Piston Height Above Deck

[ursus]

To satisfy my curiosity regarding my original question I dug out my menagerie of unused NORS pistons from such manufacturers as Sealed Power, Badger, Silvolite, Ohio Piston, Lynolite, and Perfect Circle. Obviously, my engine parts inventory is a trip down memory lane.

Original Ford data reportedly specify a 1.906 inch compression height, which is the distance from piston top to the center of the rod pin. Of the 6 pistons measured, all measured out at 1.906 to 1.916 inches with an average of 1.910 inches. Not much of a difference! Sorry, but I didn't have anything currently available from the vendors to test.

[alexiskai]

So just to revisit Brent's math, if the offset is 2.125/7, plus 7.498/50 for the rod length, plus 1.904/9 for the piston head, that's a total of 11.527-11.536 piston height. So then if the deck height is 11.500/05, then theoretically the stock piston pop-up would be between .022 and .036, depending on your exact combination of parts.

Worth noting here that if you pair this stock pop-up with a copper gasket (.052) and a head that has *no* flycut, that would put you under the minimum recommended squish of .040. Anyone planning to use a no-flycut head (pretty rare but you see them) should consider some kind of modification to the pistons, or do something like doubling the head gasket.

[BRENT in 10-uh-C]

Originally Posted by ursus

To satisfy my curiosity regarding my original question I dug out my menagerie of unused NORS pistons from such manufacturers as Sealed Power, Badger, Silvolite, Ohio Piston, Lynolite, and Perfect Circle. Obviously, my engine parts inventory is a trip down memory lane.

Original Ford data reportedly specify a 1.906 inch compression height, which is the distance from piston top to the center of the rod pin. Of the 6 pistons measured, all measured out at 1.906 to 1.916 inches with an average of 1.910 inches. Not much of a difference! Sorry, but I didn't have anything currently available from the vendors to test.

Not much difference by numbers ...however back in the day, enough of a difference for a manufacturer to stretch the truth promoting their brand of pistons are higher compression than factory pistons. Then they tell the customer that higher compression gives more power. You & I both know what the truth is, but many consumers likely didn't.

Regarding squelch or squish, I can lengthen my rod boring machine centerline to easily add, -or subtract about 0.030" of rod length to fit my needs. If I have a head without much (-or any counterbore) then I can lower (shorten) my rod length to compensate.


Something that I will tell you is that back when we were racing Briggs powered cars, the engine builders actually allowed the piston to kiss the roof of the cylinder head. If there was ever an engine type that has been flogged to death on a dyno, the flathead Briggs is it. We were running methanol, and the idea was we needed more compression than we needed turbuence or flow. I think the difference between Model-A combustion chambers and the Briggs engines was the amount of RPMs and Volumetric efficiency that are different between the two.

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[Dodge]

Just as a side note. Just finished a Burtz motor with Silvolite pistons and the protrusion
was .030 on every hole.