I pulled each plug wire while idling, and in each case the knock did not change and a miss was evident.

I also ran through the valves this morning (cold) and was pleased to find that most of them were right at 0.019”. None were loose, but a few were a few thousands too tight. I plan to run the engine with the valve covers off later, then check clearances one more time.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

since the noise did not change when pulling off the plug wires, you could try pulling the fuel pump off and running the eng. there should be enough fuel in the carb. to run the eng. long enough so that you can see if the noise is still present.  If it is not, it is probably the fuel pump. If the noise is still there you will have to go deeper, or wait for something to break. Not a good thing.  do you have an automatic trans?  If you do does the noise change when shifter into drive or reverse, is it louder one way or the other? there is the possibility of loose torque converter bolts, or cracked  flex plate/ flywheel if the noise changes

Looks like he has a 3 speed with OD.  

Any idea if the last time the engine was freshened up if it was a complete rebuild with new pistons and ground crankshaft or was it a re-ring and re-bearing rebuild.  If the old pistons were reused, then a cracked skirt is a possibility.

Here’s a picture of a cracked piston from a 272 that just had a light knock.
  

Lorena, Texas (South of Waco)

Running the engine with the valve covers off, the knocking sounds exactly the same; only the valve train clatter is more audible. With the covers on, valve train noise is not noticeable—it’s a pretty darned quiet engine. I do not know anything about its history but nothing about it looks fresh.

As for the fuel pump, I haven’t tried removing it, but my “stethoscope” suggests the source is elsewhere.

I’m inclined to just let it be for the foreseeable future; it’s very minor and not at all evident behind the wheel above idle.

The only concern I have at this point is the vibrations at 1600-2000 rpm and the vibration of the vacuum gauge. I was thinking these might be related to the audible knock, but I’m not sure whether it merits dropping the oil pan and surveying things. Maybe if a piston (probably #1) has a broken skirt, it is causing a balance issue in the bottom end—but if that were so, wouldn’t it get worse at higher engine speeds?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

I don't know how hard you drive your car, but if you baby it, there is the possibility that there could be some carbon build up on the top of a piston.  I had this happen on the GM 454. I would have sworn that it had a bad rod , until I used some "top engine cleaner", a product made by gm. and de carboned the eng. this can be done also by using straight water, or some have used a mix of water and brake fluid with good results. A carbon build up though is usually heard even from inside the car when driving, not just at idle. As Ted suggested it could be a cracked or broken piston skirt, it could possibly cause a balance problem with a vibration depending on if a piece has broken off, and how big of a piece. if just cracked, there is a possibility that it can go a long time before complete failure.  another possibility would be a piston that is slightly under size in the skirt area, or one that has a "collapsed ",  with out being able to hear your eng. running in person, at this point I am not sure what the noise could be.   maybe Ted, or someone with more experience with Y-blocks may have more suggestions.  sorry I was not more help---  Jim

I appreciate all the help, even if it hasn’t led to resolution yet.

I did remove the fuel pump tonight and ran the engine without it. The knock was still evident, but not any louder. Looking in the fuel pump hole, everything looked normal, if not particularly clean.

The next step is probably to drop the oil pan and look near the front of the block for anything irregular. Will the pan clear the steady rest on a ‘54 Crestline? Motor mounts are new.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

The pan will miss the steady rest on my 56 but I need to remove the sway bar. #5 needs to be near TDC to it slide off. I also installed 2 set screws at the rear of the pan for ease of reinstalling it when it was off. Good luck. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I love that I was able to remove the oil pan without even jacking the car up. I can’t imagine that’s possible on very many cars!

I believe I have spotted the source of the knock. In
The end of the wrist pin bushing was visible outside the rod end. I was able to easily push it back into place with a screwdriver—basically no resistance.

Additionally, I found about 3-4 tablespoons of metal paste in the bottom of the otherwise-clean oil pan. There was also two pieces of a small spring I don’t recognize.

I checked the #1 rod bearing as well. She’s done.

I didn’t see any chips or cracks in any pistons or the crank, nor anything else I would expect to create a 1600-2000 rpm vibration. Could bearing slop account for it?




1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

That wrist pin bushing is a strong candidate for the noise you are hearing.  A set of bearings would not hurt at this point either.  No idea on the spring.

Lorena, Texas (South of Waco)