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Discussion Starter #1
I've disassembled a 2001 2.7 for two spun rod bearings. I've noticed that the pistons don't seem to "flex" very easily on the rods. In fact if I were to put a fish scale to them it would probably show 2-3 pounds of "pull" to get them to move. All the pistons I've dealt with before (non-Dodge) have flopped easily from one "tilt" to the other. The clearance specs are pretty tight and I know two of them have undergone some stresses due to the spun bearings but all six are the same. Is there a standard?
 

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I'm not an engine expert, but theory: Piston-to-pin and rod-to-pin clearances are less than 0.001" (I assume that is at "normal" room temperature: 20°F/70°C). What's the temperature when you're feeling them tight? Coefficient of thermal expansion for aluminum is much higher than for steel. Pin is steel. If piston is aluminum, the aluminum ID-to-steel OD clearance will decrease when colder. Not sure at what temp. nominal clearance goes to zero.

Do they get looser if you take them in the house and let them warm up for a couple of hours? Put one piston/rod assembly in the oven and see if it gets looser at higher temperatures. It definitely should.
 

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They should easily flex. If I had to guess there is probably some oil coking or sludge preventing them from moving freely. As far as a standard, there is an oil clearance between the connecting rod and wrist pin, in the service manual, and its a "snug fit" into the piston.
 

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Note: The pistons are a "high-strength aluminum alloy" (from FSM) - probably eutectic aluminum, which has a lower coefficient of thermal expansion than more common aluminum alloys. Connecting rods are "powdered metal".


From '02 FSM:

2.7
41269


3.5 ("0.007 in." below is a typo - should be 0.0007 in.)
41270
41274
 

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Discussion Starter #5
As always, the wealth of info is invaluable. The suggestion to bring them inside was an "ah ha' moment since the car is overnight in the "shop" during sub-freezing temps getting thoroughly cold-soaked and the days aren't much better. But after hours inside they didn't improve much. I suspect there's a fair amount of coking about the bushing/pin but this engine is relatively clean. The pistons and their rods are a balanced assembly and I don't want to break anything but can the bushings/pins be pressed out (w/ a press) so I can see, measure and clean them?
I'm at least getting the crank evaluated at a reputable machine shop and, hopefully, make it serviceable again. I'm at that place where a fresh short block would be my ultimate choice, having the machine shop fix the crank, spec the rods and reassemble it be my second choice or doing it myself after getting the pieces checked out. It's all economics.
It's a judgement call. I just like the idea of bring it back to life.
 
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