What causes the piston to fall apart?

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fidiro

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I just took apart a RedMax EB7001 and found the piston shattered at the skirt. I am still able to measure it and it comes up to about 47.48 at skirt and 47.39 above rings. There is still wear lines throughout most of piston so it did not wear out thin.

What I did find odd, and I'm not a professional mechanic, is that the piston pin is tapered thinner on ends where piston would ride and pin is thicker where piston bearing sits. Pin almost looks like it was professionally tapered equally on both sides of bearing. So it had a sloppy piston from either a worn pin or something. Piston is Zenoah, made in Japan so it is all original including pin. Is the material used to make the pins not stronger then the aluminum piston? Also, is the crankshaft supposed to slide some from side to side, I'm getting about 1/16" from side to side not up/down.

It sucks that I just picked this one up along with a EBZ8000 and was hoping to find just worn rings on the 7001 and found more then I was hoping to find which includes a small crack at the bottom of crank that pushed the gasket out. I'm hoping to JB weld that little crack to get it running.

I still have to tear down the 8000 hoping for something better but I hear a grinding and fear the bottom end needs attention.

I'm going to get my pics uploaded here in a bit so you guys can look at it.
 
Here's a couple phone pics of the damage of this 5 year old unit.

Did this piston pin wear out that led to this damage? I just can't imagine why the aluminum piston would not wear out first before the pin itself if that is the case here. What are the pins made of, is it a softer metal then the piston itself?

I know this is a blower and has nothing to do with cutting wood but I figure I would share the quality of different top manufacturers.

I have to find a piston kit for this thing to see if I can get it running to keep around for personal use. Anyone have any idea of aftermarket kits for RedMax?
 
I bought these two units from the original owner and if it was run lean I would see scoring in intake side of P/C. Jug is flawless and piston is like I described above except for the cracks it has plenty of the thin wear lines still visible throughout. Unless running lean attacks just the piston pin itself on some engines, but doubt it. If the pin is not designed to be thinner where it comes in contact with piston when installed then it is a defect in the material used to make the pins for these machines.

I've been trying to find aftermarket kits for Redmax but nothing comes up. Looks like it's going to be a dealer purchase if I can't come up with anything.
 
? I don't see any indications of lean.

What's the bottom of your intake port look like?

I just looked over the jug real quick and didn't see anything that will stop me from reusing it, it is flawless. But I will give it a better look to see if there is something that the piston caught on and post up tomorrow.

BTW, is the slight sliding of the crank normal? I can push in and out about 1/16" but no up down play.
 
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Some wrist pins are taper, but only on the inside. The pin is MUCH harder than the piston. Are you saying the piston is actually loose on the pin? Pics of the pin would be good. Your piston looks like something was injested through the intake. That's why I asked what the bottom of the intake port looks like in the cylinder.
 
Some wrist pins are taper, but only on the inside. The pin is MUCH harder than the piston. Are you saying the piston is actually loose on the pin? Pics of the pin would be good. Your piston looks like something was injested through the intake. That's why I asked what the bottom of the intake port looks like in the cylinder.

I'll get a measurement of the pin tomorrow. But it is tapered thinner on at least one outside end and I'm pretty sure it is both ends are thinner then the middle, I really need to confirm that tomorrow.

I know this is a used unit and it's 5 years old but I'm just trying to figure out what caused this so that I don't put it back together and have it happen again since jug and piston wear is not bad.

I'm going to have to throw piston back on with pin to see how much wiggle it really had, I was in a hurry to pull it all off to clean off the debris and didn't register the play of piston
 
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Redmax is a brand name common to the USA

Redmax engines are made by Zenoah of Japan.

In the rest of the world they are called Zenoah.
 
It is common to see some end (front to rear) play on the rod at the wrist pin. There should be a cupped washer on each side of the rod/wrist pin bearing. The cupped part goes towards the bearing.

Some Zenoahs seem to have some end play on the crankshaft to crankcase but they have shims available for this on some size engines.

I have a Zenoah with no end play on the crank to crankcase but the same crank in another crankcase has enough end play to feel without a shim.

The arrow on top of the piston goes towards the exhaust.
 
I'd guess Brad is right on. Something on the intake side hit the piston to cause that damage. Thats not just some normal slap.
 
Yep, Brad pointed this in the right direction. When I gave the jug a quick look I was looking over the area where rings ride and put it dow right away to start cleaning up the bottom end. Jug is still useable but there is a sign something got caught.

Pin measured 11mm @ center where bearing rides, 10.75mm at one end and 10.88 at the other end. Don't see how this size pin works to keep piston from slapping around unless it did wear out which then made the piston catch the bottom of intake. With the filter setup this thing has it's hard for something to get by unless a boot came loose from the intake and it sucked in something then. IDK, owner didn't tell me the details just said he thinks the rings were hung in there as it was locked up.

Here's a few more pics to let you guys be the judge of what could have happened. I didn't find any foreign objects anywhere inside.

Also the pic of the bottom outside crack is what I'm hoping JB Weld will take care of.
 
tapered wrist pins

Piston.jpg


wrist%20pin.jpg


The wrist pin needs to be strongest were all the force is concentrated in the middle.

Tapering of the bore of the pin is done to reduce the reciprocating mass of the engine.
 
Piston.jpg


wrist%20pin.jpg


The wrist pin needs to be strongest were all the force is concentrated in the middle.

Tapering of the bore of the pin is done to reduce the reciprocating mass of the engine.

But how can the pin get a snug fit on piston if both ends of pin are tapered thinner. This pin I can see being tapered on one side of piston but the rest of it should be exactly 11mm, then there is only one way to install it to the piston. Piston would then be 10.75 on one side and 11mm on the other. What I'm seeing with this is a worn pin on the one side with 10.88mm

I guess I'll see what the new piston and pin look like and measure when I finally order them.

If pin is worn on one side would that be enough to slap piston into catching the intake? Or is it more like something got sucked in?
 
I dont think this is your problem, but I rebuilt a jeep engine a while ago, and did not clean up well enough after re honing the bores, must have left carbrundum dust in some of the oil ways, because when it got to the big ends it lodged in the soft bearings and on a five mile test drive managed to re-grind 30 thou off a the crank shaft, and lost all oil pressure again. (I was not happy!!!):mad:
 

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