k times. Now remember, only when slid to the very front of the piston pin bore, and cocked completely to the side, to where the piston skirt touches the rod, would this slight "friction" even occur(thus the mechanical impossibility). At 20k times, this friction COULD become a problem, but given the mechanical impossibilities, this wouldn't be an issue. Is this logic flawed?

[Braden]

your thinking too hard, ive seen this before, never heard that it was a problem, install it and forget about it. if its gonna cause a problem you will knwo about it.

[datzenmike]

They all do this. The rod is press fitted (1000 to 2000 lbs) to the piston pin, very tight. The pin is free to rotate inside the aluminum piston, but it has to be a very close tolerance fit hence the 'tightness' you felt. Generally the piston to piston pin clearance is 0.0004 to 0.0008 inch. You should be able to push a pin into a piston with your thumb but it shouldn't be so loose as to fall out under it's own weight. The pin is lubricated by splash oiling and will build up some small 'coke' deposits from the heat. The piston doesn't move much on the pin and will be 'loose' in that area and may feel 'tight' at the extreme limits as you described. Sounds normal to me. I've seen them so tight they are hard to move with both hands... but only at the far end of movement where it never goes.

 

You could WD-40 them and they will loosen up as the varnish deposit dissolves, but a hundred miles down the road they will be the same. Best thing is keep the oil clean and changed to prevent build up.

 

I have a set of KA rods that are 'fully floating'. These are not pressed into the rods but have a similar clearance as the piston. The rod has a bronze bushing that the pin rides and rotate in. To prevent the pin working it's way sideways and scratching the cylinder, there are snap rings on each side of the piston pin holes to hold the pin in. Because the pin is totally free to rotate in both the piston and the rod there is some friction loss and power gain.

 

Bronze insert on KA24E rod

 

Oil hole through top of KA rod

 

 

KA piston/rods. I took these apart with just some needle nose pliers to remove the C clips. Pushed the pins out with my finger.

 

 

I don't know how much more power is made by floating pins but most serious racers use them. The Truck KA24E doesn't have them so what's the HP difference between the 240sx and a D-21???? I suppose we may never know because they also use different intakes as well. 5-10 hp?????? I don't know

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