427 Piston hit Valve
Pulled the 1966 427/425 Vette out to wash it. Hadn't been driven for a couple of weeks so I let it run on fast idle about 1200 RPM while I washed the car. After about 15-20 minutes there was a tapping sound for a few seconds then a couple of knocks and the engine turned off. Today pulled the head and have a bent intake valve and evidence where it hit the piston. The engine was rebuilt several years ago but probably less than 5000 miles since the rebuild. What in the world could have caused that?
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Re: 427 Piston hit Valve
Several things could have happened, head came off valve, valve stuck, retainer came off valve, or spring broke.Dick Whittington -
Re: 427 Piston hit Valve
Head did not come off valve. Retainer still on and valve spring not broken. Could be stuck valve, but why. See picture of rocker arm and piston where contact was made. If valve was stuck wouldn't piston just have pushed it back at contact?IMG_1217.jpgIMG_1227.jpgComment
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Re: 427 Piston hit Valve
[QUOTE=Ted Koehner (24972);685063]Head did not come off valve. Retainer still on and valve spring not broken. Could be stuck valve, but why. See picture of rocker arm and piston where contact was made. If valve was stuck wouldn't piston hjust have pushed it back at contact?IMG_1217.jpgIMG_1227.jpg[/QUOTE.
Numerous times I have seen valves stick. Lack of lube, corrosion, ate just a couple of things that could have happened.Dick WhittingtonComment
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Re: 427 Piston hit Valve
When you get valve out look at the stem for pulled metal. When new and a little tight guide it will pull metal and stick the valve then the cam will push the pushrod thru the rocker because valve won't open any more. Then piston comes up and hits the open valve. As the motor wears in the guides get looser but new they can be to tight or a bur when driving in a new guide.Comment
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Re: 427 Piston hit Valve
I have seen this happen many times when I was working at a GM dealership in Flint MI years ago (as a Mechanic) on almost every occasion the valve stuck in the guide as the engine temp began to increase to it's normal operating spec. Combination of thermo expansion and lack of valve guide lubrication. I have seen a tight/no lash adjustment cause this on a mechanical lifter set up. Same situation very tight lash and then the thermo expansion causes the valve no to close all the way resulting a slight contact.Comment
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Re: 427 Piston hit Valve
Usually happens to the exhaust valve and I think the use of the positive seal wipers that mount at the guide will not let the valve stem lube properly.
I still use the O-ring & umbrella whenever possible so the stems get oil.
You will use a quart of oil every 1000 miles and your valves will love it. The one problem was a oil build up on the intake tulip when the guide finally wore, BUT,
the guides & valve stems went over twice the life.
There was a time in the late 50's and thru the 60's when the dealerships got complaints of visable oil smoke when the new car was started, then went the chrome rings and in came the perfect circle valve seals. Solved that problem and caused other problems.
In still believe the valve stems should NOT have all the oil wiped off and should have enough oil to lube them and keep the carbon from building on the stem and guide.
DOMComment
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Re: 427 Piston hit Valve
Ted,
In your photo it looks like more than one head gasket. Why?Comment
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Re: 427 Piston hit Valve
I have this this happen on clearance being to tight.Comment
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Re: 427 Piston hit Valve
Great information and right on. Valve was stuck, evidence of pulled metal on valve stem. Valve replaced and in process of putting it together. The head seems a lot heavier when you are hanging over the fender trying to line it up than it was taking it off. Thanks for the input. TedComment
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Re: 427 Piston hit Valve
100 octane Avgas has some lead in it, the guides sure seem to like that bit of lubrication that ethanol does not supply.Comment
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Re: 427 Piston hit Valve
Ted;
When valves hit forged pistons they have a tendency too distort the piston and sometimes reduce the opening of the top piston ring grove. Which binds the top piston ring.
On a newly rebuilt engine the piston should be slightly movable laterally.And when you get the choice to sit it out or dance
I hope you dance
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Re: 427 Piston hit Valve
Most modern engines have "positive" guide seals, but the inlet is usually different than the exhaust. That's because the seal is designed to allow a "controlled leak". Since the inlet side usually has manifold vacuum across the guide that seal needs to have a lower leak rate than the exhaust side. Exhaust side positive seals may also use upgraded material like Viton because the exhaust valve stem is hotter than the inlet stem where nitrile will usually survive for a long time.
I like to see about .001" clearance on new inlet guides and .0015" on the exhaust side.
I agree that using the OE type O-ring seal and shield is a good way to go on the exhaust side. This prevents oil leaking down from the top between the stem and retainer, but the valve stem gets splash lubrication, which is limited by the shield. This system also works okay on the inlet side, but if one wants to see minimum oil consumption a properly designed and proven inlet guide positive seal is a good way to go.
DukeComment
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