I recently had the heads rebuilt on my '71 Mercury with a 400 engine. After about 2000 miles the engine developed valve train noise and a noticeable miss @ idle. Upon inspection, I found that the push rod on the #4 exhaust valve was disconnected and bent. The intake push rod was sticking all the way through the rocker arm. The heads are stock except for the heavy duty valve springs that I installed. The other rocker arms/push rods seem to be OK visually. The engine is stock except for an Edelbrock intake and a Carter AFB carb. The rocker arms were all replaced with new stock stamped steel units. I reused the original push rods. I also replaced all of the lifters with factory units. Could the higher valve spring pressure have caused this problem? I'm wondering if I should replace the stock rocker arms/push rods. Also, the heads were replaced @ the machine shop with 1973 heads. They appeared to be identical though. Thanks for any direction you can give.

the best thing to do is have someone measure the amount of seat presure you have. If the rest of the rockers are fine ,I would suspect a misfit on the one that wore through. One more thing is if the pushrods were old some dirt may have come from the inside of the rod and caused enough friction to wear the rocker out.

Yes, a stock engine only needs stock springs. Any more only ammounts to lost power and increased wear.

I'd slap a new rocker in there and go on down the road.

I just woke up and not thinking straight yet but, I wonder if his ex valve seized in the guide due to heat from an intake leak or other lean condition and caused that push rod to bend? If the ex valve happened to seize while closed, at some point you could expect that cylinder to hydralic with fuel and damage the int valve and or the piston. I doubt I am right but I think I'd take that head loose and have a look just the same.

Like I said I just woke up but, Ian Riodian, could you explain why increased spring pressure "amounts to lost power" I'd like to try and understand that.

I agree with 56Maynard. The valve probably just stuck. Replace the push rod and rocker and motor on. If it happens again then you need to find the reason. I have seen this several times on BB chevys and we always replaced the bad parts and no further problems.

What compresses the springs when the engine is running? Engine torque through the crank via chain , cam & valvetrain. The harder the spring, the more torque/horspower sent up the pushrods instead of down the driveshaft.
Valvetrain weght is also a parasite - heavier than stock is harder to lift and also costs power for the same reason.
A lot of people run a lot more spring, oil pump, and fuel pump than they need and it all costs. If your cam is stock or a factory hi/po hydraulic, use the appropriate factory spring and oil pump.
By all means use race quality fasteners in a stock engine but much race hardware has no place in a STOCK street engine. Hope this is clear enough.

Sorry, just not true about more spring pressure causing a loss in power. The more aggressive the cam profile the more spring pressure that's required to keep the valvetrain in check. The old wives tail about more spring pressure making the valve more difficult to open would only be true if you were opening and closing just one valve but in this case when one valve is being opened others are being closed so the increased force is offset. True on a flat tappet cam there is slightly more friction but the benefit of eliminating valve float outweighs the slight parasitic losses from that friction. I believe in this case you're referring to him using a stock replacement cam so I agree with using what the cam maker recommends and not springs for intended for some insane solid roller.

Good post czar. I share your thoughts on component selection also. I always let the cam grinder have his way in terms of what going to live with his lobes.

Yes on His STOCK cam, perhaps I wasn't clear. Use race parts in a race engine and stock in stock.
I do a lot of miles in my daily drivers - I have noticed far greater valve train life with the mildest appropriate spring.