What caused this damage?

[boscoe99]

Just a tiny pin pressed into a piston to keep a ring from rotating. What could have happened to cause the steel pin to be forced upward into the aluminum piston?

[robert graham]

Maybe it wasn't pressed in tightly enough, causing just a bit of up and down movement and wallowing out the hole?....

[boscoe99]

I can't imagine any normal force trying to move the pin. Normally it just sits in its hole. Do you think that inertia could be doing this? Pin has almost no mass/weight at all so I am not suspecting inertia but...

Is there any possibility that the piston top gets so hot it is near its melting point and contributing to the pin being able to deform the hole? Seems though that if the piston got that hot it would expand and score itself of the cylinder wall.

I am puzzled.

[DennisG01]

Could the ring have started by getting "just a bit" underneath of the pin, and then every time the piston went "down" the cylinder (intake/exhaust), the slight drag of the ring against the cylinder wall could push up on the pin. Thousands and thousands of cycles and it eventually created that wear/hole pattern?

[pstephens46]

I thought rings were supposed to rotate? If they get carbon fouled then they are "stuck" And then create issues. I wonder what engine this piston lived in?

[DennisG01]

I thought rings were supposed to rotate?

It was likely an attempt to keep the two rings from rotating such that the "gaps" on both rings lined up with each other, thereby allowing excessive blowby.

[pstephens46]

It was likely an attempt to keep the two rings rotating such that the "gaps" on both rings lined up with each other, thereby allowing excessive blowby.

Which would make sense as it appears to only have two rings.

[TownsendsFJR1300]

Could the ring have started by getting "just a bit" underneath of the pin, and then every time the piston went "down" the cylinder (intake/exhaust), the slight drag of the ring against the cylinder wall could push up on the pin. Thousands and thousands of cycles and it eventually created that wear/hole pattern?

+1 ^^^^ is my vote...

Never seen that before BUT that is the only force on that pin. Whether the piston got hot/soft and allowed that pin(which is HARDER THAN THE aluminum piston is a possibility)....


Note, for TWO strokes, the rings DON'T ROTATE or they'd get hung up in the cylinder ports (and break). Thus they have a pin to keep them from moving about..

Four strokes, there are no cylinder ports, so yes, there should be some movement (W/O any consequences)


.

[boscoe99]

I thought rings were supposed to rotate? If they get carbon fouled then they are "stuck" And then create issues. I wonder what engine this piston lived in?

It is from a Yamaha HPDI. Two stroke rings are pinned to keep them from rotating so they won't snag on an intake or exhaust port as they pass by.

[boscoe99]

Different piston but same movement of the pin.

[pstephens46]

One of the edges of the ring is pretty deformed. One little rounded section right at tip looks to have been doing the shoving.

Silver area on piston around pin, no carbon. Sign of great heat? Erosion. Hpdi roasting away?

[TownsendsFJR1300]

Also, notice the upper edge of the piston ring cutout, it's deformed downward.

With the heat damage (failure) of that piston, that pin started and continued to wallow out that hole upward and downward before it failed...

Wether the piston ring retainer pin got hotter than the aluminum piston itself would be the answer I'd like to know.

I wonder, if the end of the ring, over time, worked it's way UNDER the pin and eventually pushed the pin up, cracking the piston. You can see where the pin is NOW flush with the top edge of the ring..

[DennisG01]

I did not know that... about the reason for the pinning was to avoid the ports with the edge of the rings. Are ALL 2-stroke engines pinned?

[TownsendsFJR1300]

I did not know that... about the reason for the pinning was to avoid the ports with the edge of the rings. Are ALL 2-stroke engines pinned?

Yep or the rings would rotate and catch a port...

[DennisG01]

Yep or the rings would rotate and catch a port...

So even things like weedwhackers and chainsaws? Curious minds, you know?