A tale of 2 pistons

On December 20th 2015 I was on the way home from the New England Norton Owners (NENO) Christmas party, it had been warm on the way to the party, almost 65 DF, later that same day, about 4 PM it was getting down right cold so I decided to take the interstate rather then the back roads I used to get there. When I was within 2 miles of my exit I pinned the throttle, the engine was running good and had 1200 miles on it with numerous re-torqueing and checks of the various adjustments. Just as the Autometer speedo indicated 105 I felt the engine loose power. I turned on the reserve tap, but it did nothing to get my power back. I pulled into the right lane and slowed way down, by now I had lost one cylinder. I was able to nurse it home the remaining 6 miles; I knew if I stopped it would not re-start, and it was really cold now. I made it back to my shop and was impressed how well it functioned as a 375cc single. I let it cool, pulled the plugs and found zero compression in the left side, still it turned over.

I'm sure I did something wrong and was disgusted, so I let it sit; I finished another project and business has been brisk, so it has been relatively easy to ignore, but it kept eating on me. Fast forward to one year later. I pulled the top end down yesterday, here is what I discovered:

The motorcycle:
http://s173.photobucket.com/user/s70mt5 ... ort=3&o=12

The damaged piston:
http://s173.photobucket.com/user/s70mt5 ... g.html?o=1

The good piston:
http://s173.photobucket.com/user/s70mt5 ... g.html?o=3

The pair:
http://s173.photobucket.com/user/s70mt5 ... sort=3&o=8

All I can think of is an air leak, any other suggestions /thoughts?

Ouch :shock: Looks like it "kissed" a valve to me; and then a piece of the piston broke and wrecked that whole cylinder. Perhaps at the higher than normal speed and higher than normal RPM ... it had no clearance and ... "BANG" !!! Really sorry, such cool and nice lookin' Commando too 8) What do the tappet adjusters look like ? Equal distance between rocker and where it meets valve, etc. How does the head in general look ??? Sorry again

It was the best of pistons, and the worst of pistons.......
IMHO, it looks more like a seizure than valve interference. Especially with that top ring bent out of shape.

Stephen Hill

nortriubuell said:

Ouch :shock: Looks like it "kissed" a valve to me; and then a piece of the piston broke and wrecked that whole cylinder. Perhaps at the higher than normal speed and higher than normal RPM ... it had no clearance and ... "BANG" !!! Really sorry, such cool and nice lookin' Commando too 8) What do the tappet adjusters look like ? Equal distance between rocker and where it meets valve, etc. How does the head in general look ??? Sorry again

Click to expand...

The head looks a bit gnarly on the left side, the right side looks normal, the combustion chamber, on the left side, looks OK (salvageable), but is peppered with metal particles, the exhaust valve is covered with sandy looking metal particles, conceivably form the piston top. I spent a lot of cheese to achieve reliability so I am a bit freaked out by the results of what I assumed would be a "normal"full bore run. Back in the day I had a 1970 Roadster that would pull redline in high gear (19 tooth counter) indicating 115 (Smiths speedo).

The tappet adjusters are mushroom types, and all are intact. In fact, I checked the valve clearances after the melt-down and all were where I set them to at .006/.008. So, did the piston break-up or did the left cylinder run too lean, don't know. My re-build plan is to question the forged pistons with long rods, maybe to go for the oversize forged pistons, or just re-do the lower end with standard rods/cast pistons and use the Commando for 50 to 80 MPH back roads with a mellow 34mm Mikuni carb, not sure. I fantasize about commissioning Steve Maney to build me a cracking 920 street engine and just be done with my performance obsession, not sure. My body hurts; I have the money, not sure how much time I have. I told my wife if I couldn't ride, I'd learn how to fly, that didn't make her any happier...

RoadScholar said:

nortriubuell said:

Ouch :shock: ... I told my wife if I couldn't ride, I'd learn how to fly, that didn't make her any happier...

Click to expand...

Click to expand...

I agree with Stephen Hill ..... seizure. If you suffered a bent valve, that happened after the seizure.

Is the color of the failed piston actually as in the pics, or is that some effect of the Lighting? The color is weird .... I cannot put a cause on it (the color).

The failed piston seems to be carboned up, which indicates a rich mixture.

Possible causes:

failed cylinder bore .001 to .002 smaller bore than other cylinder. That is, less clearance on failed piston.
Insufficient end gap on rings

Do the spark plugs indicate a mixture variance between Cylinders?

Slick

I just read your next post: the knackered up cylinder is salvageable. Just sand smooth and go on. You mention forged pistons....... some forged alloys require more piston clearance .... .0045 is considered minimum for the usual cast alloys. What clearance did you use in your build?

You mention turning on the reserve tap after the slow down. Was the speed run done with a single tap open?

Glen

RoadScholar said:

All I can think of is an air leak, any other suggestions /thoughts?

Click to expand...

Check also that the main jet(s) and fuel supply mesh filter(s) are not blocked or obstructed, taps also.
Be good to verify that the spark timing was correct too.

Yes, you keep mentioning forged pistons - did this have these, and did they have the needed clearance ?

worntorn said:

You mention turning on the reserve tap after the slow down. Was the speed run done with a single tap open?

Glen

Click to expand...

That could very well lead to fuel starvation and seizure...especially if the tolerance was a bit 'tight' between cylinders and pistons...

The chocolate colour could indicate overheating prior to the seizure.

Sorry for you and good luck!

Sheet!!! Sorry to see that.

That's not a seizure, that's detonated it self to pieces,

I turned the reserve tap on as soon as I noticed the power loss; I let off on the throttle while I was doing this then pinned it again and quickly realized that if fuel starvation was the cause alleviating it wasn't going to be the cure. I was sure that I would be loosing the right cylinder at almost anytime and wanted to get off the interstate.

Skirt clearance was set at 5.3 thou with gapless rings. The issue wasn't seizure.

The photo color isn't super accurate, the left piston is black and cooked, torn up and uglier than 3 days in jail. I am damned lucky it didn't break up between the time it failed and the time I shut the engine down. The failed piston's top looks like it was modestly sprinkled with gold flecks. The right plug looked very healthy, light to medium brown. The plug in the failed cylinder was black and, I believe, of little forensic value as it was a "passenger" for about 6 miles.

Detonation? why wouldn't the condition be uniform with both cylinders?

Carbs were new Amal 32mm and set for Combat calibrations w/230 mains. I had used the motorcycle about every two weeks so the fluids were fresh. I had tested for a lean surge shortly after hitting 700 miles, if anything I might have been a tad rich.

When I assembled the engine I did set the deck height at flush with the jug, I suspect that the lower compression allowed the cylinders to retain more heat, not sure. But I keep coming back to the right cylinder, that looks so good. Carbs, the same, compression, the same, plugs, the same...everything the same, yet the left piston looked like it did duty as one of the ceramic tiles on the space shuttle.

Would be good to see pics of the pins, they contain clues as to the temps that were in the combustion chamber. The current pics are not detailed enough but the good piston looks to have a blue pin and the bad one a straw pin, if correct then both were overheating, one more than the other, but can't remember if blue is hotter than straw or vica versa.

Found it

http://www.anvilfire.com/article.php?bo ... rdness.htm

Blue is hotter than straw, which is odd if the colours are as they look in the pics.

If it ran great right before it went south, it was probably an air leak.

kommando said:

Would be good to see pics of the pins, they contain clues as to the temps that were in the combustion chamber. The current pics are not detailed enough but the good piston looks to have a blue pin and the bad one a straw pin, if correct then both were overheating, one more than the other, but can't remember if blue is hotter than straw or vica versa.

Found it

http://www.anvilfire.com/article.php?bo ... rdness.htm

Blue is hotter than straw, which is odd if the colours are as they look in the pics.

Click to expand...

I think the pins might be the black DLC coated variety, hence their dark appearance.

kommando said:

Would be good to see pics of the pins, they contain clues as to the temps that were in the combustion chamber. The current pics are not detailed enough but the good piston looks to have a blue pin and the bad one a straw pin, if correct then both were overheating, one more than the other, but can't remember if blue is hotter than straw or vica versa.

Found it

http://www.anvilfire.com/article.php?bo ... rdness.htm

Blue is hotter than straw, which is odd if the colours are as they look in the pics.

Click to expand...

Both pins are a dark blue/black, same as when I installed them; it's a coating called diamond like (DLC). No doubt that the left one got really hot, but looking at the pin and the small end you'd not know it. the small end of the right cylinder looks like it took more heat than the cooked cylinder; both rods are forged steel, as is the crank.

RoadScholar said:

I turned the reserve tap on as soon as I noticed the power loss; I let off on the throttle while I was doing this then pinned it again and quickly realized that if fuel starvation was the cause alleviating it wasn't going to be the cure. I was sure that I would be loosing the right cylinder at almost anytime and wanted to get off the interstate.

Skirt clearance was set at 5.3 thou with gapless rings. The issue wasn't seizure.

The photo color isn't super accurate, the left piston is black and cooked, torn up and uglier than 3 days in jail. I am damned lucky it didn't break up between the time it failed and the time I shut the engine down. The failed piston's top looks like it was modestly sprinkled with gold flecks. The right plug looked very healthy, light to medium brown. The plug in the failed cylinder was black and, I believe, of little forensic value as it was a "passenger" for about 6 miles.

Detonation? why wouldn't the condition be uniform with both cylinders?

Carbs were new Amal 32mm and set for Combat calibrations w/230 mains. I had used the motorcycle about every two weeks so the fluids were fresh. I had tested for a lean surge shortly after hitting 700 miles, if anything I might have been a tad rich.

When I assembled the engine I did set the deck height at flush with the jug, I suspect that the lower compression allowed the cylinders to retain more heat, not sure. But I keep coming back to the right cylinder, that looks so good. Carbs, the same, compression, the same, plugs, the same...everything the same, yet the left piston looked like it did duty as one of the ceramic tiles on the space shuttle.

Click to expand...

Perhaps we could draw analogy between your experience and the experience of Son of Sir Eddy this past fall at Bonneville, where a single fuel tap caused one of his cylinders to essentially vaporize the piston. The bike ran perfect and set records with the 1st set of pipes designed to provide lower RPM and higher torque (the torque set-up) and when these pipes were switched out for the high RPM pipe set (requiring higher fuel delivery to support higher rpm operation), the engine ended up running slightly lean on one cylinder, in turn increasing the cylinder temp which caused detonation and ultimately vaporized one piston. IMHO would be consistent to suggest you experienced the same outcome, with the exception that you ended up with a piston to show us whereas Son of Sir Eddy did not.

There is an indisputable truism:

"They always run best just before they blow up"

Check the flow from the tap on reserve, there may be muck or other in the bottom of the tank :shock:

Sorry the mice got at your piston :!:


I suggest that when you reassemble the bike and get it running you check the ignition timing on both cylinders, and on a dark night use a color tune to check the mixture .

Bernhard said:

Check the flow from the tap on reserve, there may be muck or other in the bottom of the tank :shock:

Sorry the mice got at your piston :!:


I suggest that when you reassemble the bike and get it running you check the ignition timing on both cylinders, and on a dark night use a color tune to check the mixture .

Click to expand...

Before I removed the tank I drained the fuel from both taps; I got a steady stream from a 1/4" ID line, it took about 10 to 12 minutes to drain a gallon from each tap. I do wonder with the reserve closed if there was an air bubble between the left carb and the reserve tap preventing full fuel flow?? Don't know.