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Pro X vs. Wiseco Piston

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I've always used wiseco forged pistons in everything (Japanese and Euro), including my current 250 SX. I don't really understand why so many KTM owners are so high on the Vertex brand (OEM supplier) as it's just another cast piston, but the C clips sure are difficult! If you want to read about cast vs. forged metallurgy and all of that, the current MXA issue has a nice discussion. As a woods rider/racer, I go with a forged piston. I'm mainly interested in durability and reliability, as my motor is going to make more power than needed whatever piston gets used. I've never noted any significant downside to forged pistons. There is a bit more noise at start-up on a chilly morning, but it goes away as soon as the motor is half-way warm.

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The most important thing for you to do is measure the bore.Wiseco only comes in #2 size, and if your bore is too tight for that, you have a problem. Vertex comes in different sizes, so you can get the proper one. It DOES make a difference.

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The most important thing for you to do is measure the bore.Wiseco only comes in #2 size, and if your bore is too tight for that, you have a problem. Vertex comes in different sizes, so you can get the proper one. It DOES make a difference.

pretty sure the wiseco only comes in standard size. In countless top-ends, I have never had piston clearance issues. Ring gap will occasionally need a tweak, but even that is very rare.

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A cast piston is manufactured by pouring molten metal into a mold. The final shape is machined to it's final exacting tolerances.

A forged piston is made by taking a chunk of metal, and beating it into shape with a die-press under enormous pressure. Like casting, the final shape is achieved through precise machine work.

The main difference between a cast and forged piston is the grain structure. A forged piston is beaten into shape, and as a result the metal stretches and compresses as the piston takes shape. The varied, elongated grain structure is like fiber reinforcing, and it makes for a very strong piston. Microscopic cracks don't readily propogate through the structure of a forged piston due to the high density and the irregularly spaced and sized grains.

A cast piston, on the other hand, is made up of grains that are all the same size, because it starts out as a liquid that, after being poured into a mold, undergoes a controlled cool-down process that allows the metal to reach a near-perfect equilibrium right out of the mold. The highly regular grain size and distribution makes them more prone to crack propogation and failure.

The break-in of the two types is very different, because the metal properties are very different. The forging process produces a lot of internal stress from beating the metal into the intended shape. The stress is trapped in the metal of the finished part. A cast piston has lower internal stress, because it was able to seek it’s own internal equilibrium as the liquid metal flowed around inside the mold and then underwent a controlled cool-down. Since a cast piston has lower internal stress, it won’t distort nearly as much as a forged piston will when heated to a high temperature. The forged piston's propensity to distort when heated is the reason they need an elaborate break-in procedure

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