Will a rebuilt engine that is too tight ever loosen up, or does it need another rebuild?

Dear Car Talk

Dear Car Talk | Jan 01, 1993

Dear Tom and Ray:

I recently had my 1969 429 cubic inch Ford V8 totally rebuilt using sixty thousandths oversized pistons. A new water pump and a new, oversized, heavy duty radiator were installed. After installation there has been an unacceptable overheating problem. The mechanic has no explanation. The thermostat was removed to help prevent overheating, but that didn't work. A direct drive for the fan was installed. That didn't work either. Can you comment on possible causes and solutions?

RAY: Our solution would be TWO heavy duty oversized radiators, and an immediate move to a far northern latitude, like Cicily, Alaska.

TOM: If all of the normal things have been checked--and it sounds as though they have--the problem is probably that the engine is too tight. And that means the rebuild was done incorrectly. The guy who did it could have over-tightened everything, or he could have installed parts that were the wrong size. Or perhaps the machine shop ground the crankshaft incorrectly, or bored the cylinders too small.

RAY: Think about it, Norm. If things are too tight, or if the cylinders are too small by a couple of thousandths of an inch, it's going to be really hard for the pistons to go up and down. And the friction created by the pistons trying to go up and down in a space that's too small for them is converted very quickly into heat---lots of heat.

TOM: Did you happen to notice if they used four or five batteries to start this thing when they finished the rebuild? That would give you a hint that it was a little too tight.

RAY: The good news is twofold. First, winter is here. Second, it's possible that the engine will loosen up over time as it breaks in. That is, if it doesn't throw a rod, seize, or melt in the mean time. So I'd just drive it, Norm. And if the car breaks down before it breaks in, take it back to the rebuilder and tell him to go back to the drawing board.

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