Exploding piston, How much damage can it do?

[Dist34_678]

Hi all,
I was ripping around on the ice on my KDX 220R and the motoro cut out.

It wouldnt kick so I knew it was bad bad news, I figured I had siezed up the crank bearings. I took the motor out and see that the intake side of the piston
was half gone =(. All the pieces that broke off of course dropped into the gearbox. I am guessing that I was running too lean for the cold temps because the ring in it didnt have many hard hours on it.

I havent split the cases but I know there is a ton of **** sitting down there. But
it does seem like the pison is made of a softer metal than then gearing components in the bike. I am hoping there is not major damage that will make me scrap the bike.

What I want to know is how much potential damage I am looking at? Im sure someone here has had the same happen and had to repair it.

[m2g4b]

Bummer on the Piston man.....I had a buddy that was riding along and the same thing happened to him. drop one whole skirt down the motor and it broked into millions of little pieces...this was on a cr250...anyway...I toke it home and took the top end off....instead of splitting the cases, I drained the oil and what not...took the engine out and then took it to a local car wash...pressure washed it out...then took it back to my shop and used a magnet on little pieces that were left and oiled everything down...it spun freely and I could see NO damage since the motor siezed quickly. I put it back together and ran new oil thru it a few times to clean the rest out and it is still running today with no problems....you might get lucky!!! hope so....

[psycho393]

Since your running a KDX and it's a two stroke here's the maximum damage you'd be looking at if your head and cylinder are still intaked. Your crank may be alright but will at minimum need a new rod, and rod bearings at both ends, and crank bearings, and a piston and rings of course. Here's what to look for, split the cases and see what the crank looks like you will have to use common sense as to what you would want running in your bike. If it's questionable replace it. I think Weisco makes cranks for your bike and you'll probably have luck finding one brand new on Ebay. Other then that after your cases are split clean out the crank area and replace the crank bearings. The remains of your piston didn't get into the tranny so don't worry about that but you may as well check for worn parts that need to be replaced while you are into the engine that far. So I'd say your looking at about 225-325 for a crank, 100 for a piston and rings and 60-80 for crank bearings. so about 400-500 bucks.

If you want to save some money pm those crank bearings just get the bearing number off of them once they are out and go to a farm impliment dealer and they will get the exact same bearing for you for about 12-15 bucks a piece. Just a thought, Good luck

[MotoMojo17]

Not to hijack the thread, but how much would the same damage cost on a thumper?

[Dist34_678]

I would be tempted to say that in a 4 stroke it would be less damage.

The reasoning behind that is that the 4 stroke piston, at least the ones I
have in my CRF have much less thin walled metal and are not nearly as tall.

To me less thin walled metel gives less opportunity for the piston to break apart.
And the short height of the piston makes it so there is less piston in there to break apart.

However, as I think about it more if and when the piston does break apart there will be thicker pieces of metal swimming around the gearbox, possibly causing more damage....

I think we need someone with experiance to chime in =)

[MotoMan]

Originally Posted by psycho393

Since your running a KDX and it's a two stroke here's the maximum damage you'd be looking at if your head and cylinder are still intaked. Your crank may be alright but will at minimum need a new rod, and rod bearings at both ends, and crank bearings, and a piston and rings of course. Here's what to look for, split the cases and see what the crank looks like you will have to use common sense as to what you would want running in your bike. If it's questionable replace it. I think Weisco makes cranks for your bike and you'll probably have luck finding one brand new on Ebay. Other then that after your cases are split clean out the crank area and replace the crank bearings. The remains of your piston didn't get into the tranny so don't worry about that but you may as well check for worn parts that need to be replaced while you are into the engine that far. So I'd say your looking at about 225-325 for a crank, 100 for a piston and rings and 60-80 for crank bearings. so about 400-500 bucks.

EXCELLENT POST. I hate to say this but it sounds as though you might have had this happen to you once or twice as well Psyco. I was ready to write this long explaination, but you already did.

[Vintage Dave393]

Let me jump in on the damage caused by a piston exploding.

On a two stroke, the crankcase area is contained. The gearbox would not sustain damage. The damage incurred will normally be limited to the piston, cylinder, crank bearings, big end bearing (rod), crank rod, and if it happens with enough force, possibly the cases could crack.

On a four stroke, add all of the above and you can add valve train damage, which would include valves, camshaft, rocker arms, cam chain, and since the oil is shared on Yamahas, the gearbox.

Normally a piston will crack from a lack of maintenance. This can be an air filter that is too dirty to filter or one that is not sealing well. It can also be just a case of running a piston too long or be the result of jetting that is too lean. Piston slap is normally the forerunner of a piston failure.

[ossagp]

I would side with VD on both the 2 and 4 stroke. I have broken skirts where all I needed was a piston, rings, gasket and small end bearing to get it going. So start with that. then just throw in head, cylinder, crank, bearings,,,and cases.


Kind of the same with the 4 stroke, except things can get between the gear shafts,,,,don't leave out clutch basket damage. Kind of getting the hint not to ever let it get that far?

andy

[MotoMojo17]

How would you know with a four stroke though?

Oh yeah, and what does piston slap sound like? I guess the compression would be low on four strokes when they need top ends, too?

[Vintage Dave393]

You would know if you did periodic top end tear downs and measured the clearance. As far as what periodic means, it would depend on how much and what type of riding you do.

Piston slap is an abnormal sound from the top end of the engine. Ususally it will be more pronounced when the engine is cold and won't go away completely when it is warmed up. You can identify it best at idle. Sometimes it gets confused with detonation under load. There is no good reason for a piston to break from slap. Slap is just the result of poor maintenance or abuse of the engine.

[psycho393]

Originally Posted by MotoMan

EXCELLENT POST. I hate to say this but it sounds as though you might have had this happen to you once or twice as well Psyco. I was ready to write this long explaination, but you already did.

SSSSHHHHHH who told?

[ossagp]

to note someone mentions bearings from an implement store. make sure the store you are buying from understands classifications and not just the part numbers that relate to the size and seal arangement of the bearing. Classification deals with the amount of alignment offset etc that the bearing will be capable of handling. So if you ask if he is selling you a class 1, 2, 3 bearing and he doesnt know, you had better be familiar with it yourself.

Most bearing houses that specialize in bearings and related items know this. Your John Deere dealer may not. Your AutoZone, Schuck's, NAPA probably wont.