Grenade!!!!!!!!!!!!!!!!! Warning, GRAPHIC content!!!

[Sprintcar]

Sorry Brad, I've got to go with my gut on this one. It appears to be impact damage. Get the saw back and measure the OEM skirt against the BB 046 piston skirt. In looking at the remains of that piston and the intake side skirt, it looks like high RPM crank counter weight damage.

Just my WAFG on this, but thats what it looks like to me.

 

[blsnelling]

I remove a fair amount tapering the windows, which often uncovers the pin. In between the windows, on the bottom side, I only smooth out casting imperfections.

 

[blsnelling]

Sorry Brad, I've got to go with my gut on this one. It appears to be impact damage. Get the saw back and measure the OEM skirt against the BB 046 piston skirt. In looking at the remains of that piston and the intake side skirt, it looks like high RPM crank counter weight damage.

Just my WAFG on this, but thats what it looks like to me.

The skirt is way shorter than a 440 piston. I've got a Wiseco 440 piston and a aftermarket 460 piston I'll compare it to.

It definately wasn't high RPMs. This was the first startup of a new topend and was only being heat cycled. It saw no WOT or H tuning, only a couple mild blips of the throttle.

 

[Sprintcar]

The skirt is way shorter than a 440 piston. I've got a Wiseco 440 piston and a aftermarket 460 piston I'll compare it to.

It definately wasn't high RPMs. This was the first startup of a new topend and was only being heat cycled. It saw no WOT or H tuning, only a couple mild blips of the throttle.

Then it had to be a flaw in the piston around the wrist pin bosses. The good race car engine builders are using dye penetrate and other non-destructive inspections on both pistons, wrist pins and rods at each engine shop visit. You would be amazed as to what that will show in micro fractures.

Great thread, thanks for starting it.

 

[ironman_gq]

Is it possible that the locating pin loosened up after the bore was uncovered for some reason, maybe a casing flaw? That might allow the ring to spin like it had and drop an end into a port. It does look a lot like the bottom ring caught first judging by how badly the lands are broken up between the rings.

 

[blsnelling]

I greatly appreciate yours, and everyone's, input.

In the picture below are the 046BB on the left, Wiseco 440 in the middle, and white box aftermarket 460 on the right. The measure left to right respectively, 1.440", 1.555", 1.556".

 

[blsnelling]

Is it possible that the locating pin loosened up after the bore was uncovered for some reason, maybe a casing flaw? That might allow the ring to spin like it had and drop an end into a port. It does look a lot like the bottom ring caught first judging by how badly the lands are broken up between the rings.

Neither locating pin is out of pisition.

 

[Sprintcar]

Are all the wrist pin compression heights the same? I would measure from the base of the skirt to the center of the wrist pin boss and compare the three.

Jerry

 

[Tzed250]

The way the pin bosses are fractured shows that only a force perpendicular to the fracture line could have caused the break. The only thing that is above the pin bosses that could have applied such force is the wrist pin.

Nobody has mentioned the difference between a four-stroke and two-stroke piston. The pin bosses on a 4T piston have to accept loads from above and below. The above forces come from compression and combustion. The forces from below are the negative acceleration near TDC on the exhaust stroke.

The 2T piston on the other hand does not ever see the decel loads near TDC, as it is always in the compression phase at this point. The lower area of the pin bosses on a 2T piston can be made very thin as no significant loading should normally take place in this zone. The pin bosses were fractured because loads were applied to the bottom of the pin bosses when the ring hung in the port. The pin did not have to leave the pin bores, it only had to apply enough force to cause plastic deformation of the metal. Castings will take very little strain before fracture.



.

 

[blsnelling]

The way the pin bosses are fractured shows that only a force perpendicular to the fracture line could have caused the break. The only thing that is above the pin bosses that could have applied such force is the wrist pin.

Nobody has mentioned the difference between a four-stroke and two-stroke piston. The pin bosses on a 4T piston have to accept loads from above and below. The above forces come from compression and combustion. The forces from below are the negative acceleration near TDC on the exhaust stroke.

The 2T piston on the other hand does not ever see the decel loads near TDC, as it is always in the compression phase at this point. The lower area of the pin bosses on a 2T piston can be made very thin as no significant loading should normally take place in this zone. The pin bosses were fractured because loads were applied to the bottom of the pin bosses when the ring hung in the port. The pin did not have to leave the pin bores, it only had to apply enough force to cause plastic deformation of the metal. Castings will take very little strain before fracture.



.

Very interesting and enlightening post. I do agree that at some point the ring hung in the intake. That's when the skirt was busted off, along with the pin bosses, as the piston travelled up. But I'm still not convinced that's where it started. I still think it started near the transfer, damage was done that then allowed the ring to fall into the intake. Just a theory though.

 

[Sprintcar]

The way the pin bosses are fractured shows that only a force perpendicular to the fracture line could have caused the break. The only thing that is above the pin bosses that could have applied such force is the wrist pin.

Nobody has mentioned the difference between a four-stroke and two-stroke piston. The pin bosses on a 4T piston have to accept loads from above and below. The above forces come from compression and combustion. The forces from below are the negative acceleration near TDC on the exhaust stroke.

The 2T piston on the other hand does not ever see the decel loads near TDC, as it is always in the compression phase at this point. The lower area of the pin bosses on a 2T piston can be made very thin as no significant loading should normally take place in this zone. The pin bosses were fractured because loads were applied to the bottom of the pin bosses when the ring hung in the port. The pin did not have to leave the pin bores, it only had to apply enough force to cause plastic deformation of the metal. Castings will take very little strain before fracture.



.

Excellent analysis.

 

[Zombiechopper]

The way the pin bosses are fractured shows that only a force perpendicular to the fracture line could have caused the break. The only thing that is above the pin bosses that could have applied such force is the wrist pin.

Nobody has mentioned the difference between a four-stroke and two-stroke piston. The pin bosses on a 4T piston have to accept loads from above and below. The above forces come from compression and combustion. The forces from below are the negative acceleration near TDC on the exhaust stroke.

The 2T piston on the other hand does not ever see the decel loads near TDC, as it is always in the compression phase at this point. The lower area of the pin bosses on a 2T piston can be made very thin as no significant loading should normally take place in this zone. The pin bosses were fractured because loads were applied to the bottom of the pin bosses when the ring hung in the port. The pin did not have to leave the pin bores, it only had to apply enough force to cause plastic deformation of the metal. Castings will take very little strain before fracture.



.

This makes the most sense of anything I've read in this thread thus far. I am not an engine guru but this logic really seems clear.

 

[Brian13]

I agree with Sprintcar that the skirt shattered on the down stroke. I just have no idea what it hit. I would think if it hit the crank both skirts would receive similar if not the same damage. And I definitely think the ,alloy was off, the breakage almost looks like it was cryogenicly froze with all the small shards. Looks to be too brittle to me. I would think there would have been more deformation and gouging at low rpms. Thats my take anyway, Im by no means an experienced mechanic but thats what I see with my limited understanding. Thanks for taking the time to post all the pics Brad.

 

[Tzed250]

Very interesting and enlightening post. I do agree that at some point the ring hung in the intake. That's when the skirt was busted off, along with the pin bosses, as the piston travelled up. But I'm still not convinced that's where it started. I still think it started near the transfer, damage was done that then allowed the ring to fall into the intake. Just a theory though.

The ring was hung in the intake when the fractured rear section of the piston was pushed down and into the intake. A ring failure may be the root cause. Remember that rings are castings also.




.

 

[blsnelling]

...This allowed the ring end to drop into the intake port. As the piston moved back up, the lower ring caught on the top of the intake port, and broke the entire skirt off.

I've said since post #26 that I believe the lower ring dropped into the intake port. It then caught the top of the port as the piston moved up. There's nothing for the ring to catch on on the way down on the intake port. Also, that would be the opposite force requred to strip the skirt off the piston. The wrist pin was pushing up, the ring stopped at the top of the intake port, and the skirt popped right off, taking the pin bosses with it.

If this were the only damage, that would be the end of the story, IMHO. But there's that big chunk that was broken off the top of the piston. It broke off as the piston was moving down. After carefully lining up the piston in the bore, I believe this happened first, and then allowed the ring to drop into the port.

 

[blsnelling]

The ring was hung in the intake when the fractured rear section of the piston was pushed down and into the intake. A ring failure may be the root cause. Remember that rings are castings also.




.

Say again? I didn't quite follow that.

Wouldn't that be something if it were a ring failure, lol! And here I used Caber rings instead of those that came with the kit, which I've never had a problem with, lol.

 

[Andyshine77]

In the video I see a long fracture under the bottom ring near the ring pin, it looks different than any of the other pieces, it looks more like it was ripped apart rather than bent.

 

[blsnelling]

TZed, after reading again, I think we're agreeing, just saying it differently. No? You believe the ring caught as the piston moved up, stripping the skirt off the piston?

 

[Tzed250]

.


Another thing to think about for those wondering about all the crumbled pieces.

Brad stated the the saw was coming back to idle or was at idle when the failure occurred. With a normal saw idle of ~2500RPM if the saw only continued to rotate for another 1/4 of a second that is an additional 10 revolutions. Plenty of time to make crumbles.


.

 

[blsnelling]

In the video I see a long fracture under the bottom ring near the ring pin, it looks different than any of the other pieces, it looks more like it was ripped apart rather than bent.

That's where the entire skirt was ripped off as the piston went up, and left the skirt behind.