Anyone have this happen to a ProX piston before?



The guy I bought the bike from said he put an OEM in the bike...apparently he did not.

Never been a big fan of Cast pistons...this is why. What a mess they make....

Had something simular on my 07 rm one month after i bought it new and about a month after rebuild. Found an almost invisable hair line crack in the cylinder between exhaust ports. Replaced the cylinder and the bike rans for two years like a dream after. The crack was so small even US Chrome missed it when they replated the cylinder.

Really worn cylinder and piston causes skirt slap and whammo

It's hard to tell with this photo but it looks like a fairly warn piston. Like Paul said skirt slap and whammo!

That's what he does to his wife too!

You nasty old men! lol

Piston actually looked really good (except for the broken part now).

Cyclinder looks nice and clean, no wear marks or discoloring.

Well, the guy I bought it from said it was a new piston, but who knows...

What does the other side of the piston look like?

I don't have a pic, but it looked good. That's why I am scratching my head a bit.

The piston looked fairly new. Crank and main bearing look great too, I am going to measure them and the cylinder tonight to see if something is off.

I am also going to really take a look at the piston. It did show some wear (chipping) on the hole were it broke.

Yeah, I almost always run OEM pistons...They have never failed me.. Only aftermarket ones I'd trust would be JE pistons or Vertex. At least that's what I'd run, other than OEM. I guess it could happen to any piston though if the cylinder and piston were worn bad...like Paul said, again,...whammo!

I have heard this from several sources now:
"Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers."

I agree Kballer ran a vertex all last year in my CR250 on pump gas, and ran it liked to be run (like all two strokes) super lean. I took it out and it looked almost brand new. I was pretty impressed with it.

I've also read the ProX and Art are the same OEM cast pistons.

For a 2-Smoke you can't go wrong with the Athena branded Wossner forged pistons however Wiseco seems to have outgrown the Siezeco moniker and makes a fine forged piston as well.

AFAIK Vertex is a cast piston.

The J&E I put in my street bike were high quality forged pistons.

Pro X are ART pistons ....which ART is OEM supplier for pistons .... prox just buys direct from ART ....did you check ring end gap yet??? that will instantly tell you if the bore is worn....

hey kimball remember that rm piston that i pulled out of wanat rm cracked skirt and all and it still ran when i pulled it !!!!

So after looking super close with a magnifying glass...I noticed a couple hair line cracks about 1/4 to 1/2 in from the break on the edge of the piston. Also, the piston wall looked just a touch thin on the leading edge of the break...almost like it wasn't cast with the correct thickness??? One other thing that was odd, any edge on the body of the piston had tiny chips missing along the edges. The piston just does not seem right. The Cylinder is in great shape..you can even still see the score marks of the original criss cross pattern that comes from the factory in most of the surface. IDK...gonna throw a forged Wiseco in there and see what happens...

If you have access to measuring equipment, I'd still mic the bore. Perhaps it was done oversize. Did you check for end play in the rod? The only big thing to remember with a forged piston in a 2 stroke is to make sure the bike is WELL warmed up before romping on it. The cast cylinder and the forged piston have different expansion rates and you risk a seizure. That's kinda how Wiseco got their bad rep.

Thanks Paul...I will have it Mic'd for sure. The rod is surprisingly tight, no up and down movement with the normal 1/16-1/8 inch movement at the top of the rod. The main bearing is nice a shinny with no discoloring. I always let the 2-stroke warm up to the point that the air is pretty hot coming out of the exhaust before I go ride it...

I just had the same thing happen to my sons 04 kx250. All stock, it looked just like that. I think it was just to much time on our motor. All the pieces went to the bottom and cracked the case. Nice. I hope the ProX is ok. Just rebuilt with an oversize ProX piston. Seems to be running strong so far.

Perhaps it was the wrong piston and the skirt was hitting on the crank? Be interesting to compare the new and old?

So I took the Cylinder and Piston in to my local mechanic...who was a Team Yosh mechanic back in the 80's...

He couldn't believe that there wasn't even a scuff on the cylinder was anywhere from the piston breaking. He said the cylinder looked good, with just some normal wear in the intake area's, but nothing bad at all.

He had 2 thoughts...the piston was just a bad cast or it is the wrong size.

I get my Wiseco tomorrow and we are gong to mic it and see how it fits. Stay tuned....

So, as a follow up, this is what I did...

Cleaned the cases out (with 2-stroke gas) blew them out with compressed air. Used parts cleaner on the main bearing and all around the rod...then used compressed air again. (Amazing the residue that is on it).

My local mechanci mic'd the piston to cylinder clearance...right in the middle of the range. He stated something about .0024 clearance, which is good.

Anyway, replaced with a Wiseco (at the request of my mechanic) since he states with the cold weather of Montana the Cast are pretty brittle in colder temps. During the summer they are fine, but its not uncommon to start a race in the low 20's this time of year.

I plan on breaking it in after work today...so lets keep the fingers crossed.

That piston looked awfully smooth. Pretty worn looking to me. Everyone has their own opinions and preferences when it comes to everything from pistons to oil. So there is no clear cut answer from a mechanical or engineering stand point. Personally, I feel forged pistons are louder with more piston slap until properly warmed up. I've seen far more forged pistons (Wiseco especially) Fail, Seize, and shatter than I have cast pistons. Cast pistons generally allow for tighter tolerances. My experience has been stock is best!! Stock is cast. If a Pro-X is cheaper than Pro-X is best b/c it is a stock piston. The failure could have been a lack of lubrication. Could have been a lot of things, but the piston looks awful gray and smooth. In other words it looks pretty worn. You mentioned the way you warm your bike until you feel warm or hot air come out of the exhaust. This may explain the broken piston as well. The piston as I said looked worn. If your warming the bike the way you described you may have cold seized the worn piston resulting in the broken skirt. The exhuast coming out of the pipe will be warm far sooner than the engine is. Not a good way to warm the bike. When warming you bike up, especially with forged pistons (more critical) I would hold the throttle just above idle and let it warm up intil the left radiator is fairly warm/hot to the touch. The cylinder will be fairly warm/hot as well. By this time the engine, cylinder, and piston have all had time to properly warm up and expand. If you don't take the time to properly warm the bike up and start revving the engine before it is warm the piston will heat up and expand quicker than the cylinder resulting in the cold seize. I do not mean to preach or be little. Please do no take offense. I was just using the facts givin in the post. Just my .02 I hope this helps.