Deep knocking sound

[ModelA29]

Originally Posted by Gary WA

........ All he did was change the head!?

He also increased the compression ratio adding more load to the bearings.

[Gary WA]

Originally Posted by ModelA29

He also increased the compression ratio adding more load to the bearings.

I've replaced all my A's to high compression heads and don't have any knocks! All stock engines and drive at 45-55.

[Dkrahn]

Originally Posted by Marshall V. Daut

If it truly is a deep knock and not a hollow rapping sound (a rod bearing), it's probably the center main bearing, or at least one of the main bearings. Generally, nothing goes wrong with #1 main bearing, often not even requiring adjustment. It's the center and the rear main bearings that cause the knocking.
To test, drain the oil and replace it temporarily with a very heavy oil such as straight 50wt. Buy the cheapest oil you can find because it will only be used for testing and drained afterwards. Start the engine and allow the oil to circulate. Retard the timing as far as possible. If the knock lessens noticeably, one of the main bearings is at fault, usually the center main. Doing this won't affect a rod bearing noise, which can be isolated by shorting out each spark plug one at a time. Sometimes you can short #2 and #3 cylinders at the same time and isolate a center main bearing knock. That's the one that usually goes bad first. The rear main may have too much clearance and allow oil to pass by, but it's the center main that is the noise maker when going bad or out of adjustment. If you are lucky, simply dropping the oil pan and adjusting the main bearings will solve the problem. That way, you can at least see if the Babbitt material is broken when you drop the caps. 'Can't tell about the upper bearing material in the engine block, but as a rule, the Babbitt in the main bearing caps is what goes bad.
Marshall

OK, when I retarded the timing the knock lessened. I pulled the copper connectors on each plug and no difference. Sounds like I need to drain the oil, pull the pan and look at and "adjusting the main bearing". This will be a new experience. And one I am hoping to only do once.

[Randall]

This might help and give you a little insight to what to do
https://youtu.be/8MH3HtTnshQ?si=RGbjJ5dRa5xOqvOw

[nkaminar]

In my opinion, the easiest way to take up the bearings is with the engine out of the car and up side down on an engine mount. For one thing the crankshaft will be held down by gravity. The clutch can be examined at the same time and repaired if needed. After removing the flywheel and flywheel housing the crankshaft can be lifted out of the engine to examine the Babbett. It will also keep you from getting lots of engine oil in your hair.

Remember to mark the caps so that they go back into the engine in the proper location and orientation. And keep track of the shims.

[Marshall V. Daut]

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I file a nick at the front of each main bearing cap to ensure that I re-install it correctly oriented. The rounded raised ridge is a perfect place to do this. Painting a dot or slash can result in it being wiped off by accident because unless you get the cap squeaky clean, an oil residue will remain, which makes for a poor base for retaining paint. You do NOT want to swap the caps end for end! That will throw off adjustment. This won't be a problem with the rear main bearing cap because it is unique and can only be put back in one orientation. You will need a long 3/8" breaker bar for leverage and VERY shallow 3/4" socket to break those two nuts loose. As long as you have the oil pan down, why not also adjust the rear main cap? I'll bet you don't need to remove any shims from the front main bearing. I think the most I have ever removed from that cap was one thin shim from each side. Too bad the other two main bearings can't be as long-lasting and trouble-free as the front one!
Instead of instantly pulling the engine to adjust the main bearings - although I agree that it is much easier to do it that way - I think I would first want to know if the center main cap has broken Babbitt in it. If it does, that's what is causing the loud heavy knocking noise. If the Babbitt is broken, the engine will have to come out for sure to re-do the main bearings. If the cap Babbitt is still good, it's likely the Babbitt in the block is also o.k. Not always, but usually it's the cap Babbitt that goes bad first. Check the drained oil through a strainer for pieces of Babbitt, a sure sign that one or more bearings have gone bad. Also remove the dipper tray and check for Babbitt chunks at the bottom of the pan.
On the other hand, if you cannot remove any shims or only one thin one from each side from the center cap without locking up the engine, then the Babbitt up in the block is probably cracked. The kind of noise you describe cannot be eliminated by removing only one or two thin shims from the center bearing. It sounds more serious than that. Because the noise apparently started all of a sudden, I'm betting the center bearing Babbitt is history. 'Sorry for jolt, but welcome to the world of Model A Fords. The center main bearing is the Achilles heel in the Model A engine that also affects the rear main bearing and oil leakage. Cross your fingers and pray that the cap Babbitt is not broken and that you can remove a couple shims before the engine locks up. That means you just might have fixed the problem. Otherwise, you'll have to decide whether to have the mains re-poured or convert to modern insert bearings. Those readers who are familiar with my postings on two Model A websites since the late 1990's know where I stand on THAT issue! Need I INSERT my preference here???
Marshall

[Gene F]

Shims get shaved off with a razor blade. Wax holds them together most of the time.

[Marshall V. Daut]

I have found with the shim packs available in the last few years that I usually have to lightly heat the pack with a low frame torch in order to separate them. Normally, a razor blade worked fine in the past. But the manufacturer must be using some space age crazy glue formula now to hold the layers together. It doesn't take much heat; just enough to loosen the adhesive's grip between shims.
Marshall

[1crosscut]

Originally Posted by Dkrahn

OK, when I retarded the timing the knock lessened. I pulled the copper connectors on each plug and no difference. Sounds like I need to drain the oil, pull the pan and look at and "adjusting the main bearing". This will be a new experience. And one I am hoping to only do once.

In my earlier post (#8) I posed several questions in regards to timing being too advanced and possibly causing a knock.

Is this a new problem since you installed the new head?

[nkaminar]

Crosscut, I tried to say the same thing in post #9, with instructions on how to find the knee in the timing curve. Ignition knocking may be the problem, but a more retarded ignition will reduce the force on the main bearings and thereby reduce a main bearing knock. The timing should be corrected first and then, if the bearing knock is still present, pull the engine.