Exhaust valve tips, what the hell?

racervette69

racervette69

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Stock engine, stock cam. stock rockers, springs. Was running fine. Exhaust valve tips only. Heat? Everything else in the engine looked good with very little wear.



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Rod bearings nice shape, cam bearing nice. No sign of low oil pressure. Yes all eight exhaust. Not mushroomed. Just looks like the edges of the tips flaked off.
 
is the area worn to a dimple? That would explain the excessive loading on the tips edge and cause chunks to chip off. Replace all valves and rockers. Weird that it's only the exhausts, maybe a set of worn guides giving poor heat transfer and a hot running valve? Won't know till you take it all off.
 
There is a metal to metal contact failure mode known as edge "biting" that looks like that. I've never seen it with valve stems but the theory is the same. The edge of the stem is loaded higher than the center because the rocker surface extends beyond the edge of the stem. Biting looks a lot like that but the only thing I have ever personally seen it is with injection molds.
 
Twin_Turbo said:
is the area worn to a dimple? That would explain the excessive loading on the tips edge and cause chunks to chip off. Replace all valves and rockers. Weird that it's only the exhausts, maybe a set of worn guides giving poor heat transfer and a hot running valve? Won't know till you take it all off.
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Yes, Its a mystery to me why just the exhaust valves. Bad run of valves??
 
Just an idea : I know that some of the original valves have a hardened chrome surface. I found that out when I left some of them lying around and the outer layer started to show rustspots. This isn't by any chance the chrome starting to come off is it ?
 
I also think its the hardening thickness flaking off. The idea of the dimple in rocker arm pulling it off seems possible. I really didnt want to take the heads off this engine. Since the leak down test was good. But.
 
so you think it's the surface hardened layer flaking off? maybe cut the oil filter apart and see if some of those flakes ended up in there? the pieces are probably too small to find in the pan but it should be magnetic..... why pull the heads ?
 
Twin_Turbo said:
how are you going to replace the valves otherwise?
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"Homer Simpson voice": doooh !!!

the first pic is kinda blurry, is the tip that bad that the valves have to be replaced? the rocker doesn't "look" too bad either....is the contact surface concave (lens shaped) or flat? has the tip worn the rocker? is it just the pic or is the surface blue? if it's blue it overheated....
 
No blue or sign of overheating. I have decided they are just worn out and the extra heat of the exhaust valves may had added to the wear???
 
When Mt. Saint Helens blew I put all of my cars away safe and only drove a 71 Pinto through the dust for the next couple of months. Toward the end it ran worse and worse as you'd expect but I was amazed to find all the damage was to the Exhaust lobes of the overhead cam. They were ground virtually round whilst the intake lobes looked totally normal??

I didn't mind, it was a sacrificial car but I've always been startled at the selectivity of the wear.....
 
racervette69 said:
No blue or sign of overheating. I have decided they are just worn out and the extra heat of the exhaust valves may had added to the wear???
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The force to open the exhaust valve is greater than that to open the intake valve because of high pressure in the cylinder at time of exhaust opening.
 
larrywalk said:
racervette69 said:
No blue or sign of overheating. I have decided they are just worn out and the extra heat of the exhaust valves may had added to the wear???
Click to expand...

The force to open the exhaust valve is greater than that to open the intake valve because of high pressure in the cylinder at time of exhaust opening.
Click to expand...

That's an interesting idea. I wonder how much difference, as a percentage of the total force, the cylinder exhaust pressure would make?
 
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