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  #1 (permalink)  
Old 10-10-2021, 07:44 PM
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Question Broken Valve Spring

I have a Superformance cobra, built in 1996, with a 460 crate motor. This past Saturday, I was pulling into my neighborhood and I heard a terrible noise. I limped it a few hundred feet to get it into the garage. After some diagnosing, I found a completely shattered valve spring, maybe 7 or 8 pieces. I was able to find the keepers and what was left of the seal. I have some new springs and seals on order. My biggest concern is that I canít get the valve to move. It appears to be in the closed position, with everything off of it (rocker arm and spring seats removed), but I canít get it to budge even the slightest bit. I have given it some light taps with the lead hammer, but it seems pretty stuck. I guess my main concern is if the valve could have bent, below where it sticks out of the head. Has anyone experienced anything like this?
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Old 10-10-2021, 08:57 PM
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Does the valve stem align with the rocker arm - if not, that would seem to be a good indicator of a bent stem? I would think the pushrod would be severely bent too though.
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Old 10-10-2021, 11:27 PM
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Quote:
Originally Posted by splenderleith View Post
I have a Superformance cobra, built in 1996, with a 460 crate motor. This past Saturday, I was pulling into my neighborhood and I heard a terrible noise. I limped it a few hundred feet to get it into the garage. After some diagnosing, I found a completely shattered valve spring, maybe 7 or 8 pieces. I was able to find the keepers and what was left of the seal. I have some new springs and seals on order. My biggest concern is that I canít get the valve to move. It appears to be in the closed position, with everything off of it (rocker arm and spring seats removed), but I canít get it to budge even the slightest bit. I have given it some light taps with the lead hammer, but it seems pretty stuck. I guess my main concern is if the valve could have bent, below where it sticks out of the head. Has anyone experienced anything like this?
Please post some photos of all valve stem heights with the rockers off.

A valve that seems to be in the closed position is unlikely after a valve spring breaks.
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Old 10-11-2021, 04:43 AM
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Well, its a good chance the top of the piston is damaged too. I'd prolly remove the spark plug and look around in the combustion chamber.
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Old 10-11-2021, 06:04 AM
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For me, every engine failure was an opportunity to upgrade to better parts.
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Old 10-11-2021, 06:35 AM
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As mentioned, the valve probably kissed the piston and bent the valve stem when the spring broke. You may have piston, block and cylinder head damage. You'll have to pull the offending head and inspect for damage.
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Old 10-11-2021, 07:09 AM
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Quote:
Originally Posted by undy View Post
As mentioned, the valve probably kissed the piston and bent the valve stem when the spring broke. You may have piston, block and cylinder head damage. You'll have to pull the offending head and inspect for damage.
If you have access to a bore scope, poke inside the spark plug hole
for indications of obvious damage but as Undy stated you're looking at
pulling that head to be sure.
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Old 10-11-2021, 01:23 PM
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Quote:
Originally Posted by splenderleith View Post
...My biggest concern is that I canít get the valve to move. It appears to be in the closed position, with everything off of it (rocker arm and spring seats removed), but I canít get it to budge even the slightest bit. ...
It is called a bent valve. You will likely find the guide is damaged also and needs to be replaced along with the valve. You will also need to have the valve seat redone.

If the heads are not ported or otherwise modified the cheaper way home might be to just buy a replacement casting and move your stuff to the new head. Have a valve jbb done and put it back together.


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Old 10-11-2021, 03:00 PM
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For my own curiosity and the OP, what is the likely root cause of a bent valve?
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Old 10-11-2021, 04:35 PM
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Quote:
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For my own curiosity and the OP, what is the likely root cause of a bent valve?
Engine running, cheap valve springs. Doesn't need to be high rpm.

Generally - valve float, valve spring harmonics, and then contact with a piston.

I have had twin cam 16v engines with ALL 16 valves bent.
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Old 10-11-2021, 08:05 PM
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Working on removing the head. Got the exhaust and carb off. I took all of the bolts out of the intake, but it is a little stuck on. I figured it was a good spot to stop. Iíll report back when I get the head off.
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Old 10-12-2021, 10:06 AM
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Quote:
Originally Posted by Tommy View Post
For my own curiosity and the OP, what is the likely root cause of a bent valve?
Contact with a piston.


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Old 10-12-2021, 12:56 PM
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Quote:
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Contact with a piston.


Ed
I would expect it was over revved. With all the rev limiters available today this is less likely but I would still think most common. It could be done by downshifting at too high rpm also.
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Old 10-12-2021, 06:13 PM
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Originally Posted by t walgamuth View Post
I would expect it was over revved. With all the rev limiters available today this is less likely but I would still think most common. It could be done by downshifting at too high rpm also.
Best efforts not withstanding, sadly there is always a way to circumvent safeties we attempt to build in. That said, it does not matter how high you overspin the engine or even float a valve. If a piston does not hit the valve, the valve will not bend.


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Old 10-12-2021, 06:28 PM
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Quote:
Originally Posted by eschaider View Post
Best efforts not withstanding, sadly there is always a way to circumvent safeties we attempt to build in. That said, it does not matter how high you overspin the engine or even float a valve. If a piston does not hit the valve, the valve will not bend.


Ed
Yes, that is exactly correct.

A valve head has to contact the piston to be bent. This could occur at ANY engine rpm, either from valve float at high rpm, or from valve spring failure, which could happen at the next startup, idling etc.
In the owners case, the valve spring has broken into many pieces, the valve drops to full open, the moving piston smacks the valve head, and keeps smacking it until the valve head snaps off, wedged into the valve seat. 50% of the time the piston could be damaged, especially from large valves.
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Old 10-12-2021, 08:34 PM
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I was able to remove the intake and all 10 of the head bolts. I can definitely see a bent valve from the intake port.

I havenít figured out how to get the head loose. Any ideas?
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Old 10-12-2021, 09:02 PM
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Refit 2 head bolts, one each end, at about 4 threads in.
Refit the spark plugs for that head, leave the other bank out.
Spin the engine on the starter. Head should break from the headgasket.

Otherwise, a large prybar, waterpipe etc down the intake port, and pull over.
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Old 10-13-2021, 10:25 AM
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Quote:
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I was able to remove the intake and all 10 of the head bolts. I can definitely see a bent valve from the intake port.

I haven’t figured out how to get the head loose. Any ideas?

This picture is exactly what Gary described / predicted in post #15.


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Old 10-13-2021, 12:16 PM
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Do it manually with a pry bar. Don't attempt to spin the engine, you already have damage and don't want to make it worse. Just make sure you have ALL the head bolts removed. Good luck.
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Old 10-13-2021, 01:02 PM
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Quote:
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I was able to remove the intake and all 10 of the head bolts. I can definitely see a bent valve from the intake port.

I haven’t figured out how to get the head loose. Any ideas?
You might try using air. Make sure all the head bolts are loose or removed. Put a fitting in one of the spark plug holes in an end cylinder (not the one with the broken valve) and pressurize it. Area of a 4.36" bore is 4.75 sq inches, so 100 PSI is 475 lbs pressure pushing up on the cylinder head.

2021-10-17 Edit: 460 CID standard bore is 4.36", so (4.36 ų 2)≤ x 3.14159 = 4.7524 x 3.14159 = 14.93 in≤ x 100 PSI = 1,493 lbs.

My earlier numbers were incorrect.
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