I have a 1978 Datsun 280z w/L28 motor. First heard a ticking sound then it starting running bad like one cylinder. Checked the little stuff, all looked fine. Pulled the Valve cover and found that a valve keeper had come off the first piston (closest to radiator) on top of the 2nd spring. How can this be replaced and is it likely that a valve may be bent? It does still start but runs very rough still....no longer starting in fear to damage more (like the piston).

If you mean "keeper" as in lash pad or the little button-like thing that sits on top of the spring retainer and under the end of the rocker arm, or if it's just the enitre rocker arm which has come off, then it won't harm the motor to turn it over or even to run it...but that cylinder won't fuction without the valve opening & closing and motor will run rougher. You can replace the rocker arm and lash pad easily enoung and readjust the proper clearance which I'm assuming you know how to do or just consult Haynes, but if it came off in the first place the valve could have stuck open, it was kicked off & may have damaged the cam, so check the lobe for that cylinder before you try to reassemble.

Thanks for response and yes I am talking about the little button ...didn't know the proper name. It is the lash pad and two half moon looking pieces with a grooved retainer clip looking area on both...I think holds the valve in place or something to that affect. The cam (lobe) seemed to be ok and it is not the entire rocker arm that came off. I had gone out of town for a week and when I got back and started it, that's when I heard the ticking sound that was not there before. It progressively got worse when I was gonna take it in to get it checked out, but returned home because I live 30 miles from town. I will investigate further and replace the rocker arm assembly but also check to see if the valve is not bent ... keeping my fingers crossed. Again thanks for the reply.

Same problem here, with photo:

The top of you spring retainer may be mashed up like in the photo.

http://www.classiczcars.com/forums/showthread.php?t=35354

Other suggested causes are loose valve seats.

Another link below on how to replace Valve seals:

http://www.geocities.com/inlinestroker/seals.html

I'm not fixed up yet and frankly not sure when I'll be able to get to a fix, the gathered information from this post and a friend have been very useful. Thx.