[StankBait]

I have a diesel boat engine and an intake valve broke about half way up the stem. It was a fairly clean break, just like it snapped. Of course this destroyed the #5 cylinder and parts were sucked into the #6 cylinder. I sent the valve to a company that determines the cause of this sort of thing. They said that it was "high cycle fatigue" that caused it, the valve was bending or flexing over a period of time.

What I am trying to find out is how rare this is? The boat is five years old and has 529 hours. Probably 75% of that time is slow (trolling or idle) operation.
The govenor is set at 3800 rpm and the max that the engine is run is at2800-3200 rpm. This engine has never been abused and never has been brought above idle until the temperature gauge showed that it was warm. The engine has never been operated at the cruise speed for more than two hours at a time.

I can not think of any reason for the valve to fail other than installation or a defective part. Does anyone have any ideas as to why the valve would be flexing?

 

[LMJD]

That's a wierd one, I've worked (and if not, been around) most all breeds of engine over the years and I can't recall a actual broken valve stem. I've seen lots of mushroomed tips, there's a guy on here who recommends refacing them, but unless you know what you're talking about that means you've gone through the case-hardening and are asking for worse trouble. You also see many scored stems and wallowed out guides. If you float a valve usually the "weakest link" breaks, a pushrod, rocker, valve head, but not the middle of the stem, etc. Hopefully someone else has more to add, what kind of engine is it?

 

[StankBait]

The tip shows no wear. Just a little polished spot where the rocker touches.

 

[Texascadillac42]

What kind of engine is it?

 

[Louisville_Joe]

How is the valve seat? If the seat was not concentric, or square with the guide, this can happen.

 

[StankBait]

It is a VM Motori 4.2 six. It is marinized ( not sure of the spelling) by Mercruiser.
Called by them a 4.2 D-Tronic. Before we ordered the boat we checked with a mechanic for Detroit, they were distributed in US through them then. We were told that the 4 cyl. was used in some UPS trucks and was a solid engine. The Detroit people here did the 100 hour head torque for us. Later the VM Motori was distributed by Cummings. From what I understand the engine has been in Europe for a long time . They are used in Jeeps and Land rovers there but the smaller version. This engine was the best (easiest) starting engine I have ever seen. The crank probably never turned more than one revolution before it started. It was incredible, turn the key and vroooom. It even would start right up with the broken valve. Wish my Super Duty would crank that quickly.

The seat condition is a little hard to tell now because it is broken. The valve is buried into the piston. The valve snapping at the stem is something I never thought could happen. Bent maybe. Halfway up the stem no way. This will have cost me $25,000 before I am back in business

 

[Fred Hemer]

I would think the valve guide would be worn if it fatiuged half way do the rockers look OK?

 

[StankBait]

Have not done a wiggle test on the guide to check for play with the stem. The stem is in San Antonio where I sent it to see what kind of break it had. Here are some photos of the damage.

http://img.photobucket.com/albums/v606/stankbait/6head.jpg
http://img.photobucket.com/albums/v606/stankbait/5head.jpg
http://img.photobucket.com/albums/v606/stankbait/56cyl.jpg

 

[NEC_RPM]

From what I see in the pictures, the valve was not sealing with the seat, even in a diesel the seat should be shiny in the contact area. This could indicate a seat concentricity problem which could eventually lead to the head snapping off, personally I have seen diesel engines with parts that look like they were worn out 50 years ogo still run and when the head is pulled valves rock back and forth in the guide 1/4" etc, an engine can get very very worn out and still run without failure /ubbthreads/images/graemlins/shrug.gif For how long would this engine sit without running? It is very rare for valve to piston contact to break or bend valves in diesel application because of the 0 deg valve angle, you would have to really sieze that valve in order for the piston to snap it off....do you see any galling or buried material on the stem?

 

[StankBait]

It is in a boat so it sits.Weather and time contraints limit its use. It had been running with no apparant problems. I ran 55 nautical miles from the sea buoy to go grouper fishing and was on the way home. Broke at 43NM from buoy. It had been run several times before that for 60nm round trip with some slow speed operation. It never is run for less than 60nm a day plus slow speed (trolling etc). The boat was built for us and had always been stored inside a warehouse on the trailer. opened up, spotlessly clean, fluids changed every 100 hours as per manual.

There are some spots on the stem where it is cut down that look as thiugh a chrome plating has bubbled. Very small bumps. It is not in the contact area of the stem and guide.