I've been looking at Piston Selections and have a basic, therefore probably stupid question. Most of the pistons specify rod length, why? If I know my block height, rod length, stroke and desired deck height, why can't I just choose a piston with the correct bore and compression height that meets my needs? Is this because of the wrist pin placement

Thanks.

 

wrist pin placement is correct.

 

It depends on where the wrist pin is like SDAKOVIC said. You need a higher pin placement with a longer rod to maintain the same overall height. There are many variables that go into making an engine.

 

To elaborate a little: the goal is to locate the piston crown at the desired location relative to the cylinder head in the assembled motor. Most performance street and street/strip motors aim for a "quench" (the distance between the flat part of the piston crown and the corresponding portion of the cylinder head) of 0.035-0.045". The relevant variables are the stroke, the rod length, the gasket thickness, the deck height of the block, and the "compression height" of the piston, which is the distance between the center of the pin bore and the flat part of the piston crown.

For example, many people use the Fel Pro #1074 head gasket and a "zero deck". With a zero deck, the piston crown is flush with the block deck surface. The gasket is 0.039" thick when compressed, so this combo gives a 0.039" quench, right in the ball park. Quality pistons are easily ordered with a custom compression height to allow the desired quench when all of the other variables are fixed.

Another way to think of this: (block deck height)-(1/2 the crank stroke+rod length+compression height)="piston deck", which is the distance of the piston crown from the block deck surface. This can be negative, with the piston down in the hole, zero, or a positive number where the piston protrudes slightly above the block deck surface.

Rich Krause

 

Thanks everyone, especially Dr. Krause, for your detailed response. To give some background I recently purchased a 3.800 stroke crank from a fellow board member. It's a GMPP used in their stroker crate motor which I believe to be manufactured by Cola. I already have a block which I don't have a block height for yet. The block had already been bored and looks fresh, but I don't know how much or if it's been decked. At any rate I'm going to ensure that the machine shop ensures that the block is square. Of course I'm hoping that it's in the 9.00 - 9.025 range after all is said and done. With standard off the shelf 3.750 stroke pistons, 6.00" rod and 1.125 compression height, zero deck becomes 9.025, which I think is a doubtful height for my block because it's already been bored and probably decked. At any rate I'm probably looking at custom pistons.

I'm going to go for zero deck or slightly negative to achieve proper quench. I guess I'm anticipating, using the above formula, (0.00 Zero Deck)+(.5 x3.8stroke)+(6.00"rod)+(? compression Height), needing a piston with a compression heighth somewhere between 1.100 and 1.125. in other words a block height of 9.0 to 9.025. At any rate I want to leave the block as "tall" as possible, taking off as little as possible to ensure the block is square and flat, and then of course order my rods and pistons appropriately.

Thanks again guys.