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Old 07-16-2003, 03:50 AM
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Stuck Valve

Hi Guys,

Following some advise from this site, I have just pulled a head off my 350, to find just what I was expecting, an exhaust valve stuck open by about 10mm.
The piston has obviously been hitting this valve, as there is slight damage to the surface, but nothing major, does this warrant replacing the piston? I would really rather not!

The valve is truely jammed there, do I want to hammer it out, or is there another way. Also, after taking another exhaust valve off to compare, I see that the top of the hole the valve goes into, on the rocker side, seems to be damaged, and is different to the working valve. Sorry, I know this isn't a very good description, I will try and put a picture up.

Thanks

Daved

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Old 07-16-2003, 04:53 AM
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I would replace the piston, even if the damage didn`t appear to be bad, microscopic cracks can be there and you can`t see them and a little down the road the piston will possibly come apart, if you run it, it`s gambling with time. a friend of mine years ago spun a rod bearing, so he did a rebuild, and reused the same piston, I told him to replace that one, as when a rod bearing is gone the piston is smacking the head, after a month the piston came apart, bent the rod, and the pin wore a groove completely through the cylinder into the water jacket. as for the valve being stuck, I`d just take the head to a machine shop and let them handle it, good luck.
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Old 07-16-2003, 08:13 AM
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Hey DV, I've heard two different stories now about what the actual "knock" noise is when you spin a bearing. I've heard that it's the piston hitting the head, and it's the gap of the bearing and rod making the noise. Obviously you think it's the first one, but I was just wondering why I've heard two different stories. Any ideas?
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Old 07-16-2003, 11:05 AM
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Sure GT, I`ll say this, when I was in my teens, I listened to a guy that ran a rail dragster with a small block that ran high 4`s in the 8th mile, so I thought "this guy really must know what he`s talking about" he said to me "I don`t know why people call it a rod knocking, what you hear is the piston smacking the head, since there`s no more bearing, the excessive clearance makes the piston travel up further in the bore, far enough to create the "knock" you hear, cyntrifical force keeps the rod to the crank at various points during it`s revolution, but when the sudden direction changes, you hear the rod hitting the crank, but no where near as loud due to it being inside a crankcase with oil sloshing all around" anyways, that made sense to me, but I was never really sure until my friend did the rebuild on his engine and didn`t replace that one piston and that was all the proof I needed.
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Old 07-16-2003, 07:39 PM
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Arrow

It actually can be either, depending on the rod clearances and dack height. Usually the piston noise is from a spun rod bearing which is actually when one of the bearings comes loose and overlaps the other. If the lower one spins to overlap with the top one it will cause the piston to hit the head, if it is the other way it probably isn't going to hit the head but will still knock from the crank. When there is excessive wear in the bearings it too will knock and not be touching the head. If you cancel that cylinder when it's running and the noise disappears it is at the rod/crank, if it doesn't it's probably spun a bearing and is tapping the cylinder head.
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