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Valve question - why did this happen

AZ79K5Project

AZ79K5Project

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I have a 79 350, I put heads on and a cam from Summit a few years back with a new timing set and topped it with the Atomic EFI. I had a pushrod bend right after assembly which I attributed to improper lash.

Fast forward to today and a couple thousand miles. I was running up a hill at about 3200 RPMS and I lost a cylinder and backfiring through the atomic. Nursing it home, the backfiring stopped but I new I had lost a cylinder.

This is the #5. Exhaust rocker arm was super loose, but the adjusting nut tight and the intake valve is… well you can see half missing and bent. The missing end is down in the engine.

What should I be looking for when I take the head off? What would cause this failure?

Compression on the engine was all 145-150 except for #3 and #5
#3 was 135
#5 was 40

IMG_8213.jpegIMG_8214.jpeg
 
How much lift? Coil bind or valve piston clearance, possibly bottomed the retainer on the top of the valve guide
 
Everything was spec’d through Summit on a single order. The heads came with the springs installed.

This was my first attempt at a top end. I read as much as I could and watched as much as possible. I’ve never worked on engine internals.

This is the cam specs:
image.jpg
 
Number 3 rocker doesn't look aligned properly. Did you pull all the rockers after bending the one pushrod to check all the others? Like roll on glass to verify straightness?

If one valve was maladjusted, why wouldn't others have been? Perhaps this was another pushrod that was already bent and finally let go?
 
For a visual, slowly turn the engine over by hand and watch the valve spring on another good valve. Is the spring being completely collapsed? Is the valve spring keeper bottoming out on the valve stud boss? Are the rocker lengths correct and were the push rods measured, or just something slapped in there? In other words, was this built by someone who knew what they were doing?
 
I can't see what heads you are running. The rockers are not self centering, so the push rod must be kept aligned by the hole in the head. If this hole is worn then the rocker will shift off the valve stem.
Another thing that can happen allowing loose rockers is the rocker stud can work it's way out of the casting, esp if the spring seat pressures are above stock.
with that compression test result, I expect to see a bent valve head and kissed piston top. when you get the intake off pull each lifter and inspect for damage on the face and worn cam lobe. Keep them in match lifter to lobe in case there is no damage.

If the head push rod guide hole is worn self centering rockers can solve this.

edit: also do not use hardened push rods with stock cast iron push rod guides
 
Last edited:
dyeager535 said:
Number 3 rocker doesn't look aligned properly. Did you pull all the rockers after bending the one pushrod to check all the others? Like roll on glass to verify straightness?

If one valve was maladjusted, why wouldn't others have been? Perhaps this was another pushrod that was already bent and finally let go?
Click to expand...
It does look misaligned too, I’ll be checking everything when I pull this apart.
Wes Harden said:
I can't see what heads you are running. The rockers are not self centering, so the push rod must be kept aligned by the hole in the head. If this hole is worn then the rocker will shift off the valve stem.
Another thing that can happen allowing loose rockers is the rocker stud can work it's way out of the casting, esp if the spring seat pressures are above stock.
with that compression test result, I expect to see a bent valve head and kissed piston top. when you get the intake off pull each lifter and inspect for damage on the face and worn cam lobe. Keep them in match lifter to lobe in case there is no damage.

If the head push rod guide hole is worn self centering rockers can solve this.

edit: also do not use hardened push rods with stock cast iron push rod guides
Click to expand...
thank you! I’ll pull it apart next weekend and see if I got lucky or not.
 
AZ79K5Project said:
It does look misaligned too, I’ll be checking everything when I pull this apart.

thank you! I’ll pull it apart next weekend and see if I got lucky or not.
Click to expand...
I had the same thing happen to me on dodge 318, it had been running fine for thousands of miles but when I hit a long hill towing a blazer, and sustained 3200-3500 rpm I had the same thing happen.
When I pulled the heads, I found the valve bent too so expect that.
The high rpm will find any weakness and exploit it to destroy your engine.
My engine was stock by the way with 100k miles
 
What would the impact of a too short or too long of a push rod be?

Would that cause the alignment or cause the rocker arm to push off like #3 exhaust and #5 bent/broken push rods?
 
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