My early 70's Chevy 350 is bending the push rod for the nuber 2 intake valve.  This started occuring after a valve  adjustment and has not been fixed with several subseqent valve adjustments.  It is always the same push rod, and you can see the rocker arm trying to work its way off the valve while it is running.  We removed the head, replaced the valve, verified the correct position of the rocker arm stud & valve, then reassembled it only to have it do the same thing again.  Any ideas???  This engine is in a 57 chevy that I would like to get running good for the summer. Thanks.  

What cam are you running?  What spring are you running? What is your spring pressure and distance to coil bind?  How much clearance do you have from the bottom of the retainer to the guide/seal? How exactly are you adjusting your valves?  

Revolutionary,
Thanks for your reply. I don't know the answers to all of your questions, as I wasn't in on the rebuild of this engine. To the best of my knowledge, it's a stock 350 from about 1970 with 1975 or 76 heads. The only modification that I know of is a port job done to the heads and intake. When I got the engine, it was assembled, but had not been run. The guy who built it said no valve adjustment would be needed. I'm not sure how he adjusted them. Shortly after I got it broken in, a combination of timing, dwell and carburetor adjustment misled me to think that a valve adjustment was necessary. I checked a Chevrolet shop manual that recommended the following procedure for valve adjustment:
With the engine warmed up and running, loosen the lock nuts on the rockers one at a time until the lifter starts to clatter. Then, tighten the nut until the clattering just stops and then tighten it another 3/4 of a turn.
It was after I did this that the problem started. It usually will run for about 1 to 1 1/2 hours (not necessarily all at once) before it develops a miss. Then, when the rocker cover is removed, the no. 2 intake valve pushrod is bent. I have repeated this routine several times, always with the same results in the same place. The engine has locking rocker nuts, (set screw type) so I know they are not loosening up.
I know I wasn't much help with your specific questions, but I hope this background information will give you something to go on. Thanks in advance for any help you can offer.
Best regards,
        jt4  

I have never been a big fan of the 'engine running' technique of setting valve lash.  What will work with pretty much any 4 stroke engine in the world is the following.  It is known as the EOIC method and I'm sure if you googled it you could find a better description.

Pull both valve covers and loosen the rockers a couple turns.  Rotate the engine until the #1 exhaust valve starts to open then while jiggling the intake pushrod up and down slowly tighten the intake rocker until all the lash is taken up and the pushrod just barely doesn't jiggle up and down.  That is zero lash.  Now go about 1/2 to 3/4 turn and the #1 intake is set.  You now have the option of just going down the cyl head repeating this process or do like I do and if you follow the firing order (18436572 in the sbc case) you can just rotate the engine 90 degrees to get the #8 starting to open.  Again repeat the process.  When all of the intake valves have been set, rotate the engine over until the #1 intake valve opens all the way then is about halfway closed.  Now slowly tighten the #1 exhaust rocker until there is no lash and go anoth 1/2 to 3/4 turn.  Again you can either follow the firing order to save time or you can just go down each head repeating the process.
Most people seem to have a problem with overtightening the rockers leading to bent pushrods.  Don't fall into the trap of tightening the rocker until you can't spin the pushrod anymore.  That is WRONG.  Follow the EOIC method and be the envy of your friends forever.