Broken valve reasons ???

[lakis1982]

Hello,

I have an oxyboxer 2110cc turbocharged injection engine that has been built from scratch with new parts... THe engine has :

- Cb performance panchito 40x35.5mm heads with supergrips valves and VW650 Springs (the upgrade option)
- Wiseco 94mm pistons

Unfortunately yesterday while test driving the car relatively mild at 2500rpm , I suddenly heard a knocking sound from the engine bay so i stopped immediately. After removing the  heads, things were not looking good. At cylinder 4, a cracking valve occured , the top part of the spark plug was smashed, the valve guide was mispositioned and a hole in the inlet of the head.

The engine is fairly new with only around 400km.. I have never managed to put serious stress and boost on that engine, it was still in the test driving process. Only a  few times i instantly made accelerations until 5500rpm with boost never going above 0.3bar but not yesterday..
air/fuel ratio was always on the safe side around 12.5-13.. i have never realised any detonations..i have one exhaust gas temperature sensor per cylinder and all of them always show normal temperatures around 650-700C.. All spark plugs have good color indications.. The valves appearance is also good.. Generally all temperature indications in ECU were very normal..

So the big question is how did that happen.  ??
In the attached images you can see that the valve has two cracks.. The crack on the top part of the valve looks so perfect like it was cut with a cutting wheel.  The crack on the bottom part of the valve includes serious bending as well...
My first guess to explain all this was due to an object (nut, screw or whatever) that somehow found its way to the combustion chamber.. But i never found anything inside the cylinder..
My second guess is that it could happen due to a faulty valve..Could it be that the valve was initially cracked on its top and then when the valve felt down it was smashed by the piston inside the chamber    Could you tell your thoughts about that ??  Any experiences ??


Thanks you in advance

[lakis1982]

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[lakis1982]

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[wph]

Can you upload a close up picture of the valve where it broke below the retainer, it might give a clue why it broke.
My first impression is that the stem let go at the top and the rest of the valve fell through the guide and bent when piston hit it.
Did you assemble the heads by yourself and check the seats and stems ? Are there any visible marks on the valve stem where it broke,
probably a nick caused by tool etc....     

[lakis1982]

I add three more pictures of the top crack of the stem

[richie]

My first impression is that the stem let go at the top and the rest of the valve fell through the guide and bent when piston hit it.
     

My thoughts too, as its an intake valve you can probably rule out heat/detonation etc as the exhaust valve suffers more from those things and would fail 1st. I would check all the other intake valves to see if they are bent or stretched at all and also look at the valve seats for signs of valve float[the seat will be pounded into head and leave a lip in port of squashed material]  If all this is good then it points to a faulty valve[ I have never seen one fail there so already think that is most likely, they normally fail at valve head or by valve locks/collets]

[wph]

Which camshaft and rockers? What method was used to setup valve train geometry, half lift or some other?
Did you check the wiping pattern at the valve tip through the whole rocker arm movement?
There's something on the first picture of the broken stem that caught my eye. Is the bore of the valve guide ID still round or ovaled ?

[lakis1982]

Rocker is cbperformance forged 1.3:1
Cam is Engle fk8
The half lift method was used
We didn't check that wiping pattern that you mention
The valve guide was smashed so its ID is not round anymore.. I could check the guide of the neighbor valve for its roundness..

[Martin S.]

Sorry to see that! 
Get on the phone with CB and let them know what happened. A friend is personally connected with them and has high praise.
In the 2nd pic you can just see the surface of the lash cap. Looks like it’s black but has a shiny mark?
How’s the rocker look?
You can check a couple things like spring installed height which is just over 1.5”.
I would think these heads would already have the guide boss clearance ready to go.
Any clues on the spring side of the guide?
Where was the guide, in the rocker box, or in the cylinder?
What was the initial valve adjustment like, anything odd, how much adjustment did they need after a quick run of the motor at the beginning?

[Flakeyjake]

 , picture number 10, you can see some material which was under the valve spring and if this material is "closed" to the head and not from the damage, it can caused the damage!! It bend the valve and then the valve break like yours .
The CB 650 valve springs are heavy and if you have only one place with more pressure, the valve bend every time !!! So, it is an installations fault...

[Martin S.]

Good eye dude!
I see that mashed down chunk of metal that was under the spring. 

[Flakeyjake]

Good eye dude!
I see that mashed down chunk of metal that was under the spring.  

Thanks, i am 100 percent sure !!!.... and you can see some black marks on the other side off the valve spring side.....

[wph]

Good eye dude!
I see that mashed down chunk of metal that was under the spring. 

Thanks, i am 100 percent sure !!!

I noticed the bending on the broken stem, valve spring bottoming or close to on one side could be the cause.
Looks like a blob of weld? to my eye, good catch...

[Martin S.]

Ask for Pat Downs. 

[lakis1982]

How’s the rocker look?

the rocker looks good

Where was the guide, in the rocker box, or in the cylinder?

the remaining part of the  valve guide that was not smashed was mispositioned towards the cylinder ..

What was the initial valve adjustment like, anything odd, how much adjustment did they need after a quick run of the motor at the beginning?

they needed adjustment once, nothing odd..


i must confirm that on picture 10 the small piece of metal that can be seen happened due to the damage.. underneath that location the valve was hitting and smashing this area..