Broken Intake Push Rod #7 Cylinder. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-08-2011, 11:43 PM
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Broken Intake Push Rod #7 Cylinder.

Howdy Everyone,

I have a question about a busted pushrod in a newly rebuild Big Block Chevy.

After the rebuild the engine was run on a test stand to break in the new cam, check temps, fluid levels, timing and leaks. Everything went as it should have, very quiet engine and very responsive. Drained the oil and installed it in my truck, filled all fluids, new oil, antifreeze, checked trans as it was rebuild also.

Once installed in my truck and fired up the engine seemed to "load up" with exccesive fuel so I revved it to clean it out.....it rev's VERY easy, almost TOO easy. It started making a "ggggrrrrrddddd" rattle in the top end during the rev.....I shut it down. I do not have a tach installed in the truck as of yet so I am not sure of the rpm this happened.

Shaking my head I popped the drivers side valve cover and discovered #7 intake pushrod snapped about an inch from where it once rested in the lifter, I pulled the intake and luckily the broken peice was sitting in the lifter valley next to the lifter. The result was a broken pushrod and a bent valve (at least). Luckily the valve did not touch the piston.

Everything was brand new, broken in on the stand at a varying 2000-2800 rpm for 20+ minutes. My timing was set at the 5 deg called for on the cam sheet and no more, (GM calls for 4 deg stock on this engine) the thing fired right up no questions asked and ran very smooth on the stand and in the truck until the "rattle".

What are possible causes for this? I am assuming I over revved it and am looking at installing a rev limiter. I have replacement heads, cam, timing set, and lifters to go back in.

Both cams, the one in it when it "rattled" and the new one are Edelbrock #2162 cam and lifter kit. The heads were and are completely rebuild with new seats, guides, seals, valves, 3 angle valve job on seats and valves and the Edelbrock springs to match the cam.(spring #5862 non-rotator) Installed spring height and interference was checked and rechecked. I am also using Comp Cams Magnum Roller Tip Rockers, 3/8 pushrods, guideplates and new rocker studs.

This is not a huge power rebuild, mostly stock except for the Edelbrock cam, lifters, valve springs, intake, and carb. (1406) (Edelbrock package, so they are all matched.)

Is it possible that my timing "slipped" or the vacumm advance is bad allowing it to continue to rev?

Thanks in advance for any suggestions or advice.

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Old 07-09-2011, 10:40 AM
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What bent the valve?That is why the pushrod broke.
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Old 07-09-2011, 10:46 AM
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Quote:
Originally Posted by SDLuck
What bent the valve?That is why the pushrod broke.
X2, the valve had to have hit something IMO to bend, if you were revving it high, maybe you floated that valve and it did kiss the piston. You have the head off?
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Old 07-09-2011, 12:25 PM
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Quote:
Originally Posted by 68NovaSS
X2, the valve had to have hit something IMO to bend, if you were revving it high, maybe you floated that valve and it did kiss the piston. You have the head off?

Quote:
Originally Posted by SDLuck
What bent the valve?That is why the pushrod broke.


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Old 07-09-2011, 12:48 PM
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You said #7, is that the corner where the chain from your engine hoist was attached? (that bent the valve).
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Old 07-12-2011, 12:49 AM
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I do have the heads off, no marks at all on the piston.

I had the intake on and used an intake mounted engine lift bracket to install the engine. The valve covers were on when I installed it in the truck fresh off the engine test stand.

I will take another look at the piston. In my initial inspection I did not see any marks. Is it possible the valve floated and the push rod jumped out of the lifter causing it to break in turn bending the valve? Not sure if that makes sense actully, the springs would have to have a great deal of return rate to bend the valve...hmmmm

Maybe I will just pull the thing and look it over again on the stand.

Thnk you gents, now I am wondering big time.
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Old 07-12-2011, 01:47 AM
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Thai is right.
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Old 07-12-2011, 09:29 AM
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I have seen where there is a problem with new valve guides and the valve sticking in them and hitting a piston,but either way there is some mark.
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Old 07-12-2011, 12:23 PM
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I did some investigating, talking to the gentlemn in the shop that built my heads. They built both sets at the same time so I was curious as to where they were getting the parts, I as many would assume they are going to use quality over quantity, one would think anyway for as much as we pay to have these types of operations performed.

It seems they do not order specific valves for rebuilds unless specifically asked to do so. The fella who built my heads was explaining that on occasion (as in my case) some of the valves are not up to snuff. They pass all the quality checks at the factory but once in awhile there will be one or even a dozen that are "bad" as he described it.

I had taken the bent valve with me to show him I wasn't pulling his chain and we looked at where the head meets the shaft.......under a magnifying glass (I dont own one myself) there is a crack. Explaining that in some instances the metal does not do what it is suppose to do during the construction period, "metallurgical impurities" as he decribed.

He explained that with even the slightest imperfection will cause issues, at this point I have to beleive him as my engine sits idle (no pun intended). He said that the valve, as most everyone has mentioned kissed the piston and there is a chance I wont see a mark.

He is going to completely redo both sets of heads at no cost to me. I am skeptical to have them do them but I told him specifically NOT to use the valves used before. I requested stainless steel one peice replacements.

I am wondering if I should also have them replace all of the springs in that head or would they most likely be ok?

The gentleman who is doing the work directed me to this page to read about bad valves. I thought I might share the info, the information itself is copy writed but the link is not.

http://www.aa1car.com/library/ar1192.htm

Thank you guys as usuall, I found what I was looking for from the great minds and experience of this site.
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