VCCA Home Forums TECHNICAL - 1945 AND NEWER 1946-1948 Crankshaft knock

I have a knock in my 47 Chevy coupe that I believe is a main bearing knock. I adjusted the mains with plastigauge and a jack to support the crankshaft. The adjustment that I set them at varied from 0.0015 to 0.002 inches. The knock is much less now but is still there at speeds of 40 mph or more when under load. Do you think that I should adjust them closer?

There is a slight engine vibration at those speeds which I believe is unbalanced pistons. I plan to correct that next spring but the vibration is not that great and I really don't think that should cause a crankshaft knock. But I sure wouldn't know.

If the mains are actually at those specs. They should not make noise.--I still prefer the rolling resistance method the older shop manuals used.How many shims did you pull out of the mains? Have adjusted them both ways myself.The vibration may be caused by the clutch or flywheel.When they are out of balance the vibration usually peaks around 40-45 MPH.

Did you do anything with the rods?Try removing one plug wire at a time and driving it.If its a main it may change the knock but if a rod it will eleminate it completely when wire removed.

Thanks Chevgene

I've usually adjusted them by feel. In fact, I did just that before and found when I used plastigauge that 2 were rignt on. One I recall was a little over and another a little more so. The knock does seem to get worse as my speed increases and does not peak at any speed.

I'm not sure how to check the clutch or pressure plate sort of taking them out and then to a machinist for testing. Is that how you would check them?

To check for balance a weight could be placed under one of the pressure plate bolts-can move it around to all six and see what happens.

If the engine ever had a piston replaced and a truck piston was used it could throw the balance off.The truck pistons are 1 1/2 oz. heaiver than the cars.They can be identified by a small forged boss on the lower center of the pin boss.Something to check if you ever tear the engine down.

A main will make more noise as the engine warms up and the oil thins.Also you may hear it give a couple of faint knocks when first starting the engine-after it sat for a bit.It will knock until the oil get to the bearing in quanity-usually three or four times.

Thanks again Chevgene for your suggestions. I'll try them shortly and let you know how it works out. A flywheel is a pretty heavy chunk of metal, I don't see how it could get out of balance. The pressure plate is probably a rebuilt one and may not have been balanced, certainly not by me. The pistons are a new set. I had the engine bored oversize .020 some time ago. The pistons are not GM and I didn't have them balanced. A mistake I've not made on any engine since. The engine does make more noise as it warms up but not when starting no matter how long it has sat.

Since your pistons are not GM, are they aluminum? If so, who made them? :confused: :confused:

Making noise when it warms up would possibly indicate loose pistons.Either cast iron or aluminum could cause this.Aluminum most likely.Would be at the low RPM range -at idle or under low speed pull.Thats why I would try shorting out each cylinder as its also possible that its a rod bearing.Either way it would pin point it to which cylinder it guilty.

Thank you all that responded to my question. The pistons are cast iron but I don't remember who the manufacturer was. It's been some time since I rebuilt the engine. For various reasons I didn't have much time to fool with the car and as a result it has only about 600 or 700 miles on the engine. If I recall correctly I purchased the rods at a local auto parts store. They would have been rebuilt ones. My experience since is that rebuilt rods tend to be very much out of balance. I do have a set of NOS rods that have been balanced to 1 gram which I intend to install later.

I question whether the vibration and engine knock are related but they may be. The vibration and knock happen at speeds over 40mph. The knock only when under load. At idle and speeds and up to about 40mph the engine is smooth as silk with the only quiet noise from the tappets. I've heard a lot of knocks from the cast iron wonders in my time but this one is not that is recognizable to me. Since it does appear to be a very solid knock I thought the main bearings most likely but I certainly don't know. I think that I may be in over my head.

could be a worn camshaft timing gear! Good luck!

I agree with Walt on the timing gear.

I had a friend that rebuilt his 1939 engine some years back.Installed rebuilt rods.Engine had a strange knock that couldn't be shorted out.It turned out that there was a bent connecting rod.Noise was simular to what you describe.

Have checked the weight of Chevrolet connecting rods.Find that they can vary by more than 1/2 oz. each.Acording to Chevrolet they selected rods and pistons of "like weight" to use in an engine.That was there state of art balancing back then....Have seen more than one pressure plate that caused a vibration.The trick of the day was to reposition it by loosening and rotating it-a trial and error job.

A bent connecting rod is a possibility. I had one in another car that knocked once although if I remember right it would knock almost all the time. I've had connecting rods that the machinist said were too twisted to use. Apparently someone put the cheeks of these rods in a vise while tightening the rod bolt.

I'm going to check the flywheel as suggested by Chevgene. Would an ordinary washer be too heavy?

Depending on the weight of the washer I would say an ordinary washer wouldn't be enough.I would find something that weighs about an ounce and another about a 1/2 Oz.What about straightening out the clip on a wheel weight?

The shop manual says to check rod alignment when ever the rods are removed-or even new rods when installing.Very few rods were new.Even most of the rods Chevrolet sold were rebabbited.Thats why they had a core charge on them.I still prefer the genuine ones as they were redone with the proper material and correct groves and oil pocket etc.The "high volumn" shop I worked in during the '50s may not have even had the rod alignment tool-at least I never saw anyone ever use it.

A wheel weight sounds like a good idea. I have a lot of them. I'm going out of town early next week and won't be able to check out the balance problem until after that. By the way, the timing gear was brand new when I installed it.

This is an EXCELLENT thread!!

Regarding connecting rod knock, I have seen an instance where the babbit in the cap was OK, but the rod's was bad. It knocked at idle, but when you shorted the spark plug, it gave a double knock ( knock..knock.............knock..knock...)

OK, Durnit.... David, :confused: I will bite....Who's there?
:confused: :confused: :confused:

I have been watching this thread for some time and am surprised that no one even mentioned a wrist pin. I understand that it does not become noticeable until the engine speed increases and heats up. Chevrolet sixes are known to have wrist pin noise that can exist for many years without failure.

They also did not mention using a large ***** driver, rod, stethoscope or other listening device to help locate the source of the noise. Placing the end against the the engine at various places and your ear on the other end is an excellant method to locate strange noises. You will also find other interesting noises coming from inside the engine. It is better than rock music or rap too.

That said I am on leaning toward the side of a bent rod. Rap, Rap, Rap...........Knock, knock, knock!

In my experiance I have learned that wrist pin noise is most noticeable at idle.When engine is first started and oil hasn't been "thrown" up to the pins they will be at their loudest.When engine is reved up and they fill with oil they will quiet down.Will become noisy again when oil warms up and "thins".This is the deep "clucking" in the engine sound at idle.Sometimes thought to be valve noise but a much deeper noise.If very loose can be heard when backing off at 40-50 MPH.The noise can get really loud on the 1929-36 engines with the long stroke and heavy cast iron pistons.Most of these engines had pin noise by the time they had 25-30,000 miles.The '37 and up engines-while having cast iron pistons usually didn't develope pin noise until 40-45,000 miles.Most were overhauled by 50,000 at which time new pins were fitted.If not and rings were replaced and rods tightened the pin noise grew worse.Engines operated at low speeds and idled a lot developed pin noise sooner.When going to meets etc. 80% of the castiron year cars that I hear run have some degree of pin noise. Its one noise that will cause no harm.

Once again Gene,I believe you've solved a problem for me. My 50 with 68,000 miles is exactly how you've described.I've adjusted rods, mains, tappets a few times, and still had a little noise at startup. Sort of like when you change the oil on a car and first start it up. It goes away in a second or two,but once she's warmed up good, at idle you can still here it faintly. I've been all over the engine with a broomstick to the ear but since it goes away so quickly and is very faint when it's warm, I couldn't really locate it.Like Chipper says though,it does make beautiful music!!

My 1950 (with hydralic lifters) has just a faint amount of pin noise when the engine is fully warmed up.The pin lubrication was improved slightly in 1950-The oil grove in the pin bushing was moved from the top to the sides of the bushing to give more bearing area where it was needed.

I will always remember a fleet of 1950 taxi cabs we serviced in our shop.They developed a lot of pin noise because the engines were idled for long periods of time while waiting for fares.I don't know if it was the oiling system or the heavy pistons but those engines would develope pin noise.

In 1962 we had a batch of small block V8s that had the pins fit to loose from new.They developed pin noise at idle.It was necessary to replace the pistons on those as oversize pins were not made by Chevrolet.This was due to the fact that the pins were pressed into the rods rather than clamped.

Just a thought on engine noises.With out actually hearing the noise "in person" in may be a little misleading as to go by a description made by another person.The "cast iron" years of the Chevrolet engine did have their own distinctive noises.An experianced mechanic years back never had a problem determining just as to what it was.Usually the most he would do is short out a cylinder to see what cylinder it was coming from.

With out actually hearing the noise trying to make an accurate diagnosis would be like your Doctor trying to diagnose a medical problem over the phone.I know that there has been some long threads on this before-and I always find them intresting.