Thread: Chevy 350 build question

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Building a motor is no different than "paint by numbers". You begin at A and end with Z, as outlined by Mr. Vizard.
Most fellows though, will stop short of doing it properly. This entails squaring the block and cutting the decks to fit the "stack" of parts you will use.

Pistons are built with a certain "compression height", or CH. This is the meaurement from the centerline of the wrist pin to the crown of the piston, just above the top ring. The blueprint dimension for a 350 Chevy is 1.560" (one inch and five hundred and sixty thousandths). There are pistons that are shorter, down to 1.540" (one inch and five hundred and forty thousandths) and there are pistons that are 350 pistons that are taller, up a thousandth or so at 1.561" (one inch and five hundred and sixty one thousandths).

Small block Chevy rods, with the exception of the 400, are 5.700" (five inches and seven hundred thousandths) in blueprint length. Stock 400 rods are 5.565" (five inches and five hundred and sixty five thousandths) in length and are generally not used in any other motor except the 400. In other words, they are not generally used in hot rod motors except for a stock 400 build. Some fellows will use a longer rod in a 350 build, like a 6" rod (6.000"), but I doubt you'll see much difference in power on a dyno. 5.7" rods will work for most 350 builds. A longer rod would require a shorter piston (6.000" rod length, 1.260" piston compression height and 1.740" crank radius = 9.000" stack height).

The blueprint stroke of a 350 crankshaft is 3.484" (three inches and four hundred and eighty four thousandths), generally shown as 3.48". Half the stroke, or the radius of the crank, is 1.740" (one inch and seven hundred and forty thousandths).

Now, I'll get to the point I've been leading up to. The stack of parts to be used in the rebuild is added together....
1.56" plus 5.7" plus 1.74" equals 9.000", so that is the "stack" of parts that will be used in the rebuild. If, for instance, you used a "fosdick" (my term for anything that is screwed up or cheezy) rebuilder piston with a 1.540" compression height instead of the 1.560" compression height, your stack would measure 8.980".

The blueprint block deck height of a Gen I 350 motor is 9.025" (nine inches and twenty five thousandths), so when you bolt in a stack of parts that measures 9.000" (nine inches), there will be a piston deck height (measurement from the crown of the piston to the block deck where the heads bolt on) of 0.025" (twenty five thousandths of an inch). When Chevrolet built these motors, they used a shim head gasket that measured about 0.020" (twenty thousandths of an inch) thickness. The gasket thickness added to the piston deck height would put the clearance between the crown of the piston and the bottom of the cylinder head at about 0.045" (forty five thousandths of an inch). This 0.045" dimension would be called the "squish" dimension. Us hot rodders generally aim for a squish of between 0.035" (thirty five thousandths of an inch) and 0.045" (forty five thousandths of an inch). This near miss of the piston to the underside of the head squishes, or jets the mixture that is above the piston crown, over to the chamber side of the piston. This jetting of the mixture across the chamber and towards the spark plug creates turbulence that homogenizes the air/fuel mixture and contributes to a better burn of the mixture, creating less tendency for detonation and making more power. A tight squish will also allow operation of the motor on a lower octane fuel without detonation.

Problem with using a shim head gasket on a rebuild is that many times either or both the block and/or heads have been subjected to the heat of operation and may not be as flat and parallel as they were when they were bolted together at the factory. If there is/are any non-flat surface(s) on either the block or the heads, the pressure of compression can find a way out by breaching the gasket and you have a "blown head gasket". Fellows who plan to use a thin shim gasket will generally make certain that the block and heads are flat and parallel by using a precision straightedge and a thin feeler gauge blade to insure that there is less than 0.002" (two thousandths of an inch) variance from one end of the block/heads to the other. If the block or heads are bowed up or down more than that, then you should either cut them on a mill back to parallel or use a thicker composition gasket to compensate for the difference in factory blueprint dimensions. Composition gaskets will begin at around 0.028" (twenty eight thousandths of an inch) installed thickness and run up to around 0.071" (seventy one thousandths of an inch).

Now, we have determined so far that the proper squish (measurement between the piston crown and the underside of the cylinder head with the piston at top dead center and with the head gasket in place) is 0.035" to 0.045". Now, let's assume that the block measured 9.025" and the stack measured 9.000". That leaves 0.025" piston deck height. If we then add a composition gasket to the mix, even with the thinnest gasket we are over 0.050" (fifty thousandths of an inch) and are beyond the design spec of 0.035" and 0.045" for squish. We need to do some cutting on the block.

Have the machinist inspect and measure the main bearing bore for each hole to be round and parallel with the other four holes of the main bearing bore. If the mains are valid, great. If not, have them corrected by the machine shop. You must begin with a valid block or you are just kidding yourself. Once the mains are valid, chuck the block up on the main saddles and cut the block decks according to the stack dimension that you will use. Let's say you will use a 9.000" stack. Cut the block decks to 9.000" and use a 0.040" thickness composition gasket to set the squish at 0.040".

What I have just written for you is mostly unknown to fellows who are building their first few motors. You have a jump start. Good luck.

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