Vexing problem.

Bassman/NZ

Bassman/NZ

Well-known member
I've been trying to find a knocking noise which is driving me nuts. I rebuilt my C59a engine about 9 months ago and gave it a hone, new rings, rod and main bearings, new valve guides and faced the valves. I had the heads milled to give me 50thou piston/chamber clearance, but otherwise is stock. It runs like a champ, but has an intermittent knocking noise which I am chasing. Using a length of rubber hose as a stethoscope, I determined the valves on #5 are very noisy. All other cylinders are very quiet in the valve area, but putting the hose on the valve area on #5 is incredible. Compression is same as all the other cylinders.
I pulled the head, had a visual, checked the lash on both, all seemed OK, retainers both in the correct place. I pulled both valves, they looked OK, springs OK, guides OK, pulled the Isky lifters, OK. I rummaged through my parts and found two solid lifters the needed length and installed them, just in case there was something wrong I couldn't see. Reassembled everything, taking care to make sure the retainers were located properly, and the lash was within spec. Fired it up, dang! That noise is still there!
I've run out of ideas, so I'm looking for inspiration here. I know sometimes it's hard to see the wood for the trees.
PS: I should add, the stethoscope test indicated the noise coming from above the spark plug, so I ruled out piston knock.
 
This can be a tough diagnosis request. The first thing I try to do is determine if the noise changes when the cylinder is grounded(no spark). If so, it points me toward the piston/rod area. If no results are obtained from that test I try to determine if the noise is at piston speed or at valve speed intervals. With some rigorous intelligence you will figure this out because the valves(obviously) move at only half that of the piston.

As an aside, if the noise comes and goes don't discount the fuelpump as a contributor.
 
Any carbon on the piston or valves that might be taking up the space between the head? You sure the valves aren't hitting the head or the spark plug?
 
If the noise is confined to just one cylinder, it seems unlikely that milling of the head is at fault. The noise could be a broken piston ring, or a loose piston (wrist) pin. If the plug wire is disconnected, and the noise changes or goes away, it will narrow down the possible problems.
 
I foil balled the valves when I did the heads. Also, there's no visual of any interference. No carbon build-up. JWL, no change when the lead is grounded, and the stethoscope test gives a quiet fuel pump, and very noisy valve area on #5, also a bit on #6 but I think that's spill-over noise from #5. Talkwrench, I have a vauum gauge, I'll have to make up a fitting so I can use it.
 
What RPM's are most noisy?? Any difference warm v.s. cold?? Metallic click sound or somewhat subdued??

FWIW, I had a noise that nobody I asked at a Bonneville meet could figure out and I pulled the manifold, checked the valve train, found nothing wrong. Turned out to be a broken wrist pin.

Flat32
 
valves do not usually knock. Loose flywheels may, thrust surface off a main may cause a knock..............you really have to be careful about milling heads and valve clearance, I know from sad experience years ago.
 
Flat32 said:
What RPM's are most noisy?? Any difference warm v.s. cold?? Metallic click sound or somewhat subdued??

FWIW, I had a noise that nobody I asked at a Bonneville meet could figure out and I pulled the manifold, checked the valve train, found nothing wrong. Turned out to be a broken wrist pin.

Flat32
Click to expand...

Rpm's would be low and medium, don't notice it at high revs. No difference warm or cold. Not metallic but not a deep knock like a bearing or something.
 
I came across a similar knock on a 38 coupe once....after a lot of headscratching it turned out to be the starter pinion. It was missing the little spring and pin, and the pinion at certain revs, would move forward and contact the flywheel ring gear.
 
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