I have 69 small block coupe that was rebuilt by the previous owner and was running good up until today.. I was driving today and then started hearing a loud knocking metallic noise.. I pulled over and called AAA to tow me home. I took of the right side valve cover and found part of a rocker arm stud laying in the head, but none of the rocker arm studs were missing.
I proceeded to remove the rockers and pushrods from cylinder #2 which seemed to be making the noise and the rocker stud on the exhaust valve was broken right in the middle of the nut. When I removed the rocker nut the top part of the stud came with it. Also the pushrod on the intake side was galled very bad. I replaced both rocker studs, pushrods, and rocker arms with new one and attempted to start the engine. The engine immediately backfired through the carb..
I tried bumping it again to make sure my valve lash was good and it backfired again when it got to # 2 cylinder..
Any ideas what might be going on with this??
I am pretty mechanically inclined, but when it comes to internal engine diag, not so much..

 

Too much valve lash on #2 possibly. Loosen the rocker nut some. I don't understand the part about a rocker stud lying on the head and none missing.

 

So, it appears the one making noise isn't the first one to break. Are those slotted rocker arms? I wonder if cam lift is too high, and they're bottoming out? That would also explain the galled pushrod.

 

Not a guru, but check the timing, distributor loose?

 

How are you adjusting valves? If you're spinning the pushrod and looking for resistance, you're probably over-tightening. Hold the pushrod between the first and second thumbs of one hand and rattle it up and down while you tighten the nut. Once all the up and down movement stops, you're at zero, go 1/2 turn past that. Backfire is a sign that spark is making its way into the intake, likely through a valve that's being held open.

 

I will check the lash later after work. I am guessing, after finding the mystery broken rocker stud that this has been a problem in the past.The cam is pretty much a stock grind, nothing radical..
What would cause these to break?

 

If it's not a high lift cam, then maybe the valve(s) have hit the piston in that cylinder. It could have happened during a high-RPM run before you bought the car. It's also possible the PO reused some of the original parts during the rebuild that should have been replaced.

Is the galled pushrod bent? If not, it would be tempting to replace the pushrod and broken stud and see how it does. However, if there are other issues you can't see, running the engine could do additional damage. I think I would want to use a bore scope at this point to inspect that cylinder before continuing. If you do this yourself, be sure to buy or rent one that can focus close. I don't own one myself, but I've been reading that some of them are useless for focal distances of less than five or six inches. Maybe someone here can make a recommendation for an appropriate model.

 

It was probably heat from not getting enough oil The nut would get very hot thereby compromising all of that particular cylinders valve train components. I would crank it over and make sure oil is getting to all the rocker arms

 

Oil problems would also cause galling at the rocker arm ball and seat (assuming these are standard rockers - the OP hasn't said). Then there's the possibility these are crap Chinese parts. The stud broken inside the adjuster nut is worrisome, don't think I've ever heard of anything like that. Then there's the loose stud that was left sitting on top of the head. I think someone who probably shouldn't have been in there has "worked" on this problem before. Assuming there's oil at all the arms and no visible damage in the cylinder(s), I'd probably yank all the arms and studs and replace them with new parts from GM.

 

If you take off the two rockers on the cylinder in question and pump air into the chamber through the spark plug hole it should not be coming out of the carb or exhaust. If you hear a loud air rushing noise then you have a bent valve which would cause the blowback you are getting. If the car ran fine before and you did not loosen the dist then timing should not be the issue. You can use a compression tester hose to put compressed air into the cylinder. Make sure you remove the schreader valve on the engine side end first. If this is good then you know the internal sealing is good and not the issue. Or you can just do a cranking compression test but the way I described will show which valve if that is the problem.

 

You do not have to put the piston in any certain position to do this test…just make sure your fingers are not near any belts or normally turning parts because the air pressure will most likely cause the engine to turn until the piston is at the bottom of the stroke

 

Corvettetodd- I love it when I hear "rebuilt by previous owner" and then internal mechanical parts start to break. You have broken pieces of steel in the oil inside the valve cover that sound like you are unsure how long they have been there or exactly where they came from and other parts that have galling and damage. If this were just a rocker that came off and everything was still in place you could safely just reattach the rocker. You have no idea whether steel pieces followed the oil to the oil pan or went somewhere else. You can drive the car and hope nothing else happens or -if it were my car- verify that nothing got anywhere else. Drain the oil and look for steel fragments. Pull the heads, look for steel pieces hung up in the oil return passages and check for cylinder scoring. Check your #2 cylinder for damage to the valves. Or- drive it and cross your fingers. A couple small steel pieces getting thru the screen on the oil pump pickup will jam the pump gears, stop the pump and twirl the pump drive rod like one of those twisted soda straws. Then you have to pull the engine. Don't ask how I found that out. It would be nice to get the oil pan off to check for any larger steel pieces that did not come out with the drain oil, but maybe you can get a fiber optic scope thru the drain hole and look. If you are lucky, nothing got below the head. Worst case, if stuff did get below, is sooner or later you will be rebuilding the engine. Hope you are lucky. RA

 

As was mentioned maybe oil starvation, so what does the oil pressure read? I too love it when the previous owner rebuilds, did he do it in his drive way or little metal meth shed out on the back corner of the yard, or did he tear it down, have it hot tanked, get the cyclinders bored with torque plates to spec, mic the crank shaft and if needed get it turned back into spec....ect. you get the idea, unless he gave you receipts for all the work and parts that he bought, you don't have any idea. So what came to mind with me was maybe the geometry on the rocker arms is off, if the PO had the block decked, maybe the heads resurfaced or replaced the heads with aftermarket heads or used different lifters, it could put the rocker arm geometry off and some times enough to break studs, valves. So on the valve that the rocker arm stud is broke, the very top of the valve should show a wear mark from the rocker, if the rocker arm is a roller or roller tip it would show even better, the original rocker might not leave that great of a mark as the area of contact is a lot larger. If it's roller tip or roller, there should be a line worn on the tip of the valve, if so were is that mark. If it's the original rocker, you can buy a plastic tool to measure it, but given that the studs are replaceable I'd bet that it has at least roller tip rockers. The question is were does the valve tip show wear? You want the mark as close to the center as you can and you want a thin straight line, if its to far to the out or in side the geometry is off, or if the the line is thick the geometry is probably off, or if the line is thin in the middle and thick on the ends of the wear line. Check that out and let us know, maybe if you can post a photo of the wear mark. As to the back fire through the carb, I'd figure that in the process of setting the preload on the lifters, after replacing the stud and pushrod you've gotten the intake valve set a little tight. First thing first, look to see were the wear marks are. After we get the cause figured out we can go through setting the preload. Another question, if you can list the parts to the engine like what brand and make of heads, rockers, camshaft lifters, block orignal, other small block chevy or an aftermarket block, ect. this might help us in the diagnosis. I had a rocker stud snap off one time with only .100" too short on the pushrod.

 

This thread reminds me of a Craigslist ad that ran some time back for my other favorite vehicle - a Lotus Esprit. The car wasn't running, but the seller said the motor had been rebuilt and only needed final integration to be finished. I asked who rebuilt the motor, and when he replied that he did it himself, I (very politely) asked what experience he had with Lotus motors and if he had receipts for the machine work. That really pissed him off, and he immediately went into a rant that ended with him saying he wouldn't sell me the car. I told him that was OK, I wasn't looking for junk, and that was the end of that.

To the OP: I'm not claiming any similarity between that story and your current situation. I'm sure everyone is hoping the motor in your car has only suffered a minor issue that can be easily repaired. Regardless of how it turns out, you'll find a lot of expert assistance in this forum.

 

If you're blowing rocker studs out on a well worn engine and it's back firing, it sounds like the timing chain jumped and you smacked a few valves. I would line it up on top dead center and make sure everything is in the right spot. Rocker studs usually give warning before pulling out. You'll be adjusting that rocker every few days and then you'll notice the amount of threads sticking up past the nut. I don't think I've ever heard of a back fire popping a stud up but I guess stress on a closing valve could help it along.

 

The timing chain is another option that I hadn't thought about, but I'd bet a nickle to a dollar that the geometry is off, I've seen the geometry break the studs before and unless I've missed something in the op's thread, He's replaced the broken stud, so I'm guessing that the studs are screw in, which would mean either aftermarket heads or modified heads. We should know more when the op posts his combo.

 

The previous owner has receipts for the rebuild from a local machine shop.
My neighbor who is a retired master mechanic came over and helped me tonight .. we checked the lash on the valves again and tried to fire it up. It still backfired through the carb. He had me pull the valve cover on the other side and sure enough, the rocker stud on the exhaust valve on number 1 cylinder was broken as well. I replace both studs on that cylinder and fired it up and she runs nice and smooth and quiet. My neighbor suggested replacing the rest of the studs as I bought a pack of 16 ARP studs. He thinks that the rebuilder may have put crap quality rocker studs in when rebuilding it.
I understand that over revving the engine can break these , but I have never revved it over 5,000 rpm.
The wear pattern on the valves and rockers looks normal. The oil pressure is always about 35 at idle, and 60 or so when cold.. We started the engine with the valve covers off and the top end is getting plenty of oil..

Thanks for all of the help..

 

I'm glad youve got it running, wish you could post a photo of the valve tip. Hopefully it stays good, but given it happened to you out of the blue, chances are that something is not right and it might happen again, lets hope that it was fluke and all was well.

 

Wow - what about all the OTHER parts he replaced? Is there a warranty? I remember back in the day, I'd reached the point where I had four or five good solid SBC/BBC rebuilds under my belt. So, I decided to try a few different things, just for variety. That included trying several non-GM sources for routine items that I'd previously always purchased from the Chevy dealer. It was the first time (and the last) I've ever had a head bolt shear off while torquing it down. And that was when Mr. Gasket engine parts were still made in USA. The OTC bolts at the dealership were ugly as sin, but like most OEM parts, they did what they were supposed to.

 

Are the lifters hydraulic or mechanical? It is my understanding that 69 SB were hydraulic but PO could have changed them.