I am trying to identify the cause of a knocking sound coming from the number 6 or 8 cylinder on my 427. During warm up the motor sounds fine, but after about 10 minutes or so the knocking / clicking / tapping sound starts. It seems to be louder after I do a full throttle run.

I recently changed to Edelbrock heads. When I had the other heads on the car, I didn't have any problems. I used the same cam with the matching lifters.

I adjusted the valves with the engine running. I backed each one off until I heard the clicking sound and then tightened them about 3/4's of a turn. I warmed the car up and re-adjusted the number 6 and 8 rockers three times, but the sound didn't go away. I even went so far as to turn them 1-1/4 turns, but it didn't help.

Any thoughts about what it could be? If your're think that it's a bad lifter, I find it odd that the lifter would start making the noise at the exact time I replaced the heads.

Thanks
Rick

Are you sure you don't have an exhaust leak? This can sometimes sound like a rocker tapping.

From the way you described adjusting your rockers I'm guessing your running solid lifters. If not, you adjusting hydros way wrong. Recheck your exhaust manifold gaskets as suggested above. Though that will sound more like a tic sound than a knock. Though knock sounds like you might have something in the cylinder detonating the fuel prematurely. Check the gasket, confirm the type of lifters and adjust correctly. if still there. I'd say pull the head and check that there isn't something sitting on top of the piston.

If you have any sharp edges around the combustion chamber or spark plug they can glow red hot and detonate the charge. Also, make sure the reach on your plugs isn't too long. The higher the compression, the worse the problem can be. Pull your plugs and look for tiny balls of slag on the isulator and electrodes. You'll need a good magnifying glass or 25X hand-held scope to see them. I hope it is an exhaust leak though.

It sounds like I should try replacing the header gasket first. Hopefully that's all it is.

The lifters are hydraulic. I like to adjust the lifters after the engine is warm. I thought that was an acceptable way of adjusting them?

When I switched to the Edelbrock heads, I had to grind the header flanges down to clear the tall ARP head bolts and washers. If I am going to replace the gasket anyway, I might as well do a little more grinding, just to ensure that I have good clearance.

I will let everyone know.
Rick

hmm i always adjusted my lifters cold with intake off (during assembly)

rotated pushrod while socking down the nut .
when resistance was felt turned 1/4 turn more.
never had a problem and to this day even do it with just a valve cover removed .
note: lifters have to be on the base circle to do this.

Well, I was finally able to work on my car today. I was hoping the sound was an exhaust leak, so I went ahead and replaced the header gasket. It had no effect, so I guess I can check the exhaust leak off the list.

It seems like the next logical thing to do would be to replace the lifter. Can I intall new lifter using my existing cam?

Any other thoughts on what it might be? I actually wouldn't mind getting a mechanic to listen to it and diagnose the problem. Does anyone know of a good mechanic in the Pleasanton CA area?

Thanks
Rick

I have a similar problem with my 327. The exhaust valves make noise when they close. At first I thought it was a lifter problem. I removed the rocker arms and depressed the valves. There is a definite difference in the sound of the exhaust valve closing against the new hardened valve seats. The engine sounds almost as if it has solid lifters when it is running. Other than the tapping sound it runs fine. Time will tell if it is something serious or not.

As far as adjusting the lifters, both methods are OK. According to How To Hot Rod A Big Block Chevy the method is to have the car running, then back off the adjusting nut until you just start to hear a slight clicking (zero lash), then turning 1/4 turn at a time, tighten the nut to 3/4 to 1 turn, letting the car come back up to RPM after each 1/4 turn. This sets the proper pre-load.

your rods all straight?

Yes, the rods are straight and balanced. Really, the only thing that changed was the installation of the Edelbrock heads. Before I installed the heads, the car ran fine, without the noise.

I find it odd that a lifter would go bad right after I changed the heads. I really don't know what else it could be though. As I stated before, the sound gets louder as the engine heats up, to a point. It takes 3-5 minutes or so for the sound to start.

I think you may be hearing the noise of the exhaust valves hitting the hardened seats when they close. Is it only the exhaust valves making the noise? Does the noise stop briefly when you quickly tighten the rocker shaft nut ¼ turn with the engine running? This would be because when you tighten the rocker nut the valve is held slightly off its seat until enough oil is bleed from the lifter to allow the valve to fully close and the noise to resume.

You can test individual valves for noise by depressing them one at a time. You can do this by removing the rocker arm then thread the adjuster nut back on the stud then depress the valve stem with a claw hammer. Place the claw under the nut (like it is a nail you are trying to pull) and slowly depress the valve with the top of the hammer head. I am willing to bet the exhaust closes much louder than the intake.

you can also verify a rocker is the source of the noise by pressing down on the valve end of it, with a rubber hammer handle, while the engine is running. The noise made by the culprit will change when you do that.

There's no chance you're hearing a rod knock here, is there (as in connecting rod)?
not too unusual for a bearing going bad to be quiet when the oil is cold and thick.

Well, I hope it's not a rod knocking. I had the engine professionally rebuilt about 5000 miles ago.

Again, it's just weird that it started happening only after I installed the Edelbrock heads. If it is the valve seat, is there anything I can or should do? I hate keep driving the car if I am going to cause more damage to something.

JimM, are you suggesting that it could be a rocker arm making the noise?

How loud is the sound? Can you hear it with the hood closed? How about sitting in the car? Valve train noise is more of a ticking sound and a rod more of a knocking sound.

I would first call Edelbrock and get their opinion. It could be a normal sound for their heads.

If you are getting full lift at the rockers and the pushrods are turning you can pretty much rule out a lifter problem. Almost all lifter failures are at the cam lifter interface (a flat cam) and will result in a loud ticking, deminished lift, and a poor running engine. The internal failure of hydraulic lifters is rare and usually on high mileage sludge engines.

If you don’t want to take any chances you can pull it apart and qualify all the parts. Lifters can be dissembled. They should also be slightly convex on the bottom. Make sure all the lifters go back in their original locations. If you find a bad lifter you will need to replace the cam and all the lifters. Placing a new lifter on a used cam is bad news. You will need a valve spring compressor to check all the valves and their seats.

If it was mine and it everything else looked good, full valve lift, the pushrods are all turning, no misfire, I would not tear it down but take my chances and run it.

JimM, are you suggesting that it could be a rocker arm making the noise?

The most common things to make a knocking sound are exhaust leak (rulled out, we think) valve train (excessive clearance, either out of adjustment or leaky lifter, or flat cam) or a connecting rod lower bearing.

A good clue here is that the noise happens when the motor warms up. The oil gets hot and thinner. What is your oil pressure at cold start, both idle and reved up to say 2000? How much does it change when the motor is hot?

Rods knocks generally get quieter when the rpm is rising, are constant when the rpm is constant, and get really loud when the rpm is falling. If you bring it to 3000 rpm, and then close the throttle, you'll know right away if it's a rod. Hopefully and probably it isn't.

Another thing to look for is how fast the pushrods spin. are they all similar? At idle, they should be spinning, but not whizzing around really fast.

Pressing on the valve side of the rocker while it's idling will hold the rocker tight to the valve. If there is excessive clearance, the sound will go away, or at least change.

So I took the plunge tonight and removed the intake and passenger side head. There wasn't any sign of damage to the piston or the head. So, I guess I can rule out a washer or nut sitting on top of the piston.

This is what I can rule out so far:
1) Debris on the piston.
2) Rocker arms hitting the valve cover.
3) Piston to head clearance issues.
4) Valve adjustment.
5) Header exhaust leak. (double checked clearances and replaced the header gasket)
6) The previous owner of the heads told me they ran great on his car. I know he's telling the truth.

The one thing I can't 100% gurrantee is that I put all the lifters back in the right location after I installed the new heads. If I accidently switched a couple of lifters, could that cause the sound?

What would you do from this point, now that the head is off.
Rick

I can't believe my car is still down. I missed virtually the whole summer driving season! I just haven't had time to work on it.

So at this point, I am going to put the head back on the motor. I didn't see any damage to the valves, pistons or cylinder walls.

Is there any reason to take the cylinder head to a shop for inspection, before I put it back on the motor?

I think I will also switch the rocker arms around, just to check and see if the rockers on the number 6 and 8 cylinders are making the noise.

You could swap the heads to see if the noise moves to the other side of the engine.