VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Procedure to determine engine knock?

I just got home from the Mid-Western Regional Meet in Emporia, Kansas. On the two tours my '34 Standard finally logged some miles (200+) beyond the 50 or so it had ran since being overhauled by my father-in-law some 20 years ago. It ran fine with no problems other than a slight, light, knock at about 35 MPH, or when accelerating or going uphill. On level grade or downhill it was quiet.

I had one of the VCCA members who is a retired GM mechanic (with experience with old sixes) take a ride with me to listen. He wasn't sure what the source was but thought it is most likely a rod that is a bit loose. He also suggested that I get it fixed. I have no experience with lower engine repair so I will be looking for someone in the K.C. area to do the work.

In the meantime, I want to try to determine which cylinder (or main bearing) has the problem. I (kind of) understand how to use a stethoscope to listen for noise while shorting a plug wire and revving the engine. It is hard to hear the knock unless driving with a load on the engine. But anyone who can give me more detailed instructions or suggestions as to how I should proceed would be greatly appreciated.

Also, I learned from some discussion that 10w30 detergent oil is not good for babbitt bearings and it was suggested I go to non-detergent 30 weight. Someone even suggested that straight 30 weight might eliminate the knock. Any thoughts here also?

Not true. 10W 30 detergent oil works great in babbitt bearing engines and that is all that I have used in my cars for the past 40 plus years.

J.D. is correct...Think about it for a moment...It just doesn't make any sense...

No oil will eleminate the knock and no oil will cause the knock. I also have used 10W-30 for the last 40 years in my cars with no problems.
Shorting out the cylinder will pin-point as to which cylinder has the loose rod.Remove one spark plug wire at a time and drive the car in the knock range.
Could also be a a loose main bearing,
If it is a rod the knock will not be heard when the oil is cold and get worse as te oi gets ot and thins out.
Either way I would not continue any extensive driving till fixed.

I suspect that the knock is a main bearing and not a rod. Try letting out the clutch slowly at a fast idle with the brakes fully on to see if you can create the noise. The load on the engine should partially duplicate the acceleration or up hill load that produces the noise. If that does not produce the noise and other testing does not locate it then it might be something loose that is knocking.

The person(s) that told you that 10W-30 is not suitable for your babbitt engine, just don't know what they are talking about. Beware there is more misinformation about automobiles than sound advise.

Thanks for the replies so far. I just went down and started the engine. I revved it up and down and did not hear any knock so I'm thinking it is a rod. I will do the drive test removing one plug wire at a time. Hopefully I'll have some time in the next couple of days.

And thanks for the oil information. The person(s) who said to use only non-detergent oil said it would cause the babbitt to "flake off" slowly.

The reason I said rod is that it is common to have a rod knock after a rebuild. This is because modern "mechanics" use Plastigage to adjust the rods and you can not get the rods tight enough using this method. The shop manual has the proper proceedure for adjusting rod bearings. If they have over .002" clearence they will make noise.
You can usually hear a main bearing when the engine is first started after it has been sitting for a while . You will hear 3 or 4 deep thumps and as sooon as the oil pressure deliverd oil to the bearing it will go away. Thge mains can get by with up to .003" clearence without making noise.

I just got back from driving the car around a circuit with a good hill. At first, with all the plugs in, I could not hear the knock. There was none at cold start-up either. After the engine got hot I began to hear the knock. So I pulled one plug at a time and drove the circuit 6 times. The knock did not go away until I got to the back (6?) cylinder. The knock went away. I put the wire back on and the knock reappeared. Took it off again and drove a longer route - no knock. Put the plug back on, drove home with the knock again. Sounds like number 6 is where the problem is. Suggestions? Thanks for the replies and any forthcoming!

Looks like you will have to drop the pan and re-shim #6 rod. Not hard to do. Take a shim out of each side and re-assemble untill the rod will not move (front to back) on the crank with a light tap of a hammer handle. Then add one shim and re-assemble. Done. You could also use plastic-gauge to check it to .0015.

When removing/adding shims is it OK to re-use the same bolts?

I do

So do I unless I find that they are stretched.

Besure to consult the shop manual for the adjustment. The bearing could have had a high spot that wore away.