VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 36 rebuilt engine--barely audible knock

The engine in my 1936 has been recently rebuilt (done my brother and myself, except for machining...). After breaking in, it now has a barely audible knock or rap (my brother says inconsequential, but he hasn't really heard it) noticable at primarily higher rpms. The noise unfortunately reminds me of a connecting rod knock, but it doesn't behave like the one's I've heard before. This "rap" is heard on acceleration.

When I've heard rod knocks before, it is on de-acceleration--when letting off the gas. The knock in this case isn't heard at all when de-accelerating or simply cruising along maintaining speed.

Will a connecting rod knock only on acceleration and not at other times? At idle or slightly higher RPMs, it is quiet, so cannot test using the disconnect spark plug method (unless, as just dawned on me as typing this, take one wire off, drive on 5 cylinders and accelerate to 50 mph, and see if still hear it, would potentially take 6 tries)

The connecting rods (babbitt bearings )were set up with the shims at around 1 1/2 thousandths, maybe up to 2 thousandths using plastigauge. It has a NOS crankshaft and NOS GM connecting rods (standard size). It has 10w30 oil in it.

Would one of the two following scenarios lead to a rod knock in this case? 1) If one was a two thousandths clearance, it is knocking at the higher RPM. 2) I did not have the tool to align the oil pan tubes to accurately spray oil at the dippers. Could one rod have run dry a little?

Could it be a wrist/piston pin? Do they knock more on acceleration? Would it sound like a connecting rod? After the fact, I found out that the machinist I used wasn't the best, and he fitted piston pins to the aluminum pistons (directly on the aluminum--no bushings). At least one had a tight and loose spot as it was rotated.

Any opinions? Could a connecting rod knock only upon acceleration? Could it only be audible at higher rpms?

Thanks!

Your rods were set up too loose.

What is the clearances on the mains?

Yep, that's what I wondered for the rods--perhaps too loose. I think we set them all as close to one and one half thousandths as possible, but a couple may have been closer to two thousandths.

My brother is used to Model A's and the rods there he says can be up two thousandths. Should one thousandth clearance be the norm for Chevrolet rods? Would they potentially knock at two thousandths?

The mains are between 1-2 thousandths as well. But this is a higher pitch "rap", so doesn't sound like a main.

The engine was extremely tight after rebuild--no way the starter would turn. In fact, with the spark plugs in, it dragged the rear wheels in grass! With the spark plugs out, it would turn the engine and finally loosened up enough to tow start. It took several times of running and shutting off before the starter would turn it over to start.

So my original concern was that we set it up too tight. In fact, I remember that the crank was very stiff after putting the rods--hard to turn over by hand. It took a screw driver prying on the ring gear teeth.

Could it still be a wrist pin? This is the only thing that seem questionable when assembling--at least one had a tight and loose spot in it. Is there any way to determine if a wrist pin?

Over the winter, I guess may drop the pan and check all the rod clearances. As I understand it, with plastiguage there should be a very, very slight film of oil so that the plastic will spread evenly. Maybe something like light film of WD-40? If completely dry, (or so I've read), it would not spread evenly and perhaps give a false reading. Is this correct?

Adjust the rods as per the shop manual. This will allow you to get them as tight as possible without being too tight. .
a rod will knock under acelleraton w and be louder when the oil is hot.
It is always the case with plastigage.....knocking rods.
Now that they have been run-in you will get the best adjustment.

A loose wrist pin will make noise at a slow idle with hot oil.

Gene,

Thanks for the reply! I must admit that what the Shop Manual says to do seems fairly subjective to me. Don't have it at the computer presently. but seems like it says take out shims until the rod will will not move on the journal, and then remove shims until it will rap back and forth with a ball peen hammer. This just didn't seem very precise to me. I'm I stating the recommended method from the Shop Manual correctly?

I plan on dropping the pan to check, but will need to drive back to my parents garage to check--about 90 miles. Do you think if I take it slow that any permanent damage will be done to the crankshaft or babbitt?

Thanks again!

Just from my experiance only a very light tap with the hammer handle to move the rod. If it flops back and forth by hand its to loose.

For the 1936 engine remove shims until you can not move the rod back and forth by han but can tap it back and for with an 8 Oz. hammer.
The 1937 and up had a larger diameter journal so they said to do the same thing but then add one shim so it can be moved back and forth by hand,
The original factory rods had silver shims that were .010" and brass shims that were .002". The .001" would be for the final adjustment.
90 miles should not beat out the babbitt if you keep it at around 40 MPH.