I have what was supposed to be a fresh 400 SBC, slight cam, low miles in my 32 Ford. I drove it 1000 miles to OKC, got there and broke 2 rockers. Replaced them, reset all the valves, on the drive home broke a different one, the one that I replaced popped over sideways and that pushrod bent and broke, replaced the p/rod and broken rocker, adjusted them, drove another 150 miles and another rocker broke, replaced it ( I was carrying spares) and drove the last 120 miles OK. The rpms are 2800 @ 70 to 75, I drove 65 to 75 the whole way. Can any of you guys shed some light? Thanks, Dale
Sounds like the valve springs are binding up and breaking the rockers. Usually the pushrods bend first, but either way something has to give. The other possibilty is maybe the valve springs are too stiff and breaking the rockers.
Well....without any cam data it's tough to say...but, if I had to make a guess, it's one of two things. 1) Your piston-to-valve clearance is insufficient (less likely) 2) Your valve springs are not correct for that cam; they are reaching coil bind when the cam is approaching full lift and forcing something to break. You may also have other bent pushrods as well.... (more likely) Is it both intake & exhaust rockers? What head castings are being used?
If they're stock rocker arms, the slots in some of the stock rocker arms are kind of short. When you get a cam with more lift, you sometimes need to replace them with "long slot" rockers. With the "short slots", the rocker can only rock back and forth so far without binding. You might want to look at the balls and rockers to see how they're wearing against each other. Maybe somebody switched them all around and put them out of order, so they're not mating well to each other and wearing prematurely. If you're using heads which were meant to be used with pushrod guides, which have huge wide openings for the pushrod to dance around in, and you don't have pushrod guides, then maybe that's the problem. Most older stock heads had narrow slots that served as pushrod guides to keep the rods from slopping side to side, but some aftermarket heads are hogged out there and meant to be used only with separate pushrod guides that are held in place by the screw-in rocker arm studs.
Yep, also a possibility...though in my exp. that tends to break the retainer first. Although, if the rockers are made out of that "good" Chinese steel... With std. press-in studs the stud generally breaks before the rocker. If the pushrods are not being guided properly, you end up with bent pushrods. The std. Chevy rocker is actually a pretty tough piece & when it breaks, ***uming it's not defective, it's generally because it is caught between a cam lobe/pushrod that won't move, & a spring/retainer that can't move. Almost anything else wrong will take out the skinny stock pushrods which serve somewhat as a fuse. All these are good ideas, though, and regardless of what exactly is the cause, it is safe to say that you have a serious ***embly/geometry problem. At this point, if you're not too versed in setting up a SBC valvetrain, you may want to consult someone....especially as I would bet that the cam is fatally damaged at this point. Sorry, man...
Depending on the cam lift, another possibility is if you are using stock rocker arms the slot is not long enough. There are long slot rockers available. I've had the situation with stock rockers where the crush thread rocker arm nut was pretty worn out and the rocker nut actually turned itself tighter while driving, causing the pushrod to push through the rocker arm pushrod indentation.
You might want to measure the length of the pushrods too to see if they're the correct length. It's possible the pushrods came from some other brand of engine or were mislabeled and they're too long or too short and ****ing the rocker arm at some unnatural angle, or making it bind on one side against something. Maybe somebody used a cam meant to be used with roller lifters instead of a flat tappet cam. Something sure is goofy, because rocker arms are usually pretty tough.
I doubt it's a roller type cam...I don't think it would have made a full revolution before bindint up and seriously breaking stuff. roller lobes are "peanut" shaped (slight, but you can see it if your lookin. things to consider.... #1- if the heads were milled to within an inch of thier life, the pushrod angles get goofy. pull a valve cover, and all the spark plugs (engine warm, but not "hot" hot. what you are going to do is spin the motor over by hand, and pay attention to what the tip of the rocker is doing in relation to the valve. it should start in the middle of the valve, and not move too far towards the front or rear of the tip as it cycles through. while you are at it, look for coil bind. remember that with a hydraulic cam, max lift is acheived via oil pressure, so if it looks "close" that could be the problem right there. if everything here looks kosher, pull a rocker arm and look at the inside (closest to the valve spring) of it- paying particular attention to where the rocker sits. ANY sign of touching, and you need different rocker arms. crane makes a 1.5 roller tip that would fix it up right, at the priceof slightly more than a set of stockers- when in doubt, fix it faster.
At this point, given the symptoms....as soon as I diagnosed the problem, I would be replacing all pushrods, rocker studs, rockers, probably the cam/lifters/timing set, and whatever parts were causing the problem (i.e. springs, for example).....and inspecting the bearings & oil galleries for particles. But that's me....and I figure that the corncob who ***embled it before Dale got it probably *****ed something else up as well. He obviously didn't know jack about valvetrain ***embly & checks. At the very least, take a long, close look at the oil & cut open the filter, looking for particle glitter....and check out every single rocker stud for pullout & damage.
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