77 shell 750CC holed the piston

[KcKeeny]

my dad got this bike only a few years old and it’s the only bike I remember him owning. It’s now mine after sitting since the 80’s. I cleaned the carbs and changed fluids etc. took her on short 2 mile rides to shake her down and then felt confident to take her up to speed. About 4 miles down the Interstate at about 80mph she blew smoke out one side and died.
I let it sit for a year and took her apart today to find the hole in the piston. My question is that it has a fancy cam and what dad said was dealer installed thuit Shell kit bored over to 750- so how do I replace the piston and what caused the failure?
She’s got flat slide carbs and exhaust etc. I “borrowed” it once when I was 16 and passed a buddy in his car that said he was going 150.

 

[gggGary]

What does that valve look like? First guess is a sticking valve.

 

[WideAWAKE]

Your buddy was lying

 

[gggGary]

Your buddy was lying

We use kilometers in Wisconsin....

 

[KcKeeny]

We use hockey sticks per snow drift up here.
He was a Dr’s kid with a brand new 92 Dodge shadow that he thought was awesome. I know I was holding on for dear life the wind had blown my helmet up so I was looking under the chin of it and thought my hands were going to rip off the handle bars at any second. And I was 6’ 185# hockey player at 16.

 

[KcKeeny]

What does that valve look like? First guess is a sticking valve.

It’s black but doesn’t look like it hit.
The hole does look just like a valve though doesn’t it.

 

[atom4488]

Damage to a piston like that looks like pre-ignition to me, not a mechanical/impact failure. Retarded timing / poor grade fuel (low octane) would cause that, IMHO.

 

[atom4488]

... and the cam is clearly marked "Norris". Norris was a cam grinder back then, since defunct.

 

[Jim]

What does that valve look like? First guess is a sticking valve.

...and Gary for the win. It's faint, but you can just make out the contact mark where the exhaust valve kissed it.

 

[650Skull]

 

[Jim]

And just so we all learn... detonation holes pistons, not preignition. Common causes are too lean, too advanced, too hot... cheap gas.

 

[Jim]

View attachment 249633

Beat ya Doug.

 

[650Skull]

Beat ya Doug.

 

[Jim]

And just so we all learn... detonation holes pistons, not preignition. Common causes are too lean, too advanced, too hot... cheap gas.

I'ma expand on that a bit, 'cause it's all about learning, right?

Preignition happens when there's a hotspot in the combustion chamber that causes the fuel/air mix to ignite prematurely. Hence the name, pre-ignition. Because it's early, you get abnormally high combustion pressures in the cylinder. That can (but doesn't have to) cause enough of a heat buildup to cause detonation. So, preignition by itself typically won't hole a piston.

Detonation is when the fuel/air mixture cooks off all at once. There's no progressive flame front inside the combustion chamber... there's one big BANG... an explosion if you will. Combustion pressures go through the roof and sumpin's gotta give. Usually it's the piston that relieves pressure... in the form of a hole... right in the middle of it... because the crown is the weakest point.

 

[WideAWAKE]

We use hockey sticks per snow drift up here.
He was a Dr’s kid with a brand new 92 Dodge shadow that he thought was awesome. I know I was holding on for dear life the wind had blown my helmet up so I was looking under the chin of it and thought my hands were going to rip off the handle bars at any second. And I was 6’ 185# hockey player at 16.

Haha. I feel all those same feeling going 52 on my Lambretta.

… I was only pokin fun.

 

[atom4488]

I'ma expand on that a bit, 'cause it's all about learning, right?

Preignition happens when there's a hotspot in the combustion chamber that causes the fuel/air mix to ignite prematurely. Hence the name, pre-ignition. Because it's early, you get abnormally high combustion pressures in the cylinder. That can (but doesn't have to) cause enough of a heat buildup to cause detonation. So, preignition by itself typically won't hole a piston.

Detonation is when the fuel/air mixture cooks off all at once. There's no progressive flame front inside the combustion chamber... there's one big BANG... an explosion if you will. Combustion pressures go through the roof and sumpin's gotta give. Usually it's the piston that relieves pressure... in the form of a hole... right in the middle of it... because the crown is the weakest point.

I stand corrected having re-read the definitions of detonation and pre-ignition. With all due respect and IMHO, the extent of the damage to the pictured piston is as a result of some form of improperly controlled combustion (most likely detonation and not pre-ignition as I wrote previously), rather than valve impact. There is too much damage in the form of cracking of the crown, the apparent disintegration of the ring-land and so on.

 

[arcticXS]

I wonder what CR that piston would give. Looks like a pretty high dome to me......

 

[toglhot]

A piston holed by heat will melt. The hole is not showing any melting. Also, the hole has fractures radiating out from it, clearly the piston has hit an immovable object, like a valve. The question is - why.

 

[AUTOMAN]

Detonation-classic
Top land damaged
Valve cut out valley next to land

 

[Jan_P]

View attachment 249633

Dont know anything about this but are you Gentlemen sure about this
Assuming there is marks ( perhaps needs better spectacles )
When the hole comes metal will fly around and land somewhere.
OK It would most likely go downwards but so can the piston do High revving on the next upstroke there can the be parts of the holed piston on top
And on the next turn of the Carousel (as the proverbial ) there are parts that hits the valve

Feel free to correct ..

Secondly if it hit the valve would that not be felt ..now having bent stem