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I'm visiting my Dad and he has the engine apart on his new project, a 39 business coupe, and there is some confusion. The pistons are clearly marked front, and if you do that on these pistons the slot ends up opposite the camshaft. This is in direct conflict with the manual, which clearly states the slot is supposed to be on the same side as the camshaft.

Anyone seen this? What's right?

Thanks in advance
Ken on behalf of Doug.

Hi Ken:

Both the squirt hole in the connecting rod and the slot in the piston should be on the camshaft side of the engine.

I don't think Packard stamped pistons so they are probably not original pistons. Thus, the reason for the language in the manual.

If you mean what I think you mean the slotted side of the piston is not the thrust side. In other words the thrust side is the plain not slotted side. Can engine experts confirm this?

Here is a bit Packard wrote in 1934. IIRC, there is another bit in a later year reminding dealers the slot was to camshaft and also mentioning engineering knew it was contrary to other industry practice but they still believed it was the preferred method.

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On replacement pistons (actualy ANY piston of which one is not familiar with) then check for wrist pin OFFSET. Not all pistons have any offset. Some do. SOme don't. IF any wrist pin offset exists then fit the piston such that the wrist pin is setting closer to the THRUST side of the cylinder wall.

The slots are ffor thermal expansion of the piston. The slot should be OPPOSITE of the thrust wall of the cylinder.

In any case, it will be wrist pin offset that dictates position of the piston regardless of a slot or no slot.

As a side note:
It is better to have pistons with NO slot and just set them up with maybe .001 to .002" MORE cylinder wall clearence than specification dictates. This is especialy true on engines that are high demand service level engines.

For trailor queens or low speed engines then the slotted pistons are just fine and will probbably make for a more quiet running engine.


Alwaays inspect any USED slotted pistons for cracks at the slot. The cracks usualy run from bottom of slot to bottom of piston skirt.

If possible. post a pic of the piston showing the slot.

Definition of "major thrust face" of a piston, courtesy of Hastings Piston Ring website.

That portion of the piston skirt which carries the greatest thrust load. This is on the right side when viewing the engine from the flywheel end with the crankshaft rotating counterclockwise.

Here is a picture of the piston. You can see it's marked front, and the slot therefore ends up away from the cam, which puts the non-slotted side on the thrust. Seems right intuitively, but not what the manual says. I can't measure any pin offset, but I don't have the best of measuring tools here. It\s probably a set of replacement pistons; the engine has been apart in the past. Probably fine I suppose.

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