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350 Making Noise after bent rod!

 
McChan
I love my Chevy Chevette!
Posts: 17
Joined: 10/03
Posted: 03/04/06 04:25 PM

Hey there, got a 350 that I built, chromolly pushrods roller rockers and 64cc heads that has bent a rod.  When I put a new rod in the thing and start to tighten up the rocker I get a sound that to me would indicate that the valve on the exhaust side is hitting the piston.  I have no clue how the rod bent or whats making the sound.  The valve seems to seat fine and move ok, but the noise is terrible.  The rocker is still loose when the noise starts.  I don't think that the valve could be hitting the piston, its the only problem with the motor right now.  I can run the truck on 7 cyl. no problem.  Is there something I am missing in the lower end that could be making the noise?  How did I bend a hardened rod without valve damage?  Could the valve be bent?

Thanks in advance!

 

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 03/06/06 03:30 PM

A bent push rod unless there is a material failure of the rod itself is the result of some valve train part ahead of it binding. This can range from over revving the engine to where a piston hits a valve and slams it backward. This can result in one or more of the following: a bent stem; bent or broken rocker; a pulled out, bent or busted rocker stud; a bent push rod; damage to the hydraulic lifer to the extent that it cannot properly maintain zero lash. Even the cam lobe can be damaged.


 


Other reasons for a bent push rod concentrate around events like coil bind or insufficient clearance between the top of the valve guide and the spring retainer.


 


Coil bind can happen for a couple reasons, perhaps the most common is that a high lift cam and/or the use of 1.6 ratio rockers increase the amount of lift at the valve to where either the spring coils are compressed to where they touch each other before the cam has lifted the valve to maximum lift, this is coil bind. The condition of high lift at the valve can also result in the spring retainer being driven into the top of the valve guide. In both cases, lift has been mechanically arrested by interfering structure and the weakest link fails, usually the push rod but as pointed out in the first paragraph, there is a list of possible failures and more than one can occur. Additionally, it’s not unknown for the head to crack around the spring seat when coil bind occurs. Another cause of these types of failures occurs at high RPMs where the spring goes into what usually called “float” Basically the springs internal frequencies cause it to fall out of synchronization with the cam action. The rocker looses contact with the valve stem and or push rod and the spring is in a compressed condition where it suddenly relaxes just as the cam is taking out all of the lash and the rocker is moving to open the valve without it actually touching the valve. As the spring uncoils and rocker moves into this space and is hit mighty hard by the ascending valve stem, the result is once again bent and broken parts.


 


Bogie

 

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McChan
I love my Chevy Chevette!
Posts: 17
Joined: 10/03
Posted: 03/07/06 08:27 AM

Hey there Oldbogie, thanks so much for the info.  I can tell you right off that I am not sure what actually happened to bend this rod, but for now I would just like to get the truck up and running on all 8 again.  I have stock length chromolly pushrods, 1.5 rockers screw in studs that were tight and forged aliu. pistons.  The cam is just a simple 1500 - 5000 rpm cam that I kept in there for torque.  The heads are 64cc pro topline torquers.  Nothing super crazy.  

I can tell you when the damage happened.  Its was super cold up here around -15f and I started the truck, it kept stalling so I kind of pardon the expression "hammered on it" and then it started making noise.  Drove the truck to work and back about 10 miles and then started to tear it apart thinking it was a loose lifter.  Got in there replaced the pushrod and then got a horrible noise on the cyl. like the valve stem was rapping the top of the pistion on the compression or exhause stroke.  This was with minimal not even 0 lash on the rocker?  Same stock lenght pushrod was used and same rocker?  What could be the problem?

Any help would be great as its my daily driver and gas up here is spendy don't really want to waste it on a blown cyl.

Thanks agian.

 

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 03/07/06 12:00 PM

At -15 degrees F, it’s mighty cold and a couple things are happening. First, at these temperatures gasoline doesn’t evaporate well; it is the evaporated product that burns, liquid gas does not. So getting a light off just for this reason can be very difficult. Not to mention that the ignition is struggling to develop a voltage and distribute it to the plugs. The oil is thick and not flowing which adds to the load on the starter which then is taking more electrical power than the battery can supply to keep above the 10 volt threshold that it needs to run both the starter and the ignition. The internal moving parts are trying to move against congealed oil or perhaps no oil so friction is very high. Second the forged pistons, which have loose clearances anyway, are more subject to shrinkage when cold than the cast iron bores so the rings loose seal because the piston *** sideways in their clearance space, this breaks the ring seal and allows compression to bleed into the crankcase. The lower compression results a less dense mixture that is harder to light off and what mixture is present burns cooler and develops less pressure, thus the force on the crankshaft my be insufficient to spin it. Also, what heat that is generated at this point is quickly lost to the cold surfaces and coolant. At the temperatures you’re talking about it’s really important to pre heat the engine with block sump heaters, or best by keeping the vehicle inside a heated space.


Back in the heads, the valve guides and stems which normally run almost with no lube are essentially dry as the cold oil cannot pass around the stem seal in any quantity that comes close to the already edgy lubrication requirement.


When you “hammered on it” the possibility is really high that an unlubricated valve stuck in its guide or the over revving caused it to fall into the path of a piston. The resulting collision probably bent the push rod. However, if this happened, it’s a strong probability that the valve head is bent in regard to the stem or the stem is bent. In the first case the valve can no longer seat when closed, in the second the valve is probably hung up, as it’s jammed in the guide. In the latter case, it’s highly probable that the valve cannot be removed without damaging the guide such that it will also need to be replaced. I’d also expect to see piston damage that may be mild to severe.


You might also find a broken valve spring; these can sometimes be difficult to see when the head is on the engine. Take a mirror and a strong light and look around the spring of the offending valve.


 


 Bogie

 

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McChan
I love my Chevy Chevette!
Posts: 17
Joined: 10/03
Posted: 03/07/06 02:19 PM

Ok well I understand the whole concept and it all makes total sence.  Let me go thought it again just to be sure I got it right.  I may have stuck a valve in the guide due to the cold oil and when I reved the engine it slamed it into the piston thus bending the push rod.  I think I have that right. 


The problem I am hearing doesn't sound like a stuck valve or bent head.  I haven't checked the compression on the cyl. but the valve looks fine on the spring, the spring looks fine and when I do put the rocker on there it seems to move fine.  Again these are all just looking but not actually testing things.  I can't understand what makes that noise when I put the rocker on to get the valve to open.  Also when I put the rockers on both studs and get them close to 0 lash the motor runs fine just have that clunk on what seems to be every two truns of the cam?  I assume something is hitting the piston but I don't know how the valve could be because it doesn't look bent or lower when the rocker is off the spring.  I know some about this stuff and how it works, but I just can't figure what this noise is coming from.  Its loud and really makes me wonder where its coming from. 


Thanks so much for your help.  I really want to get this fixed even if it means pulling heads.  I just don't want to pull the head if its not necessary. 

 

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Tom91653
I love my Chevy Chevette!
Posts: 17
Joined: 01/06
Posted: 03/08/06 01:41 PM

You need to do a compression check on that cyl. Sounds like you have a bend valve. This could have happened due to the cold oil pumping up the lifter and not being able to bleed of fast enough (or at all) and then holding the valve open. Only other options I can come up with are, a broken lifter or you have too long of a bolt in the intake manifold and it's hitting the push rod when opening the valve. 

 

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McChan
I love my Chevy Chevette!
Posts: 17
Joined: 10/03
Posted: 03/08/06 04:59 PM

Ok well I will check compresson on the cyl.  What should I be looking for with that size head for a compression number.  I guess I should just check another cyl. to see if its close.  If my valve is bent I should have 0 right?  

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 03/11/06 03:02 PM

Anyting less than 120 pounds is suspect. Also, all cylinders tested against each other should not vary more than 10-15 percent.


Testing in the cold will probably show nothing but low pressures, so expect that.


Bogie

 

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