I feel like I'm waiting on a doctor to tell me if I have a terminal disease. A few days ago I noticed a tapping noise coming from the engine when it was under load, it's a 305. It was gradually getting worse and worse. Took it to my mechanic and he found a rocker arm had come half way off a push rod. The tapping turned out to be backfiring. I think it was an exhaust valve. He hasn't had the chance to dig deeper yet so the cause it coming off is still unknown. But clearly visible was an elongated push rod hole, caused by the off center rod wearing it down. It created a gap about 2mm wide to one side. The mechanic still wants to check pressure, lifters & rods, cam shaft for damage but he also suggested it might be necessary to replace the heads too. I understand taking the head off to inspect but as long as there's no loose chunks of metal would an elongated push rod hole alone warrant replacing the heads?
Elongated Push Rod Hole
- Thread starter Maliwagon
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You won't have to replace the head for the pushrod hole, any good machine shop takes the time to open the holes up and install a guide plate. More than likely you wiped off a cam lobe or junked a lifter, that allows the pushrod to be really loose and it doesn't always sit in the cup correctly and will rub the opening. If it was me I would order cam and lifters, timing chain, probably the pushrods, do a valve job on the heads to ensure it didn't beat the seats out of them and put it back together. And most guys will tell you there is no need for guide plates, but put them in it saves a ton of problems, tap the holes for screw in rocker studs, and put them in. Maybe $125 total to instal them
That's what I'm hoping for. I've read about guideplates and thought replacing the whole head might not be necessary. I should know by the end of the day how the cam is. If a lobe on that got flattened I was thinking of going ahead with installing roller lifters and cam. The car idles okay, it just backfires a hard series of pops when revved up. Is that typical of cam damage?...beside the obvious valve not opening issue I already know I have. The roller kits are pretty damn expensive though, to the point you might as well do a whole engine rebuild. Know what I mean? Crower makes a close to stock spec camshaft that I'm interested in. This is a daily driver. BTW it has a gear drive, not a chain. Not a fan of the whine but it's not too loud.
I've had to have screw in studs and guide plates installed due to an elongated/ worn pushrod guide hole. Fortunately It didn't do further damage to the valve train...only for a bent pushrod. It might have slightly scrubbed the top of the valve, but the machine shop was able to resurface it...whew! It ran fine after I reassembled it. In fact it's still running good after 20+yrs.
Any recommended guide plate brands?
If there's a bent push rod or bad lifter do you typically have to replace ever rod and lifter? Couldn't you just get a matching new one? Sorry, I know it sounds like a n00bish question 😀
The backfiring didn't happen until after I revved it really hard to test out a new tachometer. But before that I heard a faint dieseling sound while warming up. I wonder if that was the rocker arm in the valve train loosening up and the hard rev was enough to throw it off the spring.
If there's a bent push rod or bad lifter do you typically have to replace ever rod and lifter? Couldn't you just get a matching new one? Sorry, I know it sounds like a n00bish question 😀
The backfiring didn't happen until after I revved it really hard to test out a new tachometer. But before that I heard a faint dieseling sound while warming up. I wonder if that was the rocker arm in the valve train loosening up and the hard rev was enough to throw it off the spring.
I would just replace the pushrod if it's bent, or if you determine it's a bad lifter, replace the lifter. You might have to buy a set of lifters if that's the case. I'm pretty sure pushrods can be purchased individually. You'll need to match up the old one with the new.
As far as guide plate brands, just shop around Summit or Jeg's. There's a host of companies that produce them. Comp Cams, Dart, etc.
To install screw in studs and guide plates, it would require pulling the heads and having the machine shop work done on them.
The stock studs would have to be pulled out, the surfaces would need to be machined and stud holes would have to have threads tapped. Also the stock pushrod guide bores in the heads would need to be drilled out.
There is a chance that you could have hurt the cam...unless the cam was on it's way out. 305's were notorious for cam failure.
As far as guide plate brands, just shop around Summit or Jeg's. There's a host of companies that produce them. Comp Cams, Dart, etc.
To install screw in studs and guide plates, it would require pulling the heads and having the machine shop work done on them.
The stock studs would have to be pulled out, the surfaces would need to be machined and stud holes would have to have threads tapped. Also the stock pushrod guide bores in the heads would need to be drilled out.
There is a chance that you could have hurt the cam...unless the cam was on it's way out. 305's were notorious for cam failure.
Stopped by the auto shop and kinda got some good and bad news. The good news is that each cylinder has good compression. The bad news is the other rocker on the same cylinder is ridiculously loose. I guess that's not a big surprise at this point. It started up and ran okay until giving it a harder push on the gas pedal, which made the rocker come unseated again. That was before noticing the second loose push rod. I'm taking it to another shop tomorrow. This guy sounded like he didn't want to risk doing work that may not solve the problem and who can blame him. I think I found someone with a lot more experience in this sort of thing anyways. Hoping the valves didn't float and strike the piston.
Another issue is I have some wobble in my water pump pulley, or at least it looks like a wobble. Haven't taken the belt off yet to see if there's any give to it by hand.
Another issue is I have some wobble in my water pump pulley, or at least it looks like a wobble. Haven't taken the belt off yet to see if there's any give to it by hand.
All SBC parts are dirt *ss cheap, don't put just one or two pushrods, change them all. If you want to do a roller cam now is the time, but find someone who knows performance type engine mods to do the work for you
Not sure about your mechanical abilities but it sounds like a good time to learn. Taking your car to the shop is going to cost $$$. I don't know what rockers you have on your engine, but chevy
made some self aligning stamped rockers that might be a quick fix. You have good compression so I wouldn't worry about the valves. The camshaft is another story and you can check the lift
off of the push rods. As I mentioned earlier check to see if a rocker stud is pulling out - put a straight edge across them and see if it's higher. Good luck!
made some self aligning stamped rockers that might be a quick fix. You have good compression so I wouldn't worry about the valves. The camshaft is another story and you can check the lift
off of the push rods. As I mentioned earlier check to see if a rocker stud is pulling out - put a straight edge across them and see if it's higher. Good luck!
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