Bent valve

While new to the forum, I have been a kz enthusiast for over 40 years and ride the 4 I own regularly. My ‘81 GPz1100 has 30,000 miles on it and has run great until recently when cruising steady at 3000 rpm in 5th gear it suddenly lost compression in one cylinder with accompanying jangling noise. Tear down revealed a bent #4 intake valve with evidence of being slapped by the piston. All the valve springs are intact. The cams, chain and tensioner appear to be fine. I had all the good valves and seats ground as long as it was apart.  As these engines are reliable I’m sure there is a reason for that failure and I just haven’t found it. Does anyone know of a typical or common cause of this failure considering my description of events?  I don’t want to put it back on the road if such a thing is likely to happen again.

Maybe the valve seized in the guide and then the piston came up and smacked it?

 

Welcome to a great forum and even greater group of members. If you lurked a bit before joining then you know pictures are a must. 

Is valve seizure a common malady?  If so, can I visually determine that was the problem and what can I do to prevent future occurrences?  Also, I will post some pics and introduce myself in the other forum.  Thanks for any additional guidance.

wrote:

Is valve seizure a common malady?  If so, can I visually determine that was the problem and what can I do to prevent future occurrences?  Also, I will post some pics and introduce myself in the other forum.  Thanks for any additional guidance.

I have a KZ1000 engine where a valve seized and the valve head was broken off.  But I'm pretty sure this was a result of the engine sitting outside, the open valve stem rusting in place, and then someone coming along and kicking it over.  So not what you're describing, but it is an example of a seized valve.

It seems like it's worth very closely inspecting the valve guides in your head, and possibly having the guides replaced while you have it apart.  It's also worth measuring the valve to piston clearance as well.

The only other thing I can think of is that some of the J model cranks were prone to twisting.  The pieces are press-fit together, and some were too loose.  I have no idea if this twist could  be enough to shift the cam-to-crank timing enough that a valve touches the piston.