[Mike 40M]

Back from race. Third after a Suzuki T20 and a Greeves. The Honda came in heavily breathing blue smoke from the catch tank. Last race for the season, so plenty of time stripping the engine to see what has happened. Best fourstroke anyhow.
Better luck than than for a guy in a supermono class. Engine exploded at the finish line making a big oil spill. He fell off after the finish line and thus got his second place.

 

[Mike 40M]

Took out the engine. Since I have some other bikes to fix, it is now attended by a fellow racer, who has a lot of experience of tuning and preparing CB twins.

 

[Mike 40M]

Got a X-mas package. Engine back from tuner. No big problems, he changed pistons and cylinder, rest of the engine seemed OK. Never seen piston and barrel worn this way. Left cylinder has 4 deep indentions at TDC and BDC, each side. Too deep for overboring so he got another cylinder from eBay. Top piston ring in several pieces, second ring tight fit but in one piece. Oil ring quite OK. Piston pins OK. Piston crown has some signs of excessive heat. Looks as piston has melted over and under the piston pin, both sides. Right hand piston had both rings in pieces. Any suggestions why this has happened?

 

[TrialsRider]

Poor fit of the wrist pin followed by catastrophic wrist pin spring clip failure?

 

[Cyorg]

Hard to diagnose things from photos, but there are a few reasons why a circlip comes adrift. Could have been the wrong clips were installed, wrong clip diameter, wrong wire diameter so the clip doesn't sit all the way home in the groove, clip was damaged during installation or the rod is out of whack. A bent or damaged rod, or a small end that isn't running true to the big end can cause the wrist pin to move back and forth hitting against the clip. Eventually the clip and or groove will fail. The clip will erode the piston as shown in your photo and the pin will run up against the cylinder wall.

 

[Mike 40M]

Sorry, was not able to take better pictures. One of the circlips is intact. I was not told about the other one. The wristpin has not damaged the cylinder wall. When I restored a 1930 Harley 45" back in -63, a wrist pin had made a long straight cut in a barrel. In this case the dents in the cylinder is about 4mm wide and a about 1mm deep. The four dents differ from 2mm to 5mm in length. No damage worth mentioning between endpoints of stroke. If the wristpin had come loose it would have made two long cuts for all of the stroke, eventually progressing to a single big cut. The wristpin is not damaged in any way, except some discoulouring in the middle, indicating excessive temperature.
You can see the intact circlip on the photo.
If a circlip was to blame it should first make damage on one side and then went through the wristpin and made its work on the other side. And only at both end of stroke.
I think it must have been the engine gremlins.

 

[Cyorg]

I didn't realize that was the clip still in there, I thought it was light reflecting off the end of the pin. It is odd.... the only time I have seen erosion like that on a piston, it has been from either a circlip or a piece of ring. You stated that the oil ring was ok and the second ring was intact. Pieces of the top ring wouldn't likely get in there without some sort of obvious trail past the lands and the two lower rings. Have a look at the top piston ring land and see if it shows any witness marks (above the pin area) from a end piece of the compression ring being snapped off during installation. How does the circlip groove look on the other side? Are there any odd marks on the inside of the pin compared to the pin from other cylinder? Something was bouncing around in there.

 

[TrialsRider]

Then I'm sticking with tight fit on the wrist pin, the steel got hot enough to melt piston,
unless you have reason to believe there was foreign material in there.

 

[Mike 40M]

On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
I'm still curious about the four distinctive pits in the cylinder.

 

[Cyorg]

On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
I'm still curious about the four distinctive pits in the cylinder.

Again hard to tell from photos, but I assume the four pits line up with where the pin changes direction at the top and bottom of the stroke. If that is the case....When the rod is moving up or down, the debris will more or less stay up against the opposing side of the pin or pin boss, but it is still likely to leave witness marks (scoring) in the liner between the pits. When the rod changes direction, that's when the shrapnel starts to create the pits. Those eroded troughs in the top and bottom of the pin boss will force the debris out against the wall. That why you get more wear top and bottom. That area on the piston is not melted, it was eroded/peened by whatever was bouncing around in there.
Didn't the engine builder have an opinion? He took it apart, so had the best kick at the cat.

 

[XB33BSA]

Got a X-mas package. Engine back from tuner. No big problems, he changed pistons and cylinder, rest of the engine seemed OK. Never seen piston and barrel worn this way. Left cylinder has 4 deep indentions at TDC and BDC, each side. Too deep for overboring so he got another cylinder from eBay. Top piston ring in several pieces, second ring tight fit but in one piece. Oil ring quite OK. Piston pins OK. Piston crown has some signs of excessive heat. Looks as piston has melted over and under the piston pin, both sides. Right hand piston had both rings in pieces. Any suggestions why this has happened?
View attachment 96209
View attachment 96211

we need pictures that we can actually see some detail
the cylinder picture is shit,you are awesome you are not shit just the picture
but did i mention,the pictures you gave us suck ?
please make good pics for us
it is very interesting damage you have there

 

[Mike 40M]

Agree, that barrel pic is shit. I'm not good at taking pics. But since you asked, I made another try
Upper left side

Upper right side

Lower left side

Lower right side

Sorry that the lower marks are upside down.

 

[Cyorg]

So all 4 pits line up with the pin boss area of the piston at top/bottom of stroke... yes?

Who did the cylinder deglazing when that engine was assembled the last time? Whoever did it left out some longevity and hp. Maybe not much hp, but every tiny bit comes in handy if you are racing it.

What does your engine builder have to say about it? As mentioned, He is the one who gets the best shot at determining exactly what was bouncing around inside there and why. When a new engine was released, we used to keep a small stock of complete plug and play assemblies. If an internal problem ever did occur, we could ship out a complete engine and get the knackered one back unmolested. That way there was less risk of valuable information getting lost during disassembly.

 

[TrialsRider]

Motor slowly dismantling itself, very small pieces at a time commencing with an unhappy gudgeon pin offerings to the god of speed.

 

[Mike 40M]

So all 4 pits line up with the pin boss area of the piston at top/bottom of stroke... yes?

Yes.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.

 

[Cyorg]

Yes.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.

Not that it matters in the grand scheme of things, but I still believe something was thrashing around in there. At this point you'll never know what it was. Something got in there during assembly. The extent of pitting does suggest it was perhaps something more substantial than 1 circlip. I will wager a testicle that the area around that circlip is not melted. You mentioned that the pin was still a slip fit in the piston. That would be less likely if temperatures got that high. The piston in that photo doesn't show any melting of the crown or down the side above the pin boss. There is nothing that would cause that area to melt without the rest of the piston going he same route. The area around the pin is thicker than the crown, so if things were melting there wouldn't be much of a crown left. Get someone to take a macro photo of that area of the piston and post it or look at it under magnification. It will show that area has been peened, blasted, eroded away, not melted. The clip is just caught up in the rearranged aluminum.
Here is an example of a BMW piston with similar erosion.

http://bmwvmca.org/joomla/index.php?option=com_content&task=view&id=291

 

[XB33BSA]

MY THEORY AND IT MAY HOLD WATER
for whater reason a small chunk of piston alloy,that fractured loose fom the area outer to the dcirclip groove
maybe the wrist pin reached a back and for hammering that stress cracked off a small chunk
of course up and down goes the piston and at the change of direction the recip,the minor bit of metal gets wedged and rolled up to go the other direction
after couple hundred 100,000 recip motions the balled up metal is eroding iron liner material
for sure there was never anything close to melting temps

 

[TrialsRider]

Content warning, may be disturbing to some viewers:
https://cdn2.ms-motorservice.com/fileadmin/media/MAM/PDF_Assets/Piston-damage-recognising-and-rectifying_51730.pdf

 

[Cyorg]

MY THEORY AND IT MAY HOLD WATER
for whater reason a small chunk of piston alloy,that fractured loose fom the area outer to the dcirclip groove
maybe the wrist pin reached a back and for hammering that stress cracked off a small chunk
of course up and down goes the piston and at the change of direction the recip,the minor bit of metal gets wedged and rolled up to go the other direction
after couple hundred 100,000 recip motions the balled up metal is eroding iron liner material
for sure there was never anything close to melting temps

A chunk of piston alloy wouldn't erode the cylinder wall to that degree. How would a piece of cast aluminum small enough to come from that eroded area and small enough to travel through the inner diameter of the pin cause 4 divits that size? If there was a chunk of piston pin missing, I would assume he would have mentioned it.

 

[Cyorg]

Mike hasn't answered the question about who assembled that engine originally (as in who did that "cylinder deglazing"). That first photo (post 32) highlights the deglazing and suggests whoever assembled the engine was very inexperienced. The image in the photo even leaves room for speculation that the so called deglazing was done overtop of the pitting/scoring. Is that an illusion..... or did he use 60 grit stones and the crosshatch is that deep? Did the previous owner throw together a sick engine? If so, you would think that the pits in the wall would be covered in combustion crap, but........ Again, it's a low resolution photo, so who knows. Some history on the engine might help.