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How to Stroke Any Engine - Stroker Science

The long and short of building a big-inch street engine

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Crankshafts

When we talk about stroker motors, it really is all about the crankshaft. But there's more to this than just cramming a big crank in a small motor. There are many issues, but we'll limit this discussion to strength, weight, and applications.

Let's start with a realistic approach to applications, since that will have an impact on the selection process and the cost of the parts. If a mild street motor is all you're looking to build, then a cast crank can be very affordable. Scat, Eagle, and others build very affordable parts. There are plenty of stroker cranks out there for the more popular small-blocks at less than $200. From a metallurgy standpoint, nodular iron cast cranks are more flexible than they are given credit for, and rpm has a big effect on durability. The higher the peak horsepower rpm point, the stronger the crank you should buy.

Forged cranks are stronger than cast. At the top of the affordable forged list are 5140 forged-steel cranks, but they're not as strong as 4340 steel. You'll hear about both twist and nontwist cranks with theories abounding about which is better. The point is that if you buy a 4340 steel crank, it's going to be plenty strong either way. The price difference is directly related to where these cranks are forged. The lower-cost steel cranks are forged in either Argentina or China, while U.S.-made cranks such as those from Crower or Lunati, for example, cost more but are generally of higher quality. You get what you pay for. We don't need to discuss billet cranks because if you think you need a billet then you already know the price of admission.

When considering building a stroker motor, the best choice is to buy a complete rotating package. This makes the critical decisions concerning rod length, piston compression height, and balancing much easier. If you choose to purchase your parts separately, you'd better have a lock on all those (plus about a dozen more) details because it only takes one small oversight to bore a stroker-size hole in your wallet. Companies like Holley sell cranks, rods, and piston combos for nearly any engine.

Definition:
Offset-Grinding

You can increase the stroke on any crank by offset-grinding the rod journal, which moves the journal centerline away from the crank centerline. This also reduces the overall diameter of the rod journals, often requiring custom connecting rods to match.

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