Deep knocking sound

D

Dkrahn

Member
So I cracked the head and replaced it with a 6:1 head. Now I have a deep loud knock as I start it and when it is idling, and then hit throttle. What do y'all think it is, and what kind of test do I need to do to confirm what the issue is?
 
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
 
Dkrahn said:
So I cracked the head and replaced it with a 6:1 head. Now I have a deep loud knock as I start it and when it is idling, and then hit throttle. What do y'all think it is, and what kind of test do I need to do to confirm what the issue is?
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Did you use the right head gasket? All he did was change the head!?
 
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Good questions and something to consider, but Dkrahn states that the noise is a DEEP loud knock. That tells me he thinks it's coming from down deeper inside the engine, not up top in the cylinder head area, assuming he was able to isolate where the sound was coming from. If a large bore piston was hitting a head gasket's cylinder edge, the sound would be higher up, softer and not a deep knocking. I think we need to wait until Dkrahn has had a chance to short out cylinders to tell whether the knock is from a connecting rod or main bearing. If the knock continues without change while doing this, then pistons hitting the head gasket might be re-considered. That noise would be audible whether a cylinder fires or not.
'Love these Model A mysteries!!! Especially when the cause is uncovered and shared with us.
Marshall
 
I'm with Marshall. I think your middle main bearing is lose. You will probably be able to adjust it with the engine in the car.
At what speed do you drive on the open road?
 
Have heard of the generator hitting the side of the block. Engine was torn down twice before it was discovered. Best try to isolate the area before major teardown.
 
Is this something new since the head was replaced?

Have you driven it yet?

With the higher compression heads they tend to knock much easier.

Try to purposely set the timing retarded a bit more and give it a try to see if the noise goes away.

Do you have full travel of the advance lever in the distributor body slot?

Also are you positive that you had the spark fully retarded when you set the timing?
 
The best course of action when putting a new head on an engine is to put it on without a gasket and without nuts and turn the engine over by hand to see if the head rises when the pistons reach top dead center. If it does then you will need to re machine the pockets in the head for piston clearance. Best clearance is 1/16 inch. The gaskets compress so you have to figure that into your calculations.

A deep knocking is a main bearing. It will knock more when the engine is under load or being accelerated. A rod knock will happen when the engine is not under load, like at a fast idle or slowly being slowed down.

A high compression head needs far less ignition advance. If you are advancing the ignition to a point on the sector that you used with the old head you are advancing it too much. You have to search for the knee. Start with the ignition retarded and the hand throttle set to a fast idle. Advance the ignition one notch at a time until an additional notch does not increase the rpm. Retard one notch. That is where you should run the engine most of the time. You can retard one notch when the engine is lugging under heavy load and advance it one notch when cruising down a level highway. If you have a distributor with an automatic advance you will still have to search for the knee but do so by experimenting with different cam settings in the distributor. In this case a degree wheel and timing light helps.

To test whether the knock is coming from some place other than the engine, take the fan belt off and run the engine for a few minutes.
 
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Try pulling the spar plug copper feed wire from the distributor to the spark plug, and restart. One by one. If the sound goes away it's a rod that needs taken up. Easy to do, and costs nothing, so why not.
 
1crosscut said:
Is this something new since the head was replaced?

Have you driven it yet?

With the higher compression heads they tend to knock much easier.

Try to purposely set the timing retarded a bit more and give it a try to see if the noise goes away.

Do you have full travel of the advance lever in the distributor body slot?

Also are you positive that you had the spark fully retarded when you set the timing?
Click to expand...
Is this something new since the head was replaced?

Have you driven it yet?

With the higher compression heads they tend to knock much easier.

Try to purposely set the timing retarded a bit more and give it a try to see if the noise goes away.

Do you have full travel of the advance lever in the distributor body slot? Yes I do

Also are you positive that you had the spark fully retarded when you set the timing? Yes...
 
Gene F said:
Try pulling the spar plug copper feed wire from the distributor to the spark plug, and restart. One by one. If the sound goes away it's a rod that needs taken up. Easy to do, and costs nothing, so why not.
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I like that idea.will let you know results
 
TMarsh said:
Have heard of the generator hitting the side of the block. Engine was torn down twice before it was discovered. Best try to isolate the area before major teardown.
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Yes, I am trying to isolate the issue, and appreciate all the feedback. It isn't the generator hitting anything.
 
I had a deep knocking that showed up at idle and sped up when accelerating. The ring gear on the flywheel had drifted a little and was knocking against the housing. pounded it back into place and it hasn't been an issue since. Might take off the starter and look to see if this is the noise you are hearing.
 
I don't care what some will say here about a standard Model A being able to maintain 60 mph all day. They will not! Anything over about 42 -3 mph and you risk damaging the middle main bearing. I've seen it too many times. That's why I asked at what speed do you travel.
They will do it for a while, but not for long.
 
Synchro909 said:
I don't care what some will say here about a standard Model A being able to maintain 60 mph all day. They will not! Anything over about 42 -3 mph and you risk damaging the middle main bearing. I've seen it too many times. That's why I asked at what speed do you travel.
They will do it for a while, but not for long.
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Because of no oil feed? Ya know O/D saves engines.
 
Gene F said:
Because of no oil feed? Ya know O/D saves engines.
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Not because of oil starvation, because of the spidery Model A crank shafts whips and twist a lot. As someone recently said It's tough being a middle main bearing. There are changes that can be made to improve that bearing's life. Things like counterweights on the crank, pressure oil feed to the bearing, harmonic balancer. And let's not forget the driver's style/ability.
You are correct about O/D saving engines. They reduce the revs, hence the amount of punishment meted out to the bearings. I was referring to a standard engine and driveline being pushed along at 60 mph. I have an engine I built 11 years ago that has done about 75,000 miles, mostly at 50 mph with O/D. It has insert bearings, pressure oil feed to the middle bearing (25psi), counter weights, 6:1headnad oil and air filters. It is still going just about as well as it ever did.
 
I would check the timing gear. Do the timing gear check using the timing pin or a wood dowel. Remove the timing pin and run engine at idle. Insert the pin end back in hole and lightly press pin against timing gear as it rotates. If the knocking sound goes changes the timing gear needs to be replaced. I would think if the timing gear is worn the knocking sound would increase with the new higher compression head.
 
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