LT1driver

what is meant by 2 piece exhaust valve that has been breaking in other threads. explain 2-piece to me please as I must be a dummie as I have never seen or heard of this .... thanks.

Joe_Planet

i dont think they're 2 piece but they are hollow stem sodium filled or something like that unless i'm wrong

achilds

They are two pieces. A hollow stem welded to the valve. The stem has sodium in it which improves heat transfer out of the valve. Exhaust valves run at a very high temp as they only see the exhaust gas, not the nice cool intake charge into the combustion chamber. I believe that two piece hollow stem sodium valves were originally designed for aircraft engines, rolls royce comes to mind, but it might have been somebody else.

Sonic03inSC

Does anyone know what it is about them to make them break?

Joe_Planet

the weld and heat most likely

Ol'55

That is the 64k$ question right there. My guess (and it IS no more than a guess) is that more than the valve head parting from the stem causing the failure, I think the valve GUIDE galls (either through high heat, lack of lube, or both), seizes the valve stem holding it partially open and the piston closes the valve in a bad way. I'd like to read a study of this scenario with heat paths (the cooling passages in the heads)along with oiling capabilities, especially around #7. I believe heat is the root here. (But again, it's just a theory).

hoefi

Read my report here, http://forums.corvetteforum.com/c6-z...arranty-9.html Actually, I believe the guide gets looser and looser and eventually lead to valve head breaking off. I believe heat is a major factor but might not be the ONLY factor. In my case, the other six cylinders are OK.

achilds

So did you rebuild with GM two piece valves or go with one piece stainless? What about the valve guides, stock or something else?

ta-kid

Actually they are two piece once it breaks.

30YR W8T

Been thinking about this too, talking with a friend about this and he brought up a point I had not thought about (way more engine knowledge than I'll ever have) he commented about what could happen when valve springs are change out if a new cam is installed using the stock valves. More spring pressure would just compound any other issues going on. I just keep thinking about the RPM's these motors can turn and wonder what role that plays. Obviously when something goes wrong the failure can be catastrophic and take the whole motor out. So to your point about the valve possibly seizing and if this happens when internals are zinging at 7 grand, the end results would be pretty spectacular.