Bent valve at 2,000 RPM?

[Lonnie Wiggins]

So, there I was, crusing down the freeway in my '69 427/400, about 2,000 RPM (3:08 gearing), and I suddenly hear a "tick tick tick". Then, it becomes obvious that something is amiss, no pun intended.

I pretty much figured out I lost a right bank cylinder, and in fact the exhaust valve on #4 was bent. There was very light scratching on the top of #4 piston, and placing a light in the exhaust port you could see a slight amount of miss seating, causing compression loss, of course.

Any ideas on why a valve and piston should have an erotic encounter at a steady 2,000 RPM.

It's been all repaired of course, and we've replace the lifter in case of weirdness there. But any ideas would be appreciated. The engine is all factory configuration.

[Duke W.]

Re: Bent valve at 2,000 RPM?

About the only thing I can think of is that a spring, retainer or lock broke, or the valve stem retainer groove failed allowing the valve to drop into the cylinder where the piston wacked it back into the seat, bending it in the process.

This happened on the TR3 I raced in SCCA back in the seventies, but it was an Isky aluminum retainer that broke, and the valve stem was bent enough for the valve to hang open a fair amount I've never heard of an OEM steel retainer failing, but anything is possible. A broken valve spring, lock or valve stem keeper groove failure could cause the same thing.

What did the locks/retainer/valve stem retainer groove and spring look like, and was the valve actually on or near the seat or humg open. If it was on or near the seat it must not have been bent very much.

I take it from your post that a chunk of material was missing from the valve. Did this appear to be from impact or from overheating. Most exhaust valve failures are the cumulative result of thermal fatigue.

Duke

[Jack H.]

Re: Bent valve at 2,000 RPM?

ONE, cause of such is the valve rocker stud was press-fit to the cylinder head and it had insufficient interference fit to hold it in place, and lifted up/out. Most high end engine rebuilders don't take chances with these classic car engine components and either pin or thread valve rocker studs to the cylinder head during rebuild/overhaul....

But, if you've repaired (had repaired) the damage, this should have be readily evident to the mechanic and there should be no question.... This is, UNLESS, he who repaired the damage WAS the original rebuilder and it cast an integrity question on the original rebuild job.

Further, IF the rocker studs WERE re-press fit and one of them managed to work it's way free and lift, there's a good chance (my opinion) other(s) may follow in time (E.g. 'time bomb') and ALL rocker studs on BOTH cylinder heads should be re-worked to be either pinned or threaded into their seats!

[John H.]

Re: Bent valve at 2,000 RPM?

I believe all BB heads had screw-in studs - wish SB's did too

[Michael Delehanty]

Re: Bent valve at 2,000 RPM?

I don't quite buy a slipped pressed in stud which would raise the fulcrum point, thus loosening the valve clearance. It supports the tapping but not the bent valve. I would think your autopsy would have found a broken valve spring or a dropped keeper. It's possible you sucked in a piece of hardware that jammed in the intake valve for a moment, then fell in the cylinder, broke the valve,then blew out the ehxhaust but I would think you would see some marks inside. Ever find that missing wing nut for the air cleaner?

[Lonnie Wiggins]

Re: Bent valve at 2,000 RPM?

Thanks for the responses and input.

Now I'm REALLY worried. The first thing we did was to pull the valve cover, start the engine, and observe. We observed nothing unusual.

The valve itself, though bent, was intact when the head was pulled. And, as mentioned, the valve was bent so slightly you pretty much had to put a light in the port to see a slight misalignment.

The machine shop that did the repair was not the one that did the work. They reported that they could not find any problems with the valve train.

There were no foreign objects in the cylinder and no cylinder wall scoring.

We've about decided it was a communist plot, as nothing else seems to make sense.

Thanks for the input though, we'll keep working on it.

[Duke W.]

Re: Bent valve at 2,000 RPM?

It's possible that when the chunk came off the exhaust valve it bent due to the fact that is was severely overheated and a bending moment placed on the valve stem due to uneven seat force. The combination of loss of mechanical properties due to overheating and the uneven seat forces could theoretically bend the stem a bit.

The "scratches" you refer to on the piston could be due to the valve fragment bouncing around before is was ejected out the exhaust. If the valve had kissed the piston, the crown will show an arc of contact that matches the valve head exactly.

Duke

[Duke W.]

Another theory

Okay, maybe the previous theory is no good if the valve was "intact" and didn't have a missing bit of material.

In that case I would opine that the engine ingested a piece of foreign matter which got caught between the valve and the seat as the valve was closing. the bending moment cause the stem to bend slightly. On the next opening cycle the debris came loose and was ejected on a following cycle, but the valve would not longer close, so the cylinder died.

Turbulence inside the cylinder would tend to keep a small piece of debris away from the walls, but it would bounce off the piston and head a couple of times - enough to leave marks in the piston crown, and probably the head, too.

Duke