Blown Engine Diagnosis

[jacques2727]

So I talked to the guy's that rebuilt my Mercruiser 350 MPI today. The official cause of the motor needing a total rebuild was that a valve seat fell out of the head. It fell into the cylinder where it caused much damage to the block and pistons. Has anyone ever heard of this, in a boat engine, or even an automobile engine? I don'tknow enough about these engines and if this is even possible. The guy who sold me the boat seems to think this is impossible. I will try to contact Mercruiser directly, but if anyone has any wisdom on this I would appreciate it. Thanks.

[chpthril]

Yes, usually caused by running the engine hot. The valve seat is press-fit into the cylinder head. If the engine runs hot enough, the head will expand and allow the seat to loosen up and fall out.

It may have happened some time last season out and may have taken a few hours to show it's ugly head. Do you know if the engine showed any signs of cooling system repairs??? impeller, water pump, hoses?

[whitlock87]

Ok
I am confused.
The valve seat is pressed into the head. (Usually from the top)
The valve is inserted from the bottom. There is a spring that goes over the valve stem. With some sort of retaining clip. So for the valve seat to come out, doesn’t the valve need to come out first?
(the valve head is to big for the seat to go over, and the valve seat should be one piece)
Also I am not questing the repair, just trying to under stand for future refence

[chpthril]

Ok
I am confused.
The valve seat is pressed into the head. (Usually from the top)
The valve is inserted from the bottom. There is a spring that goes over the valve stem. With some sort of retaining clip. So for the valve seat to come out, doesn’t the valve need to come out first?
(the valve head is to big for the seat to go over, and the valve seat should be one piece)
Also I am not questing the repair, just trying to under stand for future refence

The valve seat is pressed into the head from the combustion chamber side, and then it is given a 3 angle grind so the valve will make a "clean" flat seat. The valve it dropped into the valve guide also from the combustion chamber side, then the seal, spring and retainers are put over the valve stem.

When a seat drops out, the valve will not fully close. When this happens, the piston will hit the valve face and usually break it off and then it will punch a hole in the top of the piston

I'll try to find an exploded view (pardon the pun)

[whitlock87]

Cool thanks for the info
(Ok not cool if it happens to you but cool to know.)

[jacques2727]

Thanks for the great info CHPTHRIL. You sure do know your motors. I just bought this boat in January, and as far as I know, there were no cooling system repairs done to the boat. The rebuild shop did say that the heads were warped slightly, which seems to fit in with the excessive heat theory. Thanks again!

[dogbert]

In case you didn't know, you need to change your impeller at least every 2 years or you'll risk overheating again.

I'd ask the guy you bought it from how often he changed it.