VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 216 Engine Knock

A friend of mine called me last week to say that when he started his 1951 Styline Deluxe coupe in the morning, he was hearing a knock in the engine that had not been present before. The engine is the original 216 with about 60,000 miles on it, and the car has been in use as a daily driver for the past eight months with no engine problems. It's been running well, with the usual noises you expect from a 60,000 mile 216 engine. I had him bring the car over to my garage, and on hearing it, I immediately guessed that it was a connecting rod bearing. It is a solid knock from the middle or lower end, synchronous with the engine rpm.

Over the weekend, we drained the oil and removed the oil pan, finding no evidence of babbitt or other metal in the oil or in the bottom of the pan. We disassembled and inspected all six connecting rod bearings, finding them all to be in excellent condition. All the rods had two to three shims on each side, and after using the Chevrolet shop manual method of adjusting the rod beaings, we had removed, at most, one shim per rod, so all the bearings had been pretty close to correct adjustment before we disassembled. All the journals looked good, and miked at about .002 under, as you might expect.

Based on the fact that there was no babbitt in the sump, and on the condition of the rod bearings, I guessed that the mains were probably OK. I pulled the cap from the #2 main and it was, in fact, in excellent condition. We did not pull the other main bearing caps.

Finding the connecting rod bearings all in good order left me with mixed feelings--it was good to know that the connecting rods and rod journals are in good, undamaged condition, but I was at a loss as to the source of the engine knock. After inspection, we reassembled, primed the oil system and started the engine, and, as expected, the knock was still present.

It would be foolish to run the car with the engine knocking like this. Anybody have any ideas on what might be causing the knock? Is there such a thing as a catastrophic failure of the wrist pin bushings in a piston?

Mark

Could it be a bent push rod hitting against the side of the push rod cover?

Wrist pin or merely piston "slap." If I remember correctly, you can ground out each cylinder (so that it does not fire) and tell if it is the bearing or wrist pin but I don't remember which one. By using this method it will only reduce the knock enough for you to tell which cylinder and the source of the knock. Maybe some of the old timers may remember and respond.

I would assume that the valve train has been inspected and that a compression test has been done to eliminate the possibility of that being the source of the knock.

Keep in mind too that after the engine is up to temps, the piston slap is significantly reduced whereas a wrist pin will probably not. So, is the noise significantly reduced by heat or is it constant throughout the heat range?

After having found that the bearings were okay, you may have pulled the head and slipped out each piston and examined the wrist pins and pistons. But it is unfair for me to chide you on this. I would have probably buttoned up the bottom just as you did.

Good luck with it and you are a true friend to help with all that work,
Charlie

When does it knock?
At idle, a steady speed or under a load. Worse when cold or hot. When shorting out a cylinder (spark plug) does it change?
I would remove the valve cover and see if its coming from that area. If all else fails would suspect the timing gear is about to go. Or carbon on top of one of the pistons or a choke or throttle plate screw on top of a piston.

Thanks for the replies. The knock was so pronounced, I didn't want to take it for a test drive for fear of doing damage. The knock remains pretty constant whether the engine is warmed up or cold. I pulled the spark plug wires one at a time while the engine was idling and could not discern any difference in the sound.

We've decided to pull the engine and disassemble it for inspection and possible partial or total rebuild.

Which brings up another question: Will the rear side mounts on the 51 support the weight of the engine while the transmission is removed, or is it necessary to block up under the rear of the engine?

Mark

Did you check for a bent push rod hitting the side of the push rod cover?

Well Mark , sorry to hear of your problems...keep us up to date on what you find .
I posted couple weeks ago about that "clock" noise coming from back 'round cyl. 5 or 6 on mine . Only at idle . So I pulled on my wires one at a time and nothing changed...noise was still there . Just had a thought...I'll see if I can "time" the noise with a stop-watch and see if it falls in line with the RPM's divided by 6 ?
I've checked my valve lash , etc. and nothing seems amiss there...there all adjusted just fine...

Before I would do anything I would remove the carburetor and check for missing choke or throttle plate screws. I still would bet there is something on top of a piston and its hitting the head when the piston is on top center.

...Even if it had a Rodchester at the time ?

All carburetors have coke and throttle plate screws. I have from experiance seen more Carter W-1's loose screws.

At dealership, I recall 6 cylinders having an "air cleaner support/bracket" mounting across the airhorn of the carb. Mounting screws (2) would loosen, fall down the throat of the carb and beat on the top of the piston.

That was for like 1957 and up