Looks like a typical swallowed valve scenario, the valve stem and valve 
head parted company, and the valve head bounced around causing other 
damage when the piston couldn't reach top of it's normal travel, 
including the sparkplug. Can actually see on the top of the piston where 
on one impact the valve head was perpendicular, probably thats the main 
damage cause  There is usually is a weld stem to head at about that 
point, plus high stress point. This looks very similar to my airplane 
engine, except the valve head broke up, and went out the exhaust pipe 
which damaged the pusher prop 2" behind the exhaust stack. When we did a 
major overhaul on the engine a few hours later, another cylinder's 
exhaust valve had a crack and small triangular chunk missing. It was a 
good decision on my part to do the engine as quickly as could be, flew 
it home, and engine torn down immediately.

Some other comments:
1: The adjacent cylinders combustion chamber color, light tan to almost 
black looks good, mixture about right, no excessive oil consumption due 
to defective piston rings.
2: On the Pic P1000884.jpg, betwen the middle (failed) and left 
cylinders, the gasket face is a much darker color than other areas. 
Could get that way from leaking gasket, combustion products leaking to 
the adjacent cylinder, or maybe that head was off for some reason at 
another time, just looks suspicious, usually the entire gasket surface 
is fairly uniform in color and tarnish.
3: On Pic P1000892.jpg, to the left of the damaged cylinder, the gasket 
surface seems missing, just the steel cylinder liner appears, I'm 
assuming this mates to the area on comment 1. It took more than a little 
time for both to happen, in particular the color on item 1, like 
hundreds or thousands of RPM's. One cycle doesn't do that. And I'm 
assuming that considerable aluminum is missing, but could just be the 
colors. That leak is combustion chamber to  water jacket where coolant 
would have leaked.

I still believe the valve failed first, and caused all the other damage, 
except that color on the gasket surface is suspicious, would have caused 
a poor running engine, coolant loss, really not related to valve 
failure, but only long term effects. Possibly caused by a warped 
cylinder head or block, maybe improperly torqued bolts, was that head 
off previously? With the damage, probably can't rely on a check like a 
straight edge on the gasket surface. The force from the valve head 
jammed could bend stuff enough.


Vitaliy wrote:
> Hi All,
>
> This is a continuation of the [EE] thread with the same subject.
>
> Carl Denk said:
>   
>> Pictures would be worth a 1000 words. :)
>>     
>
> http://www.maksimov.org/jetski/
>
> Would love to hear your thoughts!
>
> Sincerely,
>
> Vitaliy
>
>
>   
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