Engine Broken Rocker Stud...

1978Cobra2

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Mar 25, 2014
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Well, I knew going into the cam swap that there was a chance for some "issues" to occur. I was mainly worried with the studs pulling out since the engine is a 73 and has press in studs. I think it all started when I first fired up the engine for the break in. On the driver side I had (notice I said had, lol) a tapping noise as if the rocker was loose. I went through adjusting the valves while the engine was running to try an eliminate what I thought was over-tight rockers or possibly a lifter that had not pumped up causing a miss. After the car had run for about an hour or so, I took it for a 3-4 mile ride and it ran fine other than a still existing missfire and tappingfrom the driverside bank of cylinders. The next day I started it up and let it run for a bit then had my brother put it in drive so I could set the idle. Coincidentally, as soon as it went into drive a noise that I did not appreciate arrived under the driver side valve cover. Sure enough the stud broke below the boss in the head.

I am trying to figure out what caused it. When the valves are fully open the springs have plenty of room between the coils, and the push rods are still straight. I should be able to replace this one with a screw in to see what happens.

What I have noticed is that the fulcrum that sits in the rocker under the nut seems excessively worn and that also the nut is run all the way down on the threads. I talked to my boss who runs the shop I work at and offered that the lifter could be bad causing the tapping noise and therefore caused me to tighten to nut to far down on the stud stressing it.

Any ideas would be appreciated. I am not to familiar with valve train geometry or other possible causes.

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That doesn't look good.....

Well, I guess first thing would be to check that push rod to see if it's bent. Then, and you probably won't like this, but I would suggest pulling the head to remove the broken stud. Whether you do it yourself or take it in somewhere, it will be a bit easier to do with it on a drill press/mill where you can drill a hole parallel to the stud in order to get it pulled out. I really don't think you're going to want to hand drill this. Then, if you want to go with screw in studs, the machinist can go down the line and drill/tap in one setup. Anyway, besides the head, when you pull the rest of the push rods, check them all. If you did anything wrong, they will tell the story as I believe them to be the weak point in the setup. Then, when re-assembling, adjust each rocker when the adjacent cam lobe for that cylinder is applying pressure. Just tighten the nut while spinning the push rod with your fingers. When you start to feel a little more resistance, stop and move to the next one. If done properly, you shouldn't have to adjust rockers again.

Now, if this had happened to me, I would also shop around and see if locating a set of GT40 iron heads from an Explorer were available and cheap enough. They will provide a HP improvement and I believe the valve train geometry is basically already set from the factory. IIRC, the rockers are basically a bolt on deal where you just torque them down like any other fastener. That would also require you to be able to get the rockers from the donor engine as well. The other option with those heads would be to locate screw in studs that would utilize the existing threaded holes in which case you would adjust the rockers as explained above. Oh, and another issue is you would want to change the valve springs in those heads too. But, if you did that with these heads, you would be most of the way there. The problem with them, again IIRC, is that the valves require different height springs OR different spring hold downs (the top "disc" over the springs). Anyway, either way you look at it, you will probably have to spend some money to make it right.
 
I checked the push rod and it appears pretty much straight, I will double check again tomorrow. When I re-adjusted the rockers I checked all the push rods then to make sure they were straight. That was only about 30 minutes of run time before the broken stud.

As far as hand drilling/tapping in the car, I am going to limit it to this one, a family friend runs a machine shop so I am going to trust him with this one. If it does not go as planned, hopefully he should be able to help me out with his mill. I'm not opposed to the idea of GT40's mainly because I'm not looking to make a racer out of the car and aluminum heads are a bit on the high side of the price range. The thing that has me worried is that the GT40's have some where around a ~63cc combustion chamber, from what I read, on a 73 302, that might take my CR down to low 8's or maybe even in the 7.9:1 ratio.

The way you said to adjust the rockers would be fore example, adjust cylinder 1 intake while the exhaust is open? The first two times I adjusted them, I went in firing order on TDC, but I noticed the pushrod never really snugged up. they either seemed too loose or completely tight. I just ran the nut down and torqued them to about 20-25 ft-lb which ended up being just shy of one full turn.

As far as spending money, It's just the nature of building a car I guess. I need to have an emergency "in case of catastrophic failure" stash somewhere.

Thanks for the help and resource. Hopefully by the weekend I should be able to run it again and see if maybe I just had a worn out stud!
 
Yes, when adjusting one valve on the cylinder, the other one would be open assuring you that the one you are working on is closed. So, for your example of the #1 cylinder, adjust the intake while the exhaust is open. And the tightness of the push rods is hard to explain, but it won't wiggle or really have any up/down play. If you have watched the video, the lifters are essentially another spring inside a cylinder or what have you. If you adjust the rockers within the travel of the plunger inside the lifter, they fill with oil to take the lash out making them adjustment free. But the spring pressure inside the lifter isn't really high since the oil that fills the lifter is what makes everything work properly. So it is basically a feel type thing. When you feel that your push rod is engaged against the spring pressure within the lifter, you give the adjustment nut on the rocker about a half a turn more to move your push rod within the lifter's operating range and you're done. I hope this helps, I don't feel I explained it well enough for someone trying this for a first time or anything. There are a number of different youtube videos that can help walk you through it probably better than reading my explanation....
 
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That's very good advice LILCBRA. For guys like me who don't have a good feel for when the rod is engaged against the spring pressure within the lifter, you can just attempt to rotate the pushrod. If it no longer rotates freely you've made contact. After that, another half a turn or so. This also works for rockers you adjust with shims - you want about 1/2 to 1 full turn to get to the torque spec on the bolt from the point that the pushrod no longer rotates. Too many additional turns and you've collapsed your lifters and risk damage to the rockers (as well as serious oiling problems). Too few additional turns and you'll get some nice ticking (light hammering which also damages the rockers and studs long-term). Shims or longer pushrods are then required if you are out of spec.

I'd suspect an adjustment problem might have caused the problem in the first place (too tight or loose). The other possibility (hopefully not) is that the cam has wiped a lobe during break-in which greatly increased clearances and hammered the rocker stud apart (just like adjusting them too tight or loose). (That neighboring rocker also looks like it's showing an awful lot of threads above the nut.) If you're not running a zinc additive in the oil, you might want to consider one, and definitely use break-in oil. Modern oils don't work with our flat-tappet cams any longer.
 
Sorry for the late reply. I haven't had too much time to look at it this week but hopefully this weekend I can compare all the studs. JOZSE you mentioned that it could have come from an initial adjustment error. Ever since I had first started it for the break in I notice a tapping noise. After I adjusted the rockers while running and it still persisted I figure it could be a bad lifter. Do you think that maybe a lifter that did not pump up would cause the rocker to be too loose, almost as if the push rod is too short? I took off a rocker from the number 5 cylinder and I can vertically move the push rod just a little bit. Both push rods for number 6 have no vertical movement at all. then again the engine has not run for a week so there might not be much pressure in them.

Thanks LILCBRA for the help. I will follow the instructions when I go to readjust the rockers again!
 
Hi my man, it sounds like you adjusted things correctly so I wasn't assuming it was an initial adjustment error. More likely where the adjustment was in the many years before you got here. :) Someone could have over-tightened back in 1979, or the old cam could have worn for a few decades... for all we know and that stud was being stressed for the last 30 years. Once it started to stretch you were unable to torque it (it just kept stretching) so you probably got the noise. A bad lifter could refuse to pump up and give you noise, absolutely, if they're new (along with your cam) I wouldn't suspect it. It does sound like it might be weak though since you mention it feels different from the others and had the ticking problem. Personally I wouldn't pull the manifold yet (I hate doing that) -- I'd wait until the stud was replaced, re-adjust to specs, and if you still get the noise, don't try to re-adjust, but replace the lifter. Hopefully, though, it was just the stud stretching and failing from an old issue and you won't get the noise at all.
 
I like your positive outlook! I ordered new ARP studs last night and I should have it replaced by Tuesday or Wednesday next week. I agree with pulling the intake. It's hard to justify pulling the Intake/heads without being completely certain, just because the cost of new gaskets and bolts, etc. can be around 100 dollars or more! Hopefully I can get the stud replaced with no problem. I will let you know what the outcome is!

Thanks for the help and all the information so far!