Broken Valve Spring

[splenderleith]

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 can’t 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 can’t 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?

[DanEC]

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.

[Gaz64]

Originally Posted by splenderleith

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 can’t 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 can’t 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.

[t walgamuth]

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.

[Tommy]

For me, every engine failure was an opportunity to upgrade to better parts.

[undy]

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.

[Unique427]

Originally Posted by undy

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.

[eschaider]

Originally Posted by splenderleith

...My biggest concern is that I can’t 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 can’t 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.


Ed

[Tommy]

For my own curiosity and the OP, what is the likely root cause of a bent valve?

[Gaz64]

Originally Posted by Tommy

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.

[splenderleith]

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. I’ll report back when I get the head off.

[eschaider]

Originally Posted by Tommy

For my own curiosity and the OP, what is the likely root cause of a bent valve?

Contact with a piston.


Ed

[t walgamuth]

Originally Posted by eschaider

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.

[eschaider]

Originally Posted by t walgamuth

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.


Ed

[Gaz64]

Originally Posted by eschaider

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.

[splenderleith]

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?

[Gaz64]

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.

[eschaider]

Originally Posted by splenderleith

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.


Ed

[ZOERA-SC7XX]

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.

[cycleguy55]

Originally Posted by splenderleith

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 ÷ 2)² x 3.14159 = 4.7524 x 3.14159 = 14.93 in² x 100 PSI = 1,493 lbs.

My earlier numbers were incorrect.