I just bought a 76 GMC I thought the #7 exhaust cam lobe was flat by what the kid who owned it described to me but turns out the rocker arm broke at the bottom what could cause this to happen? The rockers look stock I know the motor has just ban re built new pistons rings comp cam and such just about every thing is new or machined I was thinking that maybe the cam was to big and I just need stronger rockers any one have any thoughts about what could cause a rocker to fail and split?  

hello my name is robert. I believe what probably happened is your valvespring was in coil bind. meaning your camshaft is to big for your valvetrain, or borderline fit. Or you might have a pushrod problem. Another quick way to check for coilbind is to use a paperclip and roll you engine over. Paperclip is about .060 dia. If your paperclip binds up between the valvespring coils its telling me your camshaft is to big for your valvetrain, or you have something not machined correctly in that cylinder head. If in coilbind it locks up and forces the stud to flex and eventually break off. Usually it pulls the stud out of the cylinder head. Do you have screw in rocker studs? What camshaft do you have? Need some more info to help troubleshoot. Good luck, ford guy with a lot of chevy experience. I work on them all.  

could be rocker arm was not adj properly to begin with  

The problem is I bought the truck from the owners son who knows just the basics about the truck I talked to the father briefly over the phone he dose not know the specs on the cam I am going to try and measure the cam lobe lift in a few weeks on the intake and exhaust to try and figure what it is. I am hoping it was just a bad rocker they drove the truck every day for a year before it broke I am going to check for coil bind on Saturday good idea with paper clip I will probably use a feeler gage being that I kneed one any wise but I can definitely see my self doing that. The studs are pressed in there stock 400 heads I don’t know the details on the machine work done to them. After I check to make sure springs are ok I am going to up grade to comp magnum roller tip rockers one piece pushrods and a high volume high pressure oil pump I hope this will prevent future failure. I know this is a little more than I probably kneed but I plan on replacing the heads with a set of SR torquers from world industries and doing a little mud drags and a few quarter mile passes a year so motor is going to get drove hard once in a wile.  

In reviewing this post I would tend to think that rocket failed because it was over reved and possible a valve floated, hit piston and then slammed into and broke rocker. Generally when you get to float they do not all float at same time RPM wise because of production tolerances in hardware spring rate. Stock valve train is pretty sturdy and rarely fails to reasons other than abuse (abuse meaning over reving). Nothing is going to be gained with a high volume oil pump as stock SB oiling system is quite up to task if clearances in crankshaft bearings are good. Anything over 50 PSI at higher RPM's is wasted and if you have pressure issues you really do not need a bigger pump you need to tighten up crankshaft bearing clearance. A wiser investment would be a AUX external engine oil cooler as the can extend oil and engine life under hard usage and raise average pressure a bit too. One more thing, 400 SB's do not take kindly to overheating so make every effort to keep it from loosing its cool. Also, with stock bottom end, rods and pistons I would not go over 5500 RPM regardless of cam if you want it to last a long time. They can tend to come apart around 6000 to 6500 and and do their best work at 5000 RPM. You need to spend some serious coin on rods and pistons if you want to take it higher safely and reliably. The 400 was born to give more torque with engine in de-rated emission configurations and never meant as a high output engine like 350. As long as you keep this in mind and do no push it much and keep it cool it can have a very long service life. If you want to go wild you are better off with a built 383 (350 with 400 crank) because it would hold together better and not have cooling issue that a 400 has with Siamesed cylinders with do not have full jacket cooling. A 400 with a 350 crank and rod is a 377 and a better way to go if you want to push a 400 SB because shorter stroke and longer rods improves midrange and top end power and adds about 1000 RPM to stock bottom end parts too plus you do not need 400 dampers or flywheels anymore either.  

I thought about valve float and hope that that’s not what happened I know not to spin the motor past 5,500. On the other hand if it was valve float I will have a good excuse to rebuild the motor the way I want early with out the girl friend getting mad. As far as heating goes I do plan on replacing the radiator with an aluminum piece next year with my taxes as well as a better water pump and maybe and electric fan in front or radiator. I was also thinking about pluming the rear of the motor through the intake so water flowed through the rear water passages of the head I have never done this before but it makes sense to me to do it and I don’t know why Chevy neglected to do it from the factory.  

Well I think I figured it out I went to check on my title today because it is being mailed from California and while I was at it I thought I would check to see if the push rod was bent it was not but I checked all the rockers and I could wiggle all of them but one there all to loose. So just to make sure when I do a valve adjustment and the truck is running I loosen rocker tell is starts to clatter then I adjust tell clatter goes away which should be zero lash and then turn an extra quarter turn rite?  

Valve train adjustment is correct. Some people do a 1/2 turn or 3/4 but I think 1/4 will be fine on a warmed up engine/ normal operating temp. On valve float you could have a weak spring or a valve sucking up into the seat. Look at valve stem heights and see if they are even. If one is higher than the rest thats the one that is probably floating. This will take away from your spring installed height which hurts spring pressure. On the adjustment they make a clip that bolts on to rocker to divert oil onto rocker instead of everywhere. Cheap little tool like 20 bucks if I remember correctly. Check you local auto parts shop. Works great with OEM rockers. Not so good with aluminum rockers. More info and good luck.  

reading what everyones idea is about this problem some have good ideas but if it has factory rocker arms the problem is the slot in the bottom is designed to run no more than 450 valve lift. what you need to do is buy a set of roller rockers or a set of long slot rocker arms.  

If you run lifters at zero plus 1/4 turn you will limit valve lift potential with stock rocker more than with zero plus a full turn. This is because reduced preload increase static rocker angle more and reduces additional possible travel for lift. I still think it broke because it was over rev'ed and floated and piston slammed valve into rocker.  

I find it hard to believe the rocker broke and the pushrod isn't bent. rockers do wear, I've never seen a broken or cracked one that wasn't plainly wore out.

for a daily driver low rpm engine roller rockers are a waste of money,  

I have actually seen a few broken ones in years past and they were on engines than had been over rev'ed. Seen a few pressed in studs come out too with aggressive cams and high lifts and failed ball pivots too. I do agree that roller rocker ar kinda a waste on a street truck unless maybe you are using them to increase lift ratio of rocker.  

One of the advantages of roller rockers is less friction. Meaning less heat to dissapate. Also when a roller pushes down on the valvestem it tends to push straighter than a factory rocker arm. Meanings friendly to valve guides and longer life. If your rocker arm is truly pushing down on valvestem dead center of valvestem this really helps. But if you rocker arm is a little off and doesn't push down straight your guilde wear will be affected and a roller is a little more friendly than OEM. Most people don't care but the little things are what matter. Bigger cams will affect this a little. On the .450 lift or less that is affecting your coilbind meaning you valve springs are not correct to your situation. Need more info on what cam was installed. Also need to do a pattern check on valvestem and see if rocker alighnment is good. Just more stuff to worry about.  

I checked push rod it is good the rockers are all to loose I can wiggle all from side to side except one the kid told me he and his friend did a valve adjustment once so I think that’s what did the rocker in. I don’t pick the truck up for another week I have ban waiting for the title to arrive from California because the father moved and took it with him it just got here Saturday. Not sure how I am getting truck home yet if I am driving or towing on Friday I am pulling the rocker from the intake of the cylinder that is broke and placing it on were the broke exhaust was, pulling the plug wire and cranking the motor over to see is the valve will open and close if every thing looks ok then I will just get a replacement rocker do a valve adjustment and attempt to drive home. If not then I have to tow it to storage tell I can fix because my land lord decided I cant have a broken vehicle on my property and told me he would have it towed off his land.  

I don’t know how to post pictures so here is a link to what the truck looks like. I will have more of the motor and the broken part in a few weeks but not bad for $600 if I can fix it.

http://www.cardomain.com/member_pages/show_image.pl?bg=FFFFFF&image=http://memimage.cardomain.com/member_images/12/web/3120000-3120999/3120919_1_full.jpg