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Old 05-28-2013, 10:15 PM
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Broken exhaust valve - What caused this????

Ok guys, we had a catastrophy in our new 400 SBC Sprint Car motor this last weekend. We were running 1st place in the A-Main and at lap 12, the car started blowing oil out of the left side header. It somehow still ran great to finish the race (finished 3rd). Tore it down today and here is what we found, dropped #5 exhaust valve. I have two guesses of what may have caused this. Your input is appreciated too.

(1) Incorrect valve timing - Installed straight up
(2) Roller cam walked forward - No retainer plate used

I am leaning more towards the cam walking forward and catching the ajacent cam lobe on the edge of the roller lifter and opening the valve. After close inspection, it appears that every exhaust valve has been tapping the top of the pistons but this one just could'nt take anymore.
This motor uses a dry sump oil system so I would think that the pump would keep the cam in place. I will check that when I reassemble it.

Now I have to get the AFR head fixed and buy one SRP piston. Luckily it did not do any damage to the new Dart block but the valve head is nowhere to be found, it must be laying on the track we were racing at. We did find the two slugs from the piston in the oil pan.
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Old 05-28-2013, 10:35 PM
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If the other valves kissed then I would think there's your answer. Your p to v clearance may have been tighter here or you lofted a valve or something else. I would increase your p to v clearance when you get this fixed,the others kissing is just a warming that you're too tight. I know finding that clearance can be hard but you obviously need it. Gasket, valve seats, eye brows, something has to be done to make this work better and prevent it from happening again. Adjusting your cam some may help but 4 degrees isn't much and more than that and you'll start losing significant power. The best solution may be sinking the seats if you can.
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Old 05-29-2013, 05:36 AM
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What's interesting is that on the piston you're showing that it's the intake valve that's leaving a mark.
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Old 05-29-2013, 07:18 AM
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Any valve that hit a piston is ruined and must be replaced.
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Old 05-29-2013, 07:28 AM
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Yeah that intake valve looks a little chewed around the edges.
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Old 05-29-2013, 10:48 PM
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Quote:
Originally Posted by engineczar View Post
What's interesting is that on the piston you're showing that it's the intake valve that's leaving a mark.
Definately exhaust on all pistons. Here are 5 pictures that I'm allowed to post. I really am not sure how this motor lasted 4 races at 8000 rpm before this happened if the P2V clearance was this tight to leave valve outlines on the pistons. I think (and will verify) that the roller cam walked forward too far during on/off hard acceleration and the adjacent cam lobes were lifting the wrong lifters. The reason this is plausible is because when I tore the motor down, you can see that the lifter rollers were not running in the center of the cam lobe and the edge of the lifter was very close (almost touching) the adjacent cam lobe. I would say that if the cam moved another .015-.020 that it would hit the lifter edge. Maybe I'm grabbing at things in this thought. Either way, when it goes back together, I will check it all very well.
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Old 05-30-2013, 01:31 AM
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S10,
You're theory of the lifter being pushed by the adjacent cam lobe doesn't seem plausible to me based on you describing how well the motor ran. If that was actually happenning, and to the point of piston to valve interference, then I'm confident that you would have noticed a loss of power, black smoke, different tone to the exhaust or something. If it was actually happenning then that means that you are bleeding off compression at some point and you would/should have noticed it in performance.

Also, why aren't you using a cam button, retainer, etc. to hold the cam in place? Is there an advantage to not using some type of cam retainer?

I agree with others, your piston to valve clearance is to tight and what happenened is #5 was the 1st to have a catastrophic failure. I'd check the clearance ASAP so you can find out how much/little you haveso that you can determine what your best option would be.

My 2 cents and good luck!
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Old 05-30-2013, 11:17 AM
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In your first picture of the head you can see the #3 exhaust valve has a track around the edge from hitting the piston. I would replace all the exhaust valves and do something to increase clearance.
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Old 05-30-2013, 12:10 PM
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Well during further checking, I also found this issue but I am not sure why or even how. It is not this bad on all cylinders but there is some on all of them. The picture is of #6 cylinder. I also found during some checking by putting the straightedge across the bores and measuring to the piston decks that not all of them are the same. This leads me to believe that the crank is not indexed correctly. I can understand that more than how the piston can be crooked in the bore. Any ideas?
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Old 05-30-2013, 01:05 PM
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If the engine ran for 3 races and lived, then the valve springs only lasted 3 races, or, the engine was over-revved.
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