Rod knock?

Mike Peters

Mike Peters

Well-known member
Yesterday we pulled the pan on a friend's 31 Victoria to investigate a nasty engine knock that occurs after warm up, and when accelerating. Engine was rebuilt and rebabbited 5,000 miles ago.

We checked the rod clearances with plastigage. All were within .001" to .0015."
However, we could move the rod sideways on the crankshaft journal enough to make a definite knocking sound. Several club members on hand could hear the noise 20 feet away. Cylinders 1 and 2 can be shove sideways about a tenth of an inch. We didn't attempt to check the main bearings.

We reassembled the engine and took it for another test run, and same noise after warm up. Definite knock. NOT a tick.

None of us are engine experts, so we would welcome some analysis here. Thank you in advance.
 
Video of it running? Good idea. I will forward this thread to the car's owner, and maybe he can do that.
No. We did not check any main bearings yesterday. Jim trailered the car to our house and we put it on my car lift to pull the pan. We are all getting too old to lay on our backs. Then we put it back together and ran the car back on his trailer. All in a days work.
 
This a scan of Dyke's Encyclopedia.
 

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Loose cam gear? Easy to check by inserting the timing peg, lightly will running at a idle to see if sound changes.
 
If the engine was quiet for 5,000 miles, it's probably not because of the rods. Yes, .010" is more than needed but it shouldn't cause a knock. Loose rods normally are noisy when there is no load on them, such as when cresting a hill, mains will knock under load. Good luck!
 
Bob Bidonde said:
This a scan of Dyke's Encyclopedia.
Click to expand...

Thanks Bob,
Your Dikes manual mentions end play on the crank journal of .008" to .012", plus associated end play on the wrist pin. This engine is worse than that. With the car up on the lift and pan removed, I could shake the #1 and #2 rods back and forth on the journal and get a pretty good clunking sound. Wrist pin side to side clearance was worse than the rod. The gang was not buying my end play theory, but nobody had any better ideas, either.
 
Ramman said:
Was it it's normal quiet self for the first 4,950 miles?
Click to expand...

Yes. Quiet for first 4950 miles.
Plus, on a cold start up, there is no knocking noise at all. It gets noisy after a few miles and continues thereafter.

Another thing. There was a sort of gray/black sludge at the bottom of the oil pan with what appeared to be very tiny sparkly particles. Finely ground babbit? We thought so, but as I said earlier, none of us ever made our living rebuilding engines. Just old retired old folks trying to figure out a Model A noise.
Another thing. The babbit on #2 rod end cap had a hair line crack. Not separated, but Al, who has better vision than the rest of us spotted the crack.

Tomorrow, the owner of the Victoria plans to trailer the car out to the shop that did the engine and give the guy a ride and see what he thinks.
When this saga finally ends with a definite solution, I will get back to you guys. All suggestions are taken seriously and I thank you all!
 
Mike Peters said:
Yes. Quiet for first 4950 miles.
Plus, on a cold start up, there is no knocking noise at all. It gets noisy after a few miles and continues thereafter.

Another thing. There was a sort of gray/black sludge at the bottom of the oil pan with what appeared to be very tiny sparkly particles. Finely ground babbit? We thought so, but as I said earlier, none of us ever made our living rebuilding engines. Just old retired old folks trying to figure out a Model A noise.
Another thing. The babbit on #2 rod end cap had a hair line crack. Not separated, but Al, who has better vision than the rest of us spotted the crack.

Tomorrow, the owner of the Victoria plans to trailer the car out to the shop that did the engine and give the guy a ride and see what he thinks.
When this saga finally ends with a definite solution, I will get back to you guys. All suggestions are taken seriously and I thank you all!
Click to expand...

I'm thinking the crank thrust wore prematurely.... check the crankshaft endplay.
 
Did the babbit break or get ground off of #1 and #2 on the sides of the rod bearings? That missing would explain the movement and sparkles.
 
Let it warm up so it makes the noise. Pull off one brass strip at a time (plug wire on a modern car,....). When the noise stops you have narrowed down your problem. If all 4 of them make no difference it is likely a main, or the can gear.

What did the guys in your club think when they heard it?
 
The two cylinder locations with all that side play are well beyond the .008" to .012" range. Con rod bend or twist deflection in the beam can cause rapid wear on the side thrust surfaces. It's either that or the thrust edges of the bearing material cracked and fell away into the pan. The thrust surfaces on that rear main should hold the crank endplay to the .003" or at least very close to that.

The cam bearing bores in the block can wear out over time. Cam bushings are made to allow for repair for that situation. The bores have to be machined to fit if they go beyond max wear specs. They have to be bored on a true center axis since that rear bore tends to wear oblong. There is a Service Bulletin on drilling an oil hole to feed that rear bearing on earlier blocks. Some late ones are already drilled for that.
 
Gene F said:
Let it warm up so it makes the noise. Pull off one brass strip at a time (plug wire on a modern car,....). When the noise stops you have narrowed down your problem. If all 4 of them make no difference it is likely a main, or the can gear.

What did the guys in your club think when they heard it?
Click to expand...

We were all pretty puzzled, so no expressed much of an opinion.
No chunks of babbit in the pan, but the sparkly stuff looked like finely ground babbit.

Aluminum cam gear on this car. From underneath, it looked good.

Only 5,000 miles on it. Several Model A owners in our club have had the old fiber cam gears fail on trips and leave them stranded. So we are one by one switching to aluminum cam gears, which we have found to be amazingly quiet.
I did short out one spark plug at a time with a screwdriver, but no difference in the knocking. If it really is a main bearing, then we will have to pull the engine, etc.,etc.
 
While you're checking the bottom end clearances, don't forget to check the crankshaft thrust surfaces. If clearances there are excessive, it can make a knocking sound there as well.
 
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