Any ideas on what caused this hole?

Jmoore703

New Member
On the front end of my rebuild on 71 cb500. Started with the engine first then on to the fancy stuff. Bike was running when I bought her but had a small leak on the #2 cylinder. Skip ahead and I get the jugs off to discover a 1in hole in the #2 piston. Found the chunks of aluminum in the oil pan, miracle they didn't get stuck in the gears. Good ol honda has 20k miles and was still running on three pistons.

My question what would cause the hole? My first thought was the valves getting hit but they are fine and seated. An old mechanic said a vacuum effect caused to much oxygen to get in and it exploded causing the weakest point (piston) to give. Any thoughts?
 

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Have you or the PO ever used Ether to start it? Commonly called Quick start? Meant for Diesel engines and too hot for these puppies.
 
Previous owner could have but I doubt it, as it started first kick when I picked it up. I scrolled down a few posts and saw a guy with a 350 is having a similar issue
 
Got any pics of the head & spark plugs? Might help confirm indications of mishaps with the air/fuel ratio.
 
What does the exhaust header look like from that cylinder? And yes what about the valves and spark plug for that cylinder?
 
Lean doesn't always mean you need to jet up. May have been a vacuum leak at that cylinder.
 
A hole like that, with the sand blasted look of the piston is from detonation. Lean can cause this, as well as timing or overheating and getting pre-ignition. Oddly, running rich can cause this too, as carbon builds up, then gets hot, pre-igniting the fuel, similar to a timing issue.
 
Normally I would also say detonation, but in this case it looks almost as if the piston crown is flaking off. That does happen with old pistons sometimes. check under the crown and look at the combustion chamber and down the exhaust. If everything looks like it was spray bombed with aluminum paint, it's detonation. If the sump is full of flakes, the material has deteriorated and it has fallen apart.
 
teazer said:
Normally I would also say detonation, but in this case it looks almost as if the piston crown is flaking off. That does happen with old pistons sometimes. check under the crown and look at the combustion chamber and down the exhaust. If everything looks like it was spray bombed with aluminum paint, it's detonation. If the sump is full of flakes, the material has deteriorated and it has fallen apart.
Light detonation (pinging) can hammer the crown and fatigue the metal too....I suspect pinging rather than a big bang theory....

I've seen some pretty ugly pistons too....a leak by the rings causing what looks like a river along the side of the piston. Caused by lugging the engine....The excess pressure started leaking past a section of ring, then started cutting like a plasma cutter.....

When you thing combustion temps are 1500-2500 degrees and aluminum softens at 900 deg, you winder how any engine can stay together...
 
Detonation shock wave scours the insulating laminar layer of gas off the top of the piston. This layer of gas protects the piston top from the intense heat of combustion. With out the insulating gas the piston aluminum softens and eventually blows through. No matter the cause, the easiest way to protect old motorcycle engines from detonation is using premium fuel at the pump. The only reason for the higher octane is to stop detonation before it starts.

Tom
 
I doubt that timing is the issue, since the 500 uses paired ignition with 1&4 sharing points, condenser and coil and 2&3 also being paired. If one of the pair is good and the other not then timing is not it. I find it unlikely that jetting is the problem unless that single carb is jetted dramatically different than the other 3 for some unknown reason. So to me that leaves an air leak as the likely suspect that caused that cylinder to run exceptionally hot and initiated the detonation. If too low octane fuel were the issue we would see signs in the other cylinders. Look for an intake leak. I would check the rubber connector between the manifold and the carb or for a missing screw for the carb sync port.


Ken
 
Doesn't quite work the way you think.
The polarity difference means plugs do spark at very slightly different times.
I've had similar happen on CB550 but it was no. 3 cylinder, combination of several things just happened to coincide at the same time :(
 
And valve adjustments, actual compression ratio, wear, etc. are not identical. Close, but not identical. Small differences can add up....

For instance, if one cylinder is running rich, carbon buildup can alter the compression ratio on that cylinder, causing issues.
 
All great theories, but in the final analysis, all you have to do is to fastidiously rebuild the motor after you clean and inspect every part. New pistons, valve seats cut, timing checked, air leaks eliminated, etc.
 
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