Thread: Broken valve

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Hello all...
Well I just finish building my sbc. First day I ran it for 15 minutes and everything was great. Next day, started it up and ran great for about 10 minutes. Then all of a sudden I hear a small rattling sound and then a abrupt engine stop. At this point I start sweating a lot and thinking WTF. Coolant is now running from the header. So I tear it down and to my disbelief a exhaust valve had broke off, bounced around cracked the cylinder wall. Screwed up a new piston and then jammed itself up into the head. Definitely a bad day! So here's my question. I can't seam to figure out why the exhaust valve broke to begin with.
Engine info;
350 bored 40 over.
Forged flat top Pistons.
6 inch forged rods.
Scat stroker crank.
Dart pro 1 215 heads. 2.055/1.60 valves.
Lunati 630 lift roller cam.
1.6 roller rockers

There was no evidence that the exhaust valve hit the piston to start this problem. So if anybody can help me out by letting me know what might have caused this please let me know so this doesn't happen again I would be great full. Thanks

Welcome to CHR, and sorry to hear of your problem. Just wondering, and maybe to focus the answers of more qualified guys here, what point on the exhaust valve broke/failed? Can you perhaps post a couple of pictures, showing the fracture points on the pieces, if they weren't destroyed in the process?

First, welcome!

Originally Posted by Dotrix

Lunati 630 lift roller cam.
1.6 roller rockers

Not an expert and just curious, why you are using 1.6 ratio rockers with a 630 lift cam?

Good idea. The valve broke right where it starts to taper out.

I'm using 1.6 rockers to add a little more horsepower.

How did you check piston to valve clearance, and how much did you have?

with those valve sizes the locations have to been moved over-------possibly didn't fit to piston notches anymore----------and with lift and timing specs 1.6 rockers its pretty busy as the piston nears top of stroke faster than valve is closing---

Yep, usually happens within 15 degrees of top dead center with the piston on the overlap cycle. The piston is chasing the exhaust valve back onto it's seat and this time, it got the tag. For what it's worth, I have seen motors lose power going from 1.5 to 1.6 and if there is a gain at all, it's normally only 10-12 hp. Hardly worth the expense and effort, not to mention the extra hardship that's placed on the entire valvetrain. If you want more hp, raise the static compression ratio and use more cam.

Zyglo the piston for cracks.
https://www.youtube.com/watch?v=llHg1ZEctgU

There is a possibility that this valve was sub-par also, and just fragged itself "because"......

.

Have Dart started "outsourcing" their valves to, um, the Orient, perhaps?????