I suspect you are getting getting "piston slap" which is caused by the piston rocking in the cylinder. It is common in Chevrolets as the pistons and or pins wear. It typically is loudest when first starting the engine but is reduced as the engine heats and goes away at running temperature. The engine will run for a long time with that condition.

If you get one of those engine stethoscopes (Harbor Freight has one for under $10), you can listen more closely and locate exactly where the noise is coming from while the engine is running.

When I rebuilt a '28 engine in 2017, there was an engine knock on the first startup after the rebuild. I got one of these stethoscopes and it lead me to the knocking coming from the front and middle main crankshaft bearings. After I dropped the oil pan, I found that the bearing gaps were too large. Luckily, I was able to adjust the bearing caps enough to stop the knocking noises.

Cheers, Dean

@kevin-ray Kevin, I think Chip will back me up on this, and maybe Dean too. Plastigage is not a suitable tool for checking clearance in a babbit bearing. The bearing material is soft enough that the plastigage will sink into it and give a false reading. Also, rod bolts should never be at risk of twisting off. 

Good luck with your diagnosis.

Mike

I agree with 35Mike that plastigage can only be used as an indicator, not a precise measurement when dealing with babbitt bearings. 

That's what got me into trouble with my main bearing clearances back in 2017.

Follow the instructions that are in the shop manual and continue to ask questions here. If you don't have one of those manuals, get one. They are available from multiple vendors and also on ebay.

Dean

@kevin-ray

Hi Kevin

I concur with Chip regarding piston slap as being the most likely source of the noise.

And I agree with Mike and Dean regarding the use of Plastigage on old Chevrolet bearings.

Your rods are probably fine, but if you do decide to take the pan off and have a look, use the "light tap with a small hammer" method of adjusting rod bearing clearance.  It's still the best with softer and thicker babbit bearings.

I will do the stethoscope method to see what I come up with. I notice at could start up ideal is the loudest. So at start up I rev the motor just enough so the noise is least noticeable for a little bit or I drive slow for a minute till farmed up. My thought it’s better that way instead of knocking till it gets warmer am I correct.  Hopefully I will retain what o learn and can help pass the knowledge along. As I’m 61 not young by any means but  younger then a lot here with years of experience and knowledge. Thanks

Last ? For a day. Is one cylinder more likely to have piston slap or rod problems. . I’ve heard this happens to some engines do to cooling passage location or even oiling problems just curious. 

@chip

Hello Kevin Ray,

A question for Chip please. What would cause one cylinder to display piston pin wear more than the other cylinders evenly over time? Would a lack of oil splash within a certain area in the engine contribute to this? Besides listening with stethoscope, could a suspect cylinder piston slap sound be changed/stopped by pulling a spark plug wire and/or removing a spark plug and running on five cylinders? Will piston slap sound change with lack of compression on that piston? I may be older than Kevin, but that doesn't mean that I know all there is to know. Never stop learning. 

Edit: Could a piston slap cylinder show a lower compression check number from the other cylinders? 

Piston "slap" noise is generated by piston movement. A piston is shaped similar to a bell so can "ring". The noise can be caused by a tight pin or a loose pin or gap between a piston and the cylinder bore. A tight pin resists the rotation of the piston on the rod causing it to "rock" side to side as it moves up and down. A loose piston or worn rings or rings that have lost their tension can also contribute to increased movement. The noise is the piston vibration from hitting the cylinder wall. The gap between the piston (primarily the skirt) and cylinder wall is a factor in noise volume. 

The cylinder walls, piston rings and pins are lubricated by an oil mist or droplets. The droplets are either the result of the lower rod ends (or dippers) passing through troughs and/or the oil thrown off moving parts. So the adjustment of the troughs, rods and bearings can all have a contribution. 

Most pre mid 50s Chevrolet engines will have some level of piston slap that is noticeable when first started beginning at the 10k mileage range. The noise will essentially go away as the engine warms. The good news is that the engines will continue to run for 10s of thousands of miles with the noise gradually getting louder but not result in catastrophic failure. Something else will happen causing a rebuild first. 

The reason I asked is both cars the nose seem to be past cente of the engine around the #4 cylinder.  I just find it hard to believe both motors are making the noise in the same area. I live in southern Indiana. Does anybody know of a good shop that rebuilds this type of motor. 

I suggest that you contact the VCCA region (chapter) in Indiana for mechanic recommendations. You'll find the contact info here: https://vcca.org/regions-chapters/

I hope it doesn't need a rebuild.

Dean

@rustoholic  From what I’ve learned from help here I don’t think at this time neither one needs a rebuild. I’m just wanting to get some general information. I don’t like to wait till last moment to decide about anything . Thanks