January 2012 Chevy High Performance Q&A

Kevin McClelland Nov 23, 2011 0 Comment(s)
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Piston selection for long-rod short-stroke SBC

Q. My technical question involves the accurate selection of off-the-shelf forged pistons for a long-rod small-block Chevy—although your answer may apply to all Chevy engines. I am trying to build a small-block Chevy with a 3.25-inch stroke forged crankshaft, 6.25-inch rods, and a bore of 4.030 inches. Originally, it appeared that SBC pistons for a 3.50-inch stroke and 6.0-inch rods would work because the sum of the stroke and rod length was 9.5 inches, the same as the 3.25-inch stroke crank and 6.25-inch rod combination I would be using for the build. However, when I discussed this with a performance parts retailer, they indicated that pistons for a 3.75-inch stroke crank and 6.0-inch rods were what I needed. Is there a formula to determine what stroke and rod length combinations are equivalent for piston selection? The difference in price and availability of custom pistons and off-the-shelf pistons can be substantial.

Roger
via email

A. One of the luxuries of Chevrolet engines is the interchangeability of the Gen I small-blocks and the Mark IV big-blocks. You can build many combinations with off-the-shelf components like pistons with odd rod lengths. It’s pretty simple to work through the number to see if you have a usable combination.

You have established the components that you have as the 3.25-inch stroke crankshaft, and the 6.25-inch-long rods. Next, you know what type of deck height these components must work with. A factory small-block has a deck height of 9.020 inches, and if it’s been performance built, it’s usually cut down to around 9.00. When you’re working out the compression height requirement of the piston, you take half of the stroke of the crankshaft. This is because for rod length and compression height you are working from the crankshaft centerline to the top of the deck. Half of your 3.25-inch stroke is 1.625 inches. Next, add the length of the rod (6.25 inches) and you come up with 7.875. Finally, subtract that number from your total deck height, which we’ll use as 9.020 inches for now. This leaves you with 1.145 inches. This is the piston you’re looking for. This is a very short compression height, which would put the wristpin right up against the bottom of the head of the piston. Also, with the pin this high you’re probably into the oil ring. Both of these can cause problems, depending on what you wish to use this engine for. If it’s a drag race engine, no problem. If you’re trying to build an endurance-type engine, you will get a lot of heat transferred into the pin from being so close to the head of the piston. In addition, oil control can be a challenge with oil ring segment spacers.

Back to your off-the-shelf pistons. Your performance parts retailer was dead on. What you need is a 383-cid small-block piston built for 6.0-inch rods. This piston has a compression height of 1.125 inches. In our above calculations, we used a deck height of 9.020 inches. If you clip the deck 0.020 inch down to 9.0 inches total deck, the piston will be at zero deck height. We’d bore the block and mock up these components before clipping the deck. Make sure you know how far your piston is down in the hole before you cut. Remember, measure twice and cut once. JE Pistons offers this exact piston in a flat-top, 0.030-inch-over bore under PN 181911. Give your piston manufacturer of choice a call; they will have what you’re looking for. Good look with your small-block recipe.

Source: Jepistons.com

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