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This week-end, I had to disassemble an engine from a Kawasaki versys 650 (early). The bike lost power while making ugly metal sounds. It never started again. RIP.

Context: 108.000km, mostly highway usage, maintenance OK, everything right (oil, coolant, tappet clearance...)

It looks like the cause is an exhaust valve that got cut in half, its head falling in the cylinder, obviously destroying everything.

But can this be the source of the failure? Can a valve simply break apart like this? Otherwise, what could have caused this?

The other valves unsurprisingly bent, none of them broke. The spring was all right, nothing noticeable on this side. One of the other valves had lost one "half clip thing that lock it in the spring", I don't know how it is called in english (demi-lune in french) but I think it happened during the breakdown.

Here is an imgur album with close inspection of the cylinder head: http://imgur.com/a/KyAqt

can a valve simply break in half without any reason?

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Yes it absolutely can happen like this. Many valves are actually created in two pieces (head and stem). They are "welded" together (I use the term "welded" loosely here). They can and do fail.

There is a write-up on this page which talks about it:

Breakage ... can happen to either intake or exhaust valves. Valves break in one of two places, where the head is joined to the stem, or where the keeper groove(s) are machined into the end of the stem. Either way, breakage is bad news because the pieces fall into the combustion chamber and wreak havoc on the piston and head.

The breakage can occur due to being over burdened or because of manufacturing defect. As was stated, when it does fail, it does it spectacularly, with the possibility of destroying everything in its path.

EDIT: I also found this article from Engine Builder Magazine which describes a lot about valves and may be an interesting read to you.

  • Thanks for the links, I thought valves were made form one piece of metal only. I found out that other versys 650 had similar failures. – TNCreator Sep 22 '15 at 13:51
  • @TNCreator Since this is NOT an interference engine the valve couldn't have stuck in the guide and hit the piston. The face must have broken/failed and come away from the stem, then bouncing around in the combustion chamber wreaking havoc. – DucatiKiller Dec 18 '15 at 17:33
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My experience with this has been where a valve has overheated and the valve starts sticking in the valve guide. If the valve sticks too far open, the piston will hit it and bend or snap the valve. One possibly reasons for overheating is due to the valve not sealing completely and hot gasses flowing around it. Another is that the water level was low in a water cooled engine.

  • My experience as well. – DucatiKiller Sep 28 '15 at 19:51
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Valves can fail, but generally the failure is caused by them coming into contact with something solid. They can start to float at high rpm, they can physically touch something while they're moving (i.e. something gets sucked into the intake or the connecting rod fails and the piston makes contact, or the timing can be off and a valve is open when the piston is at TDC). I've not experienced having a valve just fail on its own.

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