77 shell 750CC holed the piston

[gggGary]

That ring looks suspiciously dirty, carbonized. Very possible "some event" convinced dad something was going bad, he parked the bike and decades later junior finished the job.
Bent valves are often very subtle don't see the bend till it's removed and spun.
The debris from what ever destroyed the piston could bend a valve also. The minimal damage from such a big event is almost odd. Results of a sticking valve event in a "it just stopped running" bike I bought.

In any case might be a lesson here about how quickly to go back to WFO operation in a recently out of the barn bike?
I'm big on internal lubing and several hundred times spinning over the motor by foot or starter before firing it up.
You'll have check the bores then see what piston might be the best fit, desired compression for your use. Hint there are still big bore kits to be found at a reasonable price https://yamahaxs650.com/product/447-big-bore-kit-80mm-750cc/

 

[Jim]

@KcKeeny , A good closeup shot of that piston from various angles and good closeups of the right exhaust valve might give us some answers.

 

[gggGary]

@KcKeeny , A good closeup shot of that piston from various angles and good closeups of the right exhaust valve might give us some answers.

Or at least fuel a wonder thread-post war!

 

[Jim]

Or at least fuel a wonder thread-post war!

Ain't that the truth.

 

[gggGary]

A zero clearance squish band could be the cause also. Dad might never have revved it far enough to find out.
Could be checked on the other side. soft solder will do to check.

 

[Vincenthdfan]

A zero clearance squish band could be the cause also. Dad might never have revved it far enough to find out.
Could be checked on the other side. soft solder will do to check.

Yep, my Service Manager at Yamaha taught me that trick when I was putting a higher lift cam and high compression piston in my TT500 back in the day when I was a kid. He said dont trust the manufacturers, always clay the valve pockets at the very least.

Sage advice...I did what he said and had to dremel a bit more of a relief in the valve pockets, they wouldve surely kissed had I not done so!

 

[KcKeeny]

 

[atom4488]

I see molten metal sprayed about in one combustion chamber and not the other. Even more convinced that the piston failure was due to improperly controlled combustion, not an impact failure. This is probably detonation. Just had a very similar failure on a two-stroke outboard; plug came out sprayed with molten aluminum.

 

[650Skull]

That chamber with the metal in it is the opposite to the one with the broken piston. Can see damage to the front side of the head where it has been beaten with debrie............Be interesting to see that other piston

 

[Jan_P]

That chamber with the metal in it is the opposite to the one with the broken piston. Can see damage to the front side of the head where it has been beaten with debrie............Be interesting to see that other piston

Aeehh .hh sorry to be anti again

But in Picture #1 it is the right side piston exhaust side that has the hole ( can see the oil feed hole at the base ) if I get this right

But in the picture # 27 it needs to be flipped --along a horizontal axis
large intake valve and the studs for exhausts as well is there

So maybe the right side after all and there is damage to the valve at the inside

So mr atom can be right.

?? Taking off the spring and feel how the valve stem slides in the valve guide turning it and test again.
If no problem maybe mr atom has it

 

[KcKeeny]

Aeehh .hh sorry to be anti again

But in Picture #1 it is the right side piston exhaust side that has the hole ( can see the oil feed hole at the base ) if I get this right

But in the picture # 27 it needs to be flipped --along a horizontal axis
large intake valve and the studs for exhausts as well is there

So maybe the right side after all and there is damage to the valve at the inside

So mr atom can be right.

?? Taking off the spring and feel how the valve stem slides in the valve guide turning it and test again.
If no problem maybe mr atom has it

I’ll pull the valves and turn them. They looked like they were moving freely when checking the lash after the bomb. I’ll take the head and jugs to Millennium Technology’s in Plymouth Wi for a pro freshining before I re assemble. I have an extra motor here but it’s an early 80’s and stock. It’ll spoon in and get me some fall rides in though.

 

[Jan_P]

I’ll pull the valves and turn them. They looked like they were moving freely when checking the lash after the bomb. I’ll take the head and jugs to Millennium Technology’s in Plymouth Wi for a pro freshining before I re assemble. I have an extra motor here but it’s an early 80’s and stock. It’ll spoon in and get me some fall rides in though.

Would be interesting to hear what the shop believes
I would run the stock motor if it is OK up to standard.
But that is just me .
Back in the day the tuners did not get far .. before the " Bomb "
Old bikes .lot of cash for parts and sometimes work more power for a short time sometimes months and then tear it apart.

 

[650Skull]

 

[kshansen]

A bit OT but wonder what some of the experts would say about this piston:

Just a note that failure is on the intake pocket side of the front piston of a Sportster.

 

[Jan_P]

I am not an expert. But That looks as a hit ( Valve ) to me Clear deformation .and i reckon the marks on the top are for getting it out or so
And piston not heat scorched

 

[gggGary]

@KcKeeny Pic of the other (both) piston top(s)?

 

[kshansen]

Pic of the other (both) piston top(s)?

Are you asking about my Sportster pistons?

Do not have them anymore but the rear one only had a few nicks in the top most likely from bits' transferring from the front cylinder.

Wiseco who made the pistons claimed they tested them and found the piston that failed, the one in the pictures above, was soft due to being over heated possibly from too much advance. I did retard the timing a bit when I reassembled it.

 

[atom4488]

A bit OT but wonder what some of the experts would say about this piston:
View attachment 249737View attachment 249738
Just a note that failure is on the intake pocket side of the front piston of a Sportster.

Without making any claims of expertise, but IMO this piston has suffered damage from two sources, mechanical impact damage on either side of the piston (above the pin) and detonation damage of the ring land in front of the intake valve pocket. Detonation damage first, with the broken-up ring and ring land causing the damage to the crown??? I assume that the bare crown is as a result of it being cleaned after the failure.

 

[AUTOMAN]

Deto-pure and simple.

Classic end gas deto on hot side of chamber.

Crown also weak from heat and fractured from deto.

Cliff

btw
red deposit on opposite side looks like it WAS running MMT

 

[gggGary]

red deposit on opposite side looks like it WAS running MMT

?
Massanutten Mountain Trail?