Thread: Another newbie needing SBC 350 build help

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The first thing is yes, they need to measure the same corner to corner, but you also need to know the block deck height so that you can determine the stack of parts that you will use in the block as well as the piston deck height (measurement from the crown of the piston to the block decks where the heads bolt on with the piston at top dead center). We're pretty sure we will need to machine the block decks, but we need to know by how much.

Here's where we're headed with all this: All of the major aluminum cylinder head manufacturers specify a composition head gasket for use with their aluminum heads and most measure from 0.039" to 0.041" compressed. This compressed gasket thickness just so happens to fall right into the range of squish/quench that most all hot rod engine builders shoot for, 0.035" to 0.045". Although there are some hard-headed fellows out there who don't see the reason for composition gaskets with aluminum heads, the fact is that the aluminum heads will move around quite a lot more than the cast iron block that they will be attached to. So, in order to allow the head to move more than the block, we need the thicker gasket. Now, that's where the other part of this comes together.....

From the factory, the 350 Chevy and all other Gen I small block chevies will measure ~9.025" block deck height. That's the design blueprint measurement. In reality, the block could measure any other measurement from the factory. All it takes is for the mill operator to accidentally leave a small iron chip in the register from the previous operation and you have a catty-wampus block. And the other variable is that if you have not owned your block from the day it was manufactured at the factory, then you have no idea what other owners have done to it. Measurements on the four corners could be anything at all and if they are not the same, then the heads will sit catty-wampus on the block and the intake manifold will sit catty-wampus on the heads. If the intake manifold is not sitting square on the heads, there can be a gap either at the top of the head ports or at the bottom of the head ports. If the gap is at the bottom, you'll never find it with a combustible agent and the motor will suck oily crankcase vapors into the cylinder every time the intake valve opens. If you were to pull an oily spark plug out of the motor, you would begin to suspect that the valve guides were too loose or the valve guide seal was not doing its job, but the truth of the matter is that is could be the bottom of the ports that are not sealed up because the whole block is out of whack.

Now, since we know that we want the squish/quench to be somewhere between 0.035" and 0.045" and we also know that we want to use a thicker composition gasket to prevent fretting the soft aluminum, it makes sense that we must engineer the piston deck height (measurement from the crown of the piston to the block decks where the heads bolt on, with the piston at top dead center) to end up with what we want. Let's say that the stack of parts we want to use measures out to 9.000". That's 1.74" for the crank radius, 5.7" for the rod length and 1.560" for the piston compression distance. Add those 3 values together and you will find a stack measurement of 9.000". Now, we must cut the block decks so that we will end up with a 0.035" to 0.045" squish/quench to make the motor detonation-resistant on pump gas. For instance, if the block deck height measures out to be 9.020" on all corners, then we would want to cut the block decks down to 9.000", the same measurement as the stack. This is called zero-decking. This leaves the piston crown exactly even with the block decks and the squish/quench is then determined by the compressed thickness of the head gasket. With a zero deck, if you were to use a gasket with a compressed thickness of 0.039", then the squish/quench would be 0.039". If you were to use a gasket with compressed thickness of 0.041", then the squish/quench would be 0.041". If we used the 9.000" stack of parts in the 9.020" block deck height block, and then used a 0.041" gasket, then the squish/quench would be 0.061", too wide to control detonation on pump gas. It is also possible to find piston that have an increased compression height that might make a stack of 9.005" or 9.010" or even 9.015" If I were to encounter a block that was square and measured out to 9.020, for instance, and I had a stack of 9.015" I would probably not cut the decks, and use a 0.039" gasket for a 0.044" squish/quench.

Most machine shops will be reluctant to cut the block decks to zero because they feel that the block may need some correction down the road and they don't want to cut the block so that the piston pushes out of the block at TDC. Truth be told, there is nothing wrong with pushing the piston outside the deck surface by a few thousandths, so long as the top to the top ring is still sliding on the cylinder bore and not the head gasket. The other truth is that 350 blocks are cheap and easy to find.

This photo shows how you would measure the block deck height at home, with a 12" caliper. This photo shows using a digital caliper, but my favorite tool is a dial caliper. The problem with anything electronic is that every time you reach for it, the batteries are dead. The last 12" dial caliper I owned, I bought used on ebay for $80. You would measure all four corners as shown in the photo, LESS BEARINGS, then add the RADIUS of the main bearing bore to your finding with the caliper. the diameter of the main bearing bore for a 350 Chevy is 2.641", so the radius would be half or that, 1.3205". So, you would measure from the main bearing bore to the block deck, then add 1.3205" to your finding. That would be the block deck height for that corner of the block.
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If you can't understand what I'm trying to teach you here, say so and I'll give you my cell number so we can talk it out. If you don't want to do that, then just measure the block decks on all four corners and post the finding here. We can work this out.

If you had waited to buy headers, I could have counseled you not to buy headers with thin flanges. You want minimum 3/8" flanges to prevent the material from curling up like a potato chip and spitting out the gaskets. Slow down dude, you're messin' up because you don't know what you're doing. The other thing I would not have advised you to do is to purchase those E-Street heads. They will flow about the same as a set of L31 iron production heads.

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