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PostPosted: Wed Jul 15, 2020 9:07 pm 
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Well unfortunately after 8 yrs or so of faithful service, I have dropped a valve in my 408C. Valves were milodon stainless street valves. They are supposed to be for street and sportsman dirt/track cars. I called years ago and asked them what the upper limit is for these, and was told I would never break one. Head broke off exhaust valve and broke the probe reverse dome 16cc piston. Spring pressures were 160/388 with tool steel retainers. Car was taken to 7k regularly and 7500 a few times. Waiting for a little cooler weather to assess the total damage. Broke the CC 4 V head too. I’m hoping the block will be ok. Car made a lot of passes over the years. I did over rev it recently at the track and wonder if that may have contributed to its failure. Piston is a probe 14213-.030” over if anyone has any. Gonna take a while to get some funds to fix this. I was getting ready to swap this to a fox body soon and put the original Q code cleveland back in the Mach 1 soon. Failure happened while driving about 40 mph.


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PostPosted: Thu Jul 16, 2020 6:39 am 
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Man, sucks to hear this. 8 years of punishment. You think the springs were just weak from all the racing and the over-rev killed it?


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PostPosted: Thu Jul 16, 2020 7:33 am 
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Valve springs were replaced 1 1/2 years ago. And tool steel retainers were used to lighten valvetrain some.


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PostPosted: Thu Jul 16, 2020 8:43 am 
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Anytime I hear that an exhaust valve failed, I always look at valve float. When the exhaust valve floats, it hangs open and the piston tags it on the way up.

If you check your valve stems, my guess is that you'll find some butterfly patterns on them.

160 lbs of seat pressure isn't enough for a 7500 rpm engine.

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PostPosted: Thu Jul 16, 2020 10:11 am 
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Hate to see that for sure. I've got a few years on my Milodon valves, but they are the race valves. Probably need to have my local guy mag the things one of these days.

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PostPosted: Thu Jul 16, 2020 4:13 pm 
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Location: Orange County, NC
I hate to hear that as well.
Even after all those years it sucks when a good engine comes apart in any way.

What kind of cam? Solid flat, solid roller, hydro roller?

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PostPosted: Thu Jul 16, 2020 7:14 pm 
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It was a solid flat tappet. I normally shifted at 6800-7k. But I did take it to 7500 4 different times.
I had one recent pass That it felt like the trans C6 went from 1st to 3rd gear and I pulled it back down and Had to shift back up because it was revving too high. It was only for a second or two, but that may have caused some damage. I made 8 or so passes after that, then drove the 30 minutes back home. Do you think I had enough spring pressure to run 7k, but not much higher?


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PostPosted: Fri Jul 17, 2020 5:35 am 
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Standard stainless valves for 4bbl heads aren't exactly lightweight. Takes a lot of spring pressure to keep everything in line.

160 lbs seat at 6500-7000 rpm is ok. Going above 7000, it's not going to be enough, especially with a camshaft that has more aggressive lobes than the average camshaft. It may have been that you've lost some spring pressure over time as well. Those were just Comp springs, right?

If the head of the valve broke off, I'd lean towards valve float. When you get it apart, if you see butterfly patterns on tops of the valve stems, you'll know the spring pressure wasn't adequate. If you see a nice little straight contact pattern on all of them, then it could have been a fluke. However, I generally don't see exhaust valves break without any outside help.

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PostPosted: Fri Jul 17, 2020 9:14 am 
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blykins wrote:
Standard stainless valves for 4bbl heads aren't exactly lightweight. Takes a lot of spring pressure to keep everything in line.

160 lbs seat at 6500-7000 rpm is ok. Going above 7000, it's not going to be enough, especially with a camshaft that has more aggressive lobes than the average camshaft. It may have been that you've lost some spring pressure over time as well. Those were just Comp springs, right?

If the head of the valve broke off, I'd lean towards valve float. When you get it apart, if you see butterfly patterns on tops of the valve stems, you'll know the spring pressure wasn't adequate. If you see a nice little straight contact pattern on all of them, then it could have been a fluke. However, I generally don't see exhaust valves break without any outside help.


Yeah, easy for me to say but I've been a fan of replacing key engine parts that have been beat on before they break. As we all know, parts do have a life-cycle. Many racers (and I know yours wasn't a pure race engine) do replace springs, valves and even rockers on a scheduled basis. Pistons, rods and cranks on a longer cycle. Having beaten on mainly street engines in recent years, I'd not hesitate to spend a few bucks here and there to ensure longevity of the entire engine.


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PostPosted: Fri Jul 17, 2020 10:47 am 
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I'd bet on what Brent said in that it was over revved and probably got caught by the piston. Probably what happened is that it didn't break the valve it just tweaked it. Then after some running/flexing back and forth the head came off. I doubt that's the only one either. There are probably others that are slightly bent that just haven't broken yet.

My friend Wes had that happen on his big Ford with A460 heads and a 2.400 intake valve. The valve developed a crack across the head but did not break into two pieces. He changed all the valves and when I did the seats I found a few that were way off and all of those were non concentric in the same direction. I think it was acting just like having a bent valve where every time it came back to the seat it would bend just a little bit. It was only a matter of time before failure.


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