I found this info on thumper talk:
Chokey
Forged pistons are more durable than cast pistons will ever be. Cast pistons are very brittle, and are prone to skirt breakage as a result.
For riders that will take the time to do a proper break-in, and will discipline themselves enough to fully warm their engines before each and every ride, Wiseco pistons are a superior product to OEM. They are more durable under high-heat high-load conditions, less prone to skirt breakage from piston slap if the bore has a bit too much wear on it, and longer wearing than any cast piston will ever be.
Cast pistons don't require much in the way of special break-in, that's part of the reason they come as OEM equipment in two-strokes. Forged pistons, on the other hand, are a different story. A forged piston must be heat-cycled before it can be run hard, or you're playing roulette with your engine.
A new forged piston has to be heat-cycled several times to relieve the internal stresses left over in the metal from the manufacturing process. If this isn't done properly, the piston will distort and become out-of-round, causing it to wedge itself into your cylinder, more commonly known as a "seizure". A seizure with a two-stroke isn't a very dramatic occurance, usually the engine just stops running. You replate the cylinder, rebuild the top-end again, and you're back in action. On a four-stroke, however, a seizure is often catastrophic. The four-stroke has more rotating mass, which creates additional inertia. So, when the piston seizes, the rest of the engine tries to keep spinning. The result is, more often than not, a snapped connecting rod, broken crank, and destroyed cases. A big bill usually follows.
To better understand why forged pistons are more seizure prone, you need to understand what makes a forged piston different than a cast piston.
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 require an elaborate break-in procedure.
To relieve the internal stress, and maintain it's correct shape, the forged piston has to go through a series of heating and cooling cycles. As it heats up, the grain structure will re-distribute itself into alignment to relieve any trapped stress. As it cools, the cylinder will contain and restrain the piston, maintaining it's shape. After a few heat/cool cycles, the internal stress reaches equilibrium, and the piston will no longer distort when heated in the engine. It will maintain it's shape for the rest of it's life.
Forging produces a higher grain density than casting, making the part much more durable under high-heat, high-load conditions. As long as you are patient enough to break a forged piston in correctly, you will have a piston that is more durable under extreme conditions.
Problems with Wiseco pistons are almost always caused by one of these three issues:
1. Improper break-in-This can be a death sentence for a forged piston.
2. Improper warm-up-A great way to cold-seize the engine. This also kills crank and rod bearings, not just forged pistons.
3. Cylinder-to-piston tolerances too tight-You can't just slap a Wiseco piston into a brand new engine with no time on it. Forged pistons require about .002" more cold clearance than a cast piston. Once it's time for the first top-end job, a cylinder will usually have loosened up enough for the forged piston to have enough clearance. Most Japanese two-strokes spec .002"-.004" clearance for the OEM pistons. Wiseco specs .003"-.005" clearance.
Another less common cause of problems with all pistons is when the person doing the top-end work doesn't drill the lube holes in the piston for an engine with a central exhaust bridge. A forged piston will actually tolerate this better than a cast piston.
Anytime you rebuild the top-end, you have to measure the ring end-gap, and file the ring ends if neccessary. I rarely have a set of Wiseco rings that don't need to be relieved slightly. If the end-gap is too tight, the ring ends will hammer the locator pin right out of the piston, with devastating results. Wiseco specs .004" per inch of bore size for their ring end-gap clearance.
All of these problems are caused by the mechanic that does the work, or the rider that's too lazy to perform a proper break-in or proper warm-up. You can't blame the product for your own incompetence (although that's usually what people do).
