Maintenance & Technical > KX500 Original
This is why you don't run a CAST piston
don46:
--- Quote from: quincyman on June 24, 2007, 02:55:43 PM --- When I saw those pictures and the caption that cast pistons do this I became concerned because I have just installed my first cast piston. If it is the very nature of cast pistons to break apart then I don't plan on running one for too long. But I put the money out and didn't want to panic for no reason. Hence the question.
--- End quote ---
Just so we're clear, you say you have just installed your first cast piston. Stock pistons are cast, Wiseco are forged. With 5 bikes, I think this was a misunderstanding becuse surely you've changed pistons before now and if you've used stock, then you've used cast. For my money it's only wiseco, both 2t and 4t.
quincyman:
--- Quote from: don46 on July 06, 2007, 03:35:51 AM ---
--- Quote from: quincyman on June 24, 2007, 02:55:43 PM --- When I saw those pictures and the caption that cast pistons do this I became concerned because I have just installed my first cast piston. If it is the very nature of cast pistons to break apart then I don't plan on running one for too long. But I put the money out and didn't want to panic for no reason. Hence the question.
--- End quote ---
Just so we're clear, you say you have just installed your first cast piston. Stock pistons are cast, Wiseco are forged. With 5 bikes, I think this was a misunderstanding becuse surely you've changed pistons before now and if you've used stock, then you've used cast. For my money it's only wiseco, both 2t and 4t.
--- End quote ---
When I purchased my 1994 KX500 it was 3 years old. It had a Wiseco piston in it. I replaced it with a Wiseco everytime I did the work. Which was 2 pistons in about 8 years. But this time around I installed a newly replated cylinder in the bike. The folks who did the replating recommended stock pistons (Cast) because of closer tolerance. They claimed greater longevity if the bike is taken care of properly. So I went ahead and let them order a stock piston for the cylinder.
Then I happened to be browsing this list and I saw this thread and the pictures. I now became concerned if I had done the right thing. Not knowing if this was a trait of Cast pistons or if the fellow who blew the piston was to blame I started asking questions. My hope was that this was unusual. I still don't know the answer to that question but enough doubt has been placed in my mind that I will be monitoring this new piston closer than I ever did with any of my Wiseco pistons.
Since I have been asking questions I have found that there are those who adamantly will argue on both sides of this issue.
My bike is running great and I rarely ride in such a way that much stress is placed on the piston. Trail riding is very slow compared to desert. I do ride the KX in desert from time to time but mostly on trails. I will most likely put 1000 miles on the KX this year. It shares riding time between 4 other bikes. I will tear it down and put the magnifying glass to it after 1000 miles.
The only other 2 stroke I own is a 05 KDX220. I replaced the stock piston in it with a Wiseco immediately after talking with Fredette racing. Fredette racing claims that only the 220 needs it's cast piston changed out because the skirts will break. He says the KDX200 stock pistons do not have this problem. So according to him it is not all cast pistons. Here are two bikes that are identical except for the bore and porting.
When my four strokes start wearing out but are still running good I sell them not rebuild them. Or I trade them in on a newer model. I have never had a stock piston go out of a four stroke.
Brett:
With the process of forging the material has to have reasonable flowing qualities and therefore means are eutectic in composition (roughly 12% silicon) This material spec has a higher coefficient of expansion compared to hyper eutectic (16-18% silicon) so therefore has to be set looser when cold to allow more room for temp growth when getting to operating temp. Cast pistons are more often than not hyper eutectic, so run tighter tolerances and have less slap when cold. Hyper eutectic pistons are less ductile then eutectic, so when worn and having excessive clearance are more likely to break than eutectic. But hyper eutectic pistons are much harder wearing due to higher silicon content so will last longer and provide ring seal for longer.
Ok so thats the material comparo, now for structure/design.
Now we are talking two strokes only ignore anything you have heard/know to do with fourstrokes as fourstroke pistons are of completely different design and this design lends itself extremely well to being forged.
So think of the forging process as having a bowl and a lump of playdough in the middle now push your fist into it and the dough flows aroung your fist, now remove your fist and you are left with the dough formed into shape. The major limitation of the forging process is that the die (fist) only gets pushed in and removed from one direction severly limiting the designers ability to have webbing and the like to give it structural integrity. Now a casting die set is generally made up of 3 or 5 core inners, they are clamped together and the melt is cast around it. Then in the case of a 3 piece core set the innermost section of the core is removed, the one of outer-inners if that makes sense is slid across to where the middle one was then removed and same goes for the remaining outer-inner core. Then the 2 piece die set is opened and out comes your piston casting (with riser and ingates still attached. The casting process allows the ability of undercutting pin bosses and creating intricate webbing design.
Now due to my bike having a LA sleeve i am stuck to using wiseco (as wossner dont make em) and from my experiance they collapse (no not break they are ductile but the out of roundness which should be greater in the thrust direction changes to being greater in the pin axis direction) I believe this is due to the piston being quite big with no webbing to speak of at all and being af a more ductile material.
So in summary i recommend using a cast piston ie stock or pro-x, vertex whatever.
If someone could take a photo of the inner of both a cast piston and forged for comparo it might be better understood than my ramblings.
Oh and while your there chuck a micromoter on your old pistons and post the out of round.
BDI:
Basically what you are saying Is when a cast piston reaches the end of It's life cycle It breaks and when a forged piston reaches the end of It's life cycle It becomes deformed? How do you feel about cast pisons In boosted or high compression aplications my bike Is 15:1 and I run Vp cmp oxygenated fuel should I use cast or forged? :? I looked all over and could not find a cast piston to take a picture of but I did have a forged wiseco piston so If any one has a cast piston for comparison here is my picture.
P.S. out of round Is 0.05
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