Q&A / Tech

Mailbag: Guidelines for Choosing Rod Length and Piston Wristpin Height

You’ve got questions. We’ve got answers—the Summit Racing tech department tackles your automotive-related conundrums. This week in our Mailbag, we’re focusing on connecting rod length and piston wristpin height.

Q: I have a 1972 Ford 302, bored .060-inch over. I took a 351W crank and had its main journals machined to 2.2486 inches to fit the 302 block (351W journals are normally 3.000 inches). With a 3.500-inch stroke, the 351W crank should increase the torque over a wider rpm band.

The problem I’m having is figuring out what kind of rods I can use with flat top pistons. What rod length do I need to match the block’s deck height? What piston wristpin height do I need? Can I use 289 rods and Chevy 350 pistons if I machine the 351W crank’s rod journal to 2.1232 inches (302 rod journal size)? Will this match deck height and clear the crank’s counterweights? Is there another rod/piston combination I need to use?

A: We can’t give you a specific rod/piston combination with the information here; however, we can give you some general guidelines to help you pick the right combination for your 302. First, measure your block’s deck height—don’t use the stock figure, because your block may have been decked at some point. Then determine the rod length you want, then the stroke of the crankshaft (which you already know). With those figures, take half of the stroke measurement and add it to your rod length. Now subtract that figure from your block’s actual deck height. That final number is the wristpin height you need for your pistons.

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