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Cast vs. Forged pistons

We are VERY often asked the question of which piston our customers should choose given the choice of Forged or Cast. The answer is simple....they BOTH are Great choices and ultimately its up to the person building it. The engineering and application factors are FAR too in depth for this short discussion and for those looking to truly understand piston manufacturing and design there are volumes of research and data to be found and digested.

MCB supports our customers with the choice of BOTH styles and even within each choice we have several additional options! Dual-Ring, Single ring, Turbo, Drag race, Anodized, Coated, etc. we have you covered.

So then it becomes how does the average consumer make that choice? First we need to outline what exactly is the difference between them. A cast piston is just that, a mold is made and molten alloy is poured producing a near net shape piston. A forged piston is produced by taking a precisely heated billet and with only a few blows in a LARGE hammer press in to a die, producing again a near net final shape. This is where the strength of a forged piston is achieved. Both pistons are then machined externally (and the forging is often INTERNALLY machined) and then heat treated.

2 Stroke vs 4 stroke: In MOST ALL situations a Forged piston is the preferred choice for any 4-stroke Performance application. Its just that simple. The benefits FAR outweigh any negatives here. Much of what is written here is directed at 2-stroke choices and as such will not apply to 4-strokes.

Alloys - Both Cast and Forged pistons are typically Aluminum alloys with varying materials added to achieve the metallurgic properties specified . The choice of these alloy components is absolutely key to achieving a balance of wear properties and machinability . For instance our 2-stroke forgings are a 4032b which actually has significantly less rapid expansion than many of the other forged pistons. This allows a forged piston to utilize almost cast piston bore clearances. The final heat treat of these forgings are also a patented and highly proprietary process that GREATLY helps with the stability under heavy loading.

Ductility and Tensile strength - These are KEY terms to describe the strength of a final piston product. Forged in most cases wins all around here as these properties are often 25-30% better BUT...and this is important, in a Two-Stroke application (particularly in cold environments) the cast piston will often retain its final shape at wider temperature swings. Remember though that once a forged piston is brought to chamber temps it DOES NOT grow any more and studies show the piston to be MUCH more consistent with the distribution of heat within the forging. This is one of the reasons why the forging is more effective at transferring heat into the cooling system.

Severe duty - This again is where you may want to look at using a forged piston. Turbo, High Rpm, Heavy Mountain loads, Nitrous, Drag Race, etc. all impart a significant ADDITIONAL load on things like pin bosses, ring lands, and skirts. I have seen dozens of our custom Forged pistons after a season of use under these conditions and even if worn the key is they are still in ONE PIECE. If you rattle a cast piston under these conditions you will likely shatter the casting causing the loss of a crank or worse. This is also why Rotax adopted the Diesel indutry design of a Ductile Iron Ring liner to try and keep the ring lands stable thus increasing life.

Is there any benefit to using a forged piston in typical stock, trail use? Yes and no. If there are issues with the OEM design and we have changed critical features or optimized the piston for its intended use then YES use our custom forging. For instance the modern ArcticCat carbed Race 600 I spent the better part of a year with design and testing to produce a piston that we feel is the best Competition parts made for that engine. Another example is the Polaris 600CFI, we changed several key offsets and final sizes with the results being the customers always mentioning how much smoother the engine operates.

When should I use a cast piston? If you prefer the OE style piston, or the machine is gently used, or will be operating in EXTREME cold temps, then you may decide a cast piston is for you. Ours have excellent wear characteristics and Cast is the original MCB Dual-Ring design that has been copied for many years.

Perhaps the MOST important consideration when deciding which piston will provide the best results is NOT the piston itself but rather the KEY dimensions of Bore SIZE, bore condition (plating wear), bore finish (hone pattern), and general engine condition. If your engine was running well and the piston you remove show no distress, and your bores are measured PROPERLY and within spec, then your piston choice is easy. If you have had a failure and think that by replacing the pistons with EITHER cast or forged is going to solve the issues then it doesn't matter what you choose. Pistons typically don't just decide to seize or fail other than simply being worn beyond functional spec.

Think of it this way, the piston is only a FUSE....if it failed, putting another fuse in will likely lead to the same problem, or worse! You must diagnose the root cause and take all necessary steps to insure your rebuild is going to last.

- Scott G.