Thread: Coolant in cylinders; fresh 400 sbc build

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Originally Posted by Bigbzc

Thought it was just the thickness of the gasket. Deck height is .036 so that makes squish .077

Do you understand that 0.077" is pretty much unacceptable and that it needs to be between 0.035" and 0.045"?

Originally Posted by techinspector1

Do you understand that 0.077" is pretty much unacceptable and that it needs to be between 0.035" and 0.045"?

I'll try to measure it again. It is in the truck so it makes it very hard to measure. However, the thickness of my gasket after being torqued down (compressed) is .041" which according to your specs isn't giving me much to work with and I know there is more than .004 deck hight. Maybe we are misunderstanding each other somewhere?

Originally Posted by Bigbzc

I'll try to measure it again. Maybe we are misunderstanding each other somewhere?

Perhaps. Let me go through it for you. When you assemble a motor, you want a flat part of the piston crown to come up against the underside of the cylinder head with just enough clearance to squeeze out the fuel/air mixture from that area and "jet" it across the chamber just as the plug is firing. This creates turbulence in the chamber and homogenizes the mixture to prevent rich and lean pockets where the mixture would burn inconsistently. Tests have shown that this tight "squish" dimension (0.035" to 0.045" including piston deck height and compressed gasket thickness) will allow the motor to operate without detonation on lesser quality fuels than a motor with a wider squish or no squish at all, as in the case of the '72 Ford D2VE cylinder heads. Ford had to drop the static compression ratio down into the 7's to prevent detonation on that motor. Needless to say, they only used that head for one year. The following year, they cast up the D3VE heads, using +/-95cc chambers and a squish pad. The clearance is just enough to prevent a piston to head collision at high engine speeds. You are setting the clearance cold with the motor at rest, but at speed, the crank flexes a little, the rod stretches a little and the piston compression height gets a little taller from the heat of combustion. David Vizard has done exhaustive experiments with the SBC and reports that he experienced contact between the piston and head at around 0.026" squish.

The best piston configuration you can use is a flat-top with valve reliefs cut into the crown. Next best, if you need to lower SCR, is to use a D-cup piston that has a generous squish pad on the piston opposite the chamber side. Either of these designs will make the best squish.

Look closely at this piston crown, the one at the top where it starts off "For budget or stock-style rebuilds". You can see that there is insufficient material on the crown to mate up with the underside of the head. The piston is cut into a "dished" configuration with no squish pad. I would not use this piston on a demolition derby car, much less a daily driver.
Sealed Power Cast Pistons - SummitRacing.com
Here's another example of a piston with no squish pad....
Sealed Power Z536NP - Sealed Power Cast Pistons - Overview - SummitRacing.com
Here's an example of a piston with a D-cup. See the nice squish pad on the left of the piston in the photo?
Keith Black/KB Pistons KB138-030 - Keith Black KB Performance Pistons - Overview - SummitRacing.com
And you don't have to spend a lot of money to get pistons that will work. Here's a flat-top for a 350 that has the proper 1.560" compression height and also a generous squish pad for under 100 bucks for the set ot 8. That is a deal!
http://www.summitracing.com/parts/SUM-17350-30/

Now, here's the way it works. The block deck height on a SBC is nominally 9.025" from the centerline of the main bearing bore to the decks where the heads bolt on. When choosing parts, you try to put together the tallest stack you can that will fit into that space. Let's use a 350 to keep it simple. The stroke is 3.480", so the crank radius is half of that, or 1.740". The rod is 5.703" center to center and the piston has a compression distance (centerline of the wrist pin to the flat crown of the piston) of 1.560". Added together, we find a "stack" dimension of 9.003. Fitting this stack into a virgin block where the decks have never been cut produces a piston deck height dimension of 0.022". If we know the decks of the block and the decks of the heads are good and flat, we can use a 0.016" compressed thickness shim head gasket and reach a squish dimension of 0.038" (0.022"+ 0.016"). You can't use these shim gaskets with aluminum heads though, so you must cut the block decks to reduce the block deck height so you can use a thicker gasket to prevent brinelling the aluminum.
So, in summation, you could use a 0.022" piston deck height with a 0.016" gasket or you could use a 10105117 GM composition gasket that compresses to 0.028" together with a piston deck height of 0.010" or you could use a 0.039" gasket with a zero piston deck height (you have to cut the block decks 0.022" in this case).