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Cast pistons in thumpers

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I want to hear some founded opinions about cast pistons in 4-stroke MX applications. I have heard they aren't as reliable as a forged piston but no one has given me any proof or theories as to why. I also heard most OEM pistons are cast, is this true? I have a 13.5:1 pro-x cast piston, fresh cylinder, and gasket kit ready to be installed in my 09' KX250F with a Pro Circuit top end (Ti Valves, cams, JE 13.5:1 forged piston). I use 50/50 mix of VP c12 and have a PC exhaust with proper jetting. Should I have any worries about putting in the cast piston? I am considering just putting new rings on the current forged piston but if it looks worn I will have to replace it. Just want to know that the cast piston will not jeopardize the integrity of my motor. Thanks :thumbsup:

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OK there are going to be alot of people who are going to have there own ideas of what is best, problem is things are not what they were 40 years ago. I have been a machinest I have owned my own shop and have over35 years welding machining and bending aluminum. I am not however a metallurgist so this link should tell you all you want to know and more. cast will be harder, forged will be softer, in other words one will be more brittle than the other,and I hate using that word because both will be malleable to point,(flex or move and return to shape). I would have no problem with eitherhttp://www.motorcycleproject.com/motorcycle/text/cows-pistons.html

Edited by thumpharder

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This month's issue of MXA discusses exactly this issue--cast vs. forged pistons in 2 and 4 stroke applications. It's a pretty good article. I didn't pay much attention to the 4 stroke part (I'm a smoker guy), but I think it said that cast pistons are not used in high-performance 4 strokes. Check it out.

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Factory piston in CRF Honda's are forged. I don't have nd expdriance with this on mx bikes but on a larger auto engine the cast pistons are more likely to break where as the forged do not. Aka cast is more brittle as mentioned above.

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I would use forged pistons over cast, and for fuel, I would use U4.4 or C16 in that bike. C12 is leaded and from what I've been told by a VP Rep, 4 strokes dont like leaded fuel. I believe it hurts the valves or valve seats.

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Unless there was manufacturing flaw, I can't see any piston breaking with proper valve clearance and regular maintenance. The piston will only be used for one season anyways. I think I will use it since it only cost $60.

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I would use forged pistons over cast, and for fuel, I would use U4.4 or C16 in that bike. C12 is leaded and from what I've been told by a VP Rep, 4 strokes dont like leaded fuel. I believe it hurts the valves or valve seats.

As far as this comment? U4.4 is also a leaded, oxygenated fuel that is an awesome fuel. C16 is a drag fuel that is commonly used in engines that are supercharged, use nitrous oxide, and compression ratios exceeding 16:1. Anyone using a fuel with an octane rating of 117 in a MX application is doing more harm than good. One of my closest friends is a large distributor for VP in the northeast. Your rep is very wrong. C12 is a great fuel, not the best, but a proven cheaper race gas that I've been using for 8+ years with great results.:thumbsup:

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As far as this comment? U4.4 is also a leaded, oxygenated fuel that is an awesome fuel. C16 is a drag fuel that is commonly used in engines that are supercharged, use nitrous oxide, and compression ratios exceeding 16:1. Anyone using a fuel with an octane rating of 117 in a MX application is doing more harm than good. One of my closest friends is a large distributor for VP in the northeast. Your rep is very wrong. C12 is a great fuel, not the best, but a proven cheaper race gas that I've been using for 8+ years with great results.:thumbsup:

I went back to VP site, you are correct, I'm wrong. As far as the C16 doing harm, I dont think that it can do any harm on your engine if jetted properly, you might waste alot of money, but thats it. I'm no fuel expert, so I take the advise of those who know or think they know what they are talking about. I was using C12 in my son's KX85 mod which is a high compression bike and requires race fuel. I recently switched to cam2 (110) which is a little bit lower in octane and I know its not the same quality as C12 but its working. I had to go up in the main jet, but thats it. Thanks for setting me straight.

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After having multiple bad experiences with cast pistons, none of which were ran past the service limit, I will not put anything other than a forged piston in my bike.

As to why forged is stronger, I'm not really sure. I believe cast pistons are made from a melted metal which is poured into a cast and left to dry, where forged is cut out of a solid block of metal which is cut and drilled unitl it is the right shape. I could be way off in left field on this one, but I do think it is correct info, so someone may want to do some research before taking this as 100% correct.

There was a video a while back showing a Wiseco piston being made, from the raw metal block to the finished product. It was quite a cool video and definitely worth checking out. I only ran Wiseco in all my 2 strokes, and now on my KX250F I will be replacing the stocker with a Wiseco or other forged when the time comes to replace the piston.

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Cast is poured, but injection molding can also be called cast in so much as it is a liquid forced into a mold, injection is a better cast than straight poured, forming the rough shape, then finish machined.

Forged is hot struck, forming a tight grain,and forming the rough shape, then finish machined.

Billet is maching the part from a sold bar that was extruded under presure.

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I want to hear some founded opinions about cast pistons in 4-stroke MX applications. I have heard they aren't as reliable as a forged piston but no one has given me any proof or theories as to why. I also heard most OEM pistons are cast, is this true? I have a 13.5:1 pro-x cast piston, fresh cylinder, and gasket kit ready to be installed in my 09' KX250F with a Pro Circuit top end (Ti Valves, cams, JE 13.5:1 forged piston). I use 50/50 mix of VP c12 and have a PC exhaust with proper jetting. Should I have any worries about putting in the cast piston? I am considering just putting new rings on the current forged piston but if it looks worn I will have to replace it. Just want to know that the cast piston will not jeopardize the integrity of my motor. Thanks :busted:

Hi PJ - Forging aluminum removes inclusions, cracks, and porosity from the casting process, condensing the material and improving the mechanical properties. The forging process also improves the ductility of the material, this is where the metal will bend before it breaks, which is why cast aluminum will nearly shatter when the mechanical loads exceed the capability of the design. Compare this to forged aluminum, which will only bend and yield.

As for any current high performance four stroke dirt bike or ATV, the OEM piston is formed using a forging process. You will only find cast pistons in lower performance engines, where the mechanical loading is lower, and the demands on the engine are less. A forged piston for these applications is an upgrade, or when the engine is modified with higher compression ratio, or a larger bore size, this is where a forged piston is most preferred.

I hope this information helps!

Dave

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