Exhaust valve break BF225

Nearly done 3000hrs and 5 years old BF225 engines are facing to breakup exhaust valve and damage full engine. what could be the cause of this problem??What impact is make that break of valve ?
till this problem happen this engines were worked fine.so nobody thinking to do major overhaul.

And would it be solved by changing only exhaust valve like after done 2000 Hrs ???



wajira

Was it only one valve..I had an issue where the timing belt pulley locked up on my 200 and I had to replace 24 intake and exhaust because the pistons bent them

yes seems to be one valve broken and then damage all,some case other all three valve still in their place and only one is break. after opening i have noticed valve stems are got corrosion,but cant think that can break the valve because valve is moving vertically by load of spring and by force of rocker arm,so their should be another impact. Is the piston hit the valve ? But How ?

wajira

Did you run it low on oil? Any extended low speed operation that could cause more oil to be created? Fuel or moisture can thin the oil and cause it to lose viscosity and lubrication quality. Maybe you spun a bearing or stretched a rod. How does the piston look?
Do you have any back story as to how the engine was running just before this happened? How did you discover the valve issue?

this engines are used for commercial fishing, 4500 rpm continue long time and like one hrs trolling, pistons are ok rings are warn out,one or two pistons oil rings are stuck but engines oil consumption didnt noticed.

before its happened their was no any major problems, after it happened engine was not crank if cranked it make horrible noise, and that broken cylinder is fully damage,

If the valve stem is corroded it may have been tight in the valve guide and not come up fast enough under the spring tension. That would cause the piston to hit the valve, and hence the broken valve. I would focus on why the valve stems are getting corrosion on them. That suggests that there is not enough oil getting to the head? Where was the corrosion on the stems? Between the valve head and the bottom of the valve guide, or where the stem is inside the spring?

What cylinder has the broken valve? Was there corrosion on that cylinder only, or was it on all of them?
As posted above. It is likely an oil issue, or a combination of that and the corrosion. If the corrosion was on the stem, then it could very well have caused the valve to drag and leave it open a split second too long, allowing it to be hit by the piston. Was the engine decarbed? Any additives put in the fuel? When were the valves adjusted last? Was there ever a compression or leak down test performed prior to the failure?

Failure analysis...not always easy. First I would want to know what sort of fuel is being primarily used. As in, what is the ethanol content? Any "coking" under intake valve heads? That stuff can break off and hold a valve open at the wrong time.
How are the engines maintained? What is/was valve clearance and when was the last time it was checked/adjusted? If it is too tight engine wide, then there is your answer. Too tight = no valve heat transfer=valve breakage. Exhaust valves can run up close to melting temperatures and fail if they don't get properly cooled.
The valve seat could be a suspect here too. If the seat is worn or, if it is loose, to the point of little to no heat transfer taking place from the valve, same issue...no heat transfer...valve overheat.
I also like aliboy's theory and would consider any evidence of contact between any other valve sets in the engine to be "tell tale" for something such as he suggests.
An experienced ENGINE machinist should probably be able to give you a pretty good evaluation of what took place.

they are some of damage parts, exhaust valve from other cylinder,we had case of No1 cylinder No2 cylinder and No 6 cylinder, this photos from recent case those valves are not much corrode but before some engines exhaust valve are badly corroded and the place i marked that place thickness become less.

Normally here mostly use 95 octain fuel. engine maintain was done accordingly,oil change every 100hrs.but valve clearance checked and adjusted long time ago like 1000 hrs before,but most of the time when we checked them clearance is increase so i dnt think tappet clearance were too tight. compression test done nearly 200 hrs before, they were fine. no any problem with engine performance.
yes i agree if no heat transfer and loss of strength of valve stem they can break. And i have noticed one more thing is valve guide,if the valve guide warn and valve stem can shake inside the guide,could valve can hit the piston? because i found with new valve also valve can shake side to side inside the guide like 0.5mm.

wajira

The valves adjustment is a big deal. I just did mine and found the intake and exhaust valves were adjusted the same, which left them way too tight.
After the issues with your valves, the heads need to be fully machined or replaced. The guides should not be that loose.

Yes I think that is correct valve clearance and the valve guide they are too loose with the new valve also.so they can move side to side. So i think if it move like this at overlap exhaust valve can touch the edge valve cut grove on the piston,

i think that how they break,

any other idea would help me lot.

So after 3000 hrs rough usage i think its better to do major overhaul with full checkup.


wajira

3k hrs sounds pretty good. I have around 1k on mine and it looks like I may be pulling heads and having them done. I am not sure how the previous owner took care of them and I know the valves were not adjusted correctly, for who knows how long. Compression is ok but not where I would like it. My leak down gauge is not working right, but it does sound like I have some air getting past the valves. It's a decent job to pull it apart. I just have to figure out if it is worth doing now or not.

That "necking down" is, at least to me, evidence of heat overload and the valve stem weakening. The rust may be indicative of something else undesirable happening in your engine. Since the exhaust is water cooled, there could be excessive backpressure taking place and forcing coolant water back up the manifold and on to the back side of the valve. This could cause a "thermal shock" condition that would further weaken the valve and cause it to fail suddenly.

Are these outboards mounted exceedingly low on the transom? Do you do a lot of "force backing"? Is the vessel run slowly (idled in gear...slow trolled) and frequently overtaken by large following seas? Any of these could contribute to excess exhaust backpressure.

those boats are more than 5 years old and engine too. they sue like everyday at the season,long run and trolling. 3000 hrs is too much but we have 7000 hrs run engine also without any problem and without any major repair. but usage is different.
This engines what i found is valve stem is weak valve guides are warn out, of course lack of cooling and over heated valve can burn but i don't think so it can brake by it self.need any other impact to brake the valve stem.

I am sure piston is hit the valve, but how its happened. Valve clearance,valve guide,old valve springs.
valves are broken but when i opened still remaining valve stems and valve springs are their.if you can see in the photos half of the valve stems are their. No over heat mark,burn mark on that, looks like its bend and broke.


wajira