engine question
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Senior User
Joined: Dec 2001
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engine question
last night i pulled the heads on my 390 to fix a bad exhaust manifold leak. i finally had enough of the noise!
well, i was surprised to find that the exhaust valve head in the #1 cylinder was somewhat 'oblong' and chunks of it were missing from the edges (12 o'clock and 6 o'clock position) what would cause this? i was running the heck out of this truck for some time now. could this thing have snapped on me? i NEVER saw this kind of damage before. now i dont know how to replace this. should i just buy a new valve and be done with it or should i trash the head? any input is greatly appreciated!
regards
ben
well, i was surprised to find that the exhaust valve head in the #1 cylinder was somewhat 'oblong' and chunks of it were missing from the edges (12 o'clock and 6 o'clock position) what would cause this? i was running the heck out of this truck for some time now. could this thing have snapped on me? i NEVER saw this kind of damage before. now i dont know how to replace this. should i just buy a new valve and be done with it or should i trash the head? any input is greatly appreciated!regards
ben
Dont trash the heads... Just pull em both and take them to a machine shop for a valve job
Elder User
Joined: Mar 2002
Posts: 609
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From: Watauga TEXAS
engine question
I wouldn't do a valve job if you have high miles on this engine, you'll blow out the bottom end. I have replaced and lapped in just the bad valves on almost all of my old trucks and they have lasted for years. I had a 360 that had 4 bad valves (between both heads) and it ran without a tick after replaceing them. Make sure you replace your timing chain- A worn out chain is almost always the cause of burnt valves.
Elder User
Joined: Aug 2001
Posts: 962
Likes: 2
From: Collierville, TN
engine question
Sounds like your exhaust valve is burnt. It's not that uncommon. As someone else mentioned, improper valve timing can cause it. If you have a lot of miles on the engine, your timing set probably needs to be replaced. When I tore down my 390 last weekend, the timing chain was so loose that I was able to lift it off the sprockets and remove them with the timing sprockets still mounted to the crank and camshaft. That's bad! Also, the original timing sets had nylon components that wear and shatter. I found a handful of chewed up timing gear teeth in my oilpan.
Another possible cause of a burnt exhaust valve is an engine that is running too lean. Check your carb mixture.
I agree that having the heads rebuilt entirely doesn't make sense if this is a high mileage engine. You have two options -- have the damaged valve or valves replaced, put the heads back on with new gaskets, and keep driving until the next thing breaks, OR you can do a complete rebuild. Even if you just replace the damaged valve, though, you should check your timing set and replace it if it's worn out.
Another possible cause of a burnt exhaust valve is an engine that is running too lean. Check your carb mixture.
I agree that having the heads rebuilt entirely doesn't make sense if this is a high mileage engine. You have two options -- have the damaged valve or valves replaced, put the heads back on with new gaskets, and keep driving until the next thing breaks, OR you can do a complete rebuild. Even if you just replace the damaged valve, though, you should check your timing set and replace it if it's worn out.
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