Crushing Valves

[cudacarl]

I have a 73' cuda 4-speed with a steel crank x-head 340. I had the heads rebuilt and plained about 2 years ago. I used the MoPar P/n P4120094 head gasket.The gasket is steel and compresses to .024"-.028" The cam is the purple cam shaft with .474 lift and 280 duration p/n P4452992 that I installed when i did the heads. The engine runs very strong but after about 4 months when i got on it pretty hard, one of the pistons bent one of the exhaust valves. I had the valve replaced and everything was fine. I thought it must have been just a fluke thing being i had put about 2000 (most times hard) miles on the engine with no problems. It wasn't a fluke. This past weekend almost exactly one year later I bent another exhaust valve. The valve geometry seems o.k. eyeballing it. Its a stock setup. I don't Know what pistons I have in the engine. The valve is being replaced and the heads checked out. I'm going to use the Felpro p/n 1008 head gasket this time. I wonder if I'm just on the edge of hitting the valve and if the thicker felpro will keep me safe or maybe i just shouldn't drive the car during the month of may. Any ideas?

[blk66sat]

Did you get valve springs for that lift of cam thay might be to weak of springs for that cam or you might be geting float you might check your preload on the lifters also

[amoparguy]

I agree with blk66sat, it sounds like valve float. You didn't mention what valvesprings you are using. If they are stock type springs they are probably not up to the task. Since the heads are off the car, now's the time to replace 'em. Also, speaking from personal experience, if you suffer piston to valve contact you better check your pushrods for being bent.

[cudacarl]

I had called the mopar hotline and they picked the cam and the springs for me. I don't recall what springs they were but i used the ones that they recommended.
As far as the pushrods I replaced all of them with new ones after I bent the valve the first time. But i does also sound like a floating valve to me too. It seems to only happen at high rpm. 5000 - 5500.

[rumblefish360]

I suffered the same problem. 4 pistons were dented. The springs were 8 years old and sat everywinter for months. Time took its toll. Old age came around for the spring. They just got tired. A new set fixed the problem and new life and feel of performance returned.
The stiffer the spring and stouter the cam, the shorter the life span.
The extra gasket thickness should help. Fel-Pro also makes a thicker head gasket as well. The perma torque blue is .054 crushed.

[mopar_man]

Thank GOD, somebody else has heard of valve float.
I replied to a thread AHHHHHHHHHH a while back to give my opinion on why his car died while drag racing. he said he had a tired old 383 that sputtered and shut off. i said it colud be valve float.
6pakman came back at me and said in all his years of turning wrenches , he had never heard of valve float .
I built my first engine when i was 12 yrs old ,that 40 yrs ago, i was beginning to think i had lost my mind.
Anybody that hasn't heard of valve floating have very little experience with hyd. valve trains!
Whats your opinion?

[cudacarl]

The Spring is Part Number MoPar P4120249. This Should Be Good For Lifts of .430/.500 . The springs Are Only 2 Years Old. My Lift is .474/.474 . Do You think I should go up to the spring .490/.535 eventhough the Mp Catalog says its for Larger cams than mine?

[Dr. Mopar]

Originally posted by mopar_man
Thank GOD, somebody else has heard of valve float.
I replied to a thread AHHHHHHHHHH a while back to give my opinion on why his car died while drag racing. he said he had a tired old 383 that sputtered and shut off. i said it colud be valve float.
6pakman came back at me and said in all his years of turning wrenches , he had never heard of valve float .
I built my first engine when i was 12 yrs old ,that 40 yrs ago, i was beginning to think i had lost my mind.
Anybody that hasn't heard of valve floating have very little experience with hyd. valve trains!
Whats your opinion?

I would say that those of you that suggest the springs are the problem are correct, remember the piston chases the exhaust valve shut, so if you have a incorrect or weak spring that is not closing the valve fast enough it will get hit by the piston. That explains why it only happens at high rpm.

[cudacarl]

Found Spring P/N
The Spring is Part Number MoPar P4120249. This Should Be Good For Lifts of .430/.500 . The springs Are Only 2 Years Old. My Lift is .474/.474 . Do You think I should go up to the spring .490/.535 eventhough the Mp Catalog says its for Larger cams than mine?

[SmokeDog]

A couple questions. Is it the same valve getting bent. Why the heck havent you checked valve to piston clearence. If the heads are off already check it by all means. Get a week spring,something you can compress with you'r fingers,and install it on valve. Install head without gasket. Rotate moter and check max clearence,valve to piston. Id say to check them ALL. While the head is off go down the cylenders ,bringing each to top dead center,and mesure debth of pistons in bore. This would do two things. Check for deck hight vareation,deck not true front to back. Or find that when rebuilt one rod may be a toutch longer then the rest. This happens when remaned to maney times,man reconing rod haveing bad day. I just find it hard to swallow only one valve has problem and you need springs. If it is in fact same valve could be as simple as valve guide (if bronze) hasent enough clearence,needs honed more for clearence. Just that i would cover all bases while it was apart,so i wasent just guessing. Best of luck SMOKEDOG

[cudacarl]

The First Time it Was the valve on the drivers side head. No 5 exhaust. The second time it was the passenger side head No 2 Exhaust . May be Coincidence same valve on each head. When i first put this together i took the car on long hard drives with no problems. Don't think its a clearance problem. It bent the first valve 3 months after getting it running. It bent the second valve 1 year later.Lots of hard miles in Between. I've taken it on 600 mile trips to tampa and homestead with no problems. It must just happen at very high revs. Think I'm going to put in the P5249847 spring that is for larger cams than mine.

[PRO]

Remember anytime you mill the heads youll need to shim the rocker shafts.otherwise youll increase your lifter preload which will cause a pumped up condition in the lifter leading to...........you guessed it.........valve float!!!!!!! lifter preload is: how far down the lifters plunger is depressed at 0 lift,chrysler reccomends .030-.060,most builders agree that even .010 is sufficent but when you mill your heads you ADD to this preload!!! and anything over .060 is NOT acceptable,it will cause valve floating which bends valves,cause excessive cam lobe wear,bend pushrods and even snap rockershafts so be sure to check it.........PRO........

[70gtx]

when checking piston to valve clearence check at least 20 degrees before and after top dead center,you will find the closest spot will be a few degrees before top dead center on exhaust valves.

[CaptNemo]

Have you checked seat height in the head? If 1 or 2 of the seats are higher in the heads than the rest ( very possible if there have been seats replaced, like hard seats for unleaded fuel ) Check the valve spring tension in a VS tester and see if they have lost pressure.
Checking the retainer height may give you a clue to seat height, look for large variations in height, and more and / or thicker spring shims.