Another Intermittent Engine Knock Question

Greg Jones

Greg Jones

Well-known member
I hesitate to post this, but it is yet another "what could this knock be" post. I have an intermittent knock, especially noticeable at idle, that sounds like it is coming from the distributor/oil pump drive train. One minute it sounds like "run-run-run-run-run-knock-knock-run-run, etc." Then no knock for a few minutes, then the same pattern, sometimes with a few (or even just one) knock. I don't hear it on the road, and the engine otherwise runs strong.

Thinking it time to check out the bottom end on this new-to-me Cabriolet, I meticulously checked all mains and rods. Took out a shim here and there; I have adjusted bearings on babbitt engines for a long time so this is not my first bearing adjustment. In fact, the engine appears to have been a nice rebuild with new babbitt, done by a reputable builder in Georgia. Buttoned it back up and same intermittent knock. Cam walk, I reasoned next, or maybe distributor or oil pump not turning freely. Of course I checked the cam plunger spring and thought, aha, it is too wimpy so installed a heavier duty spring. Still have the intermittent knock although it was almost nonexistant for about 200 miles of driving, then returned. I removed the timing gear side cover, and notice no movement when idling and hearing a knock. I installed a rebuilt distributor from Bert's that spins nicely by hand and checked the oil pump-it also spins easily. The ONLY thing that was different is that when putting the oil pan back on, it did not seem like there was much tension on the oil pump spring. Other rebuilds I have had to use longer bolts to get the oil pan started due to the rather high spring tension on the new oil pump. Could a weak spring on an oil pump allow it to flop around and possibly cause such an intermittent knock? Anybody run into this? I searched the forum for "knocks" and studied the posts for several days, looking for clues, but I seem to have covered all the moving parts that I can think of. Chasing down these noises can make a person crazy......
 
As best one can while it is in the car and under valve spring tension; lash at .003". Also, it had a fiber gear and I replaced it with an aluminum gear (the .003 lash is the aluminum gear). The engine appears to be a low mileage rebuild (judging by appearance and those who know the history of the car) and the crank gear looked new'ish as well. I replaced the timing gear primarily because the locating pin hole was in the wrong location-one of those gears.
 
Haven't tried the timing pin thing, but when I put the new aluminum gear on and tightened it believe me it was tight. I have replaced the distributor drive gear with a new one, then a different new one of the proper diameter, no change.
 
Nice to have a very articulate, knowledgeable poster.

1. I am sure you checked valve lash
2. If you have not done so, remove fan belt and see if anything changes
3. Shorting out plugs one at a time can sometimes help narrow things down, sometimes not.
4. While you had the pan down I am sure you inspected the bottom of the bores for scoring and there was none.
5. Can you obtain a bore scope and inspect for scoring at the top of the bores? If the bores are scored then the pistons are also, and you may be getting piston slap from a shrunk piston. (search my recent posts for discussion on this ad nauseum)
6. Sounds like only a few miles on a recent rebuild? Can you talk to the builder and ask what he uses for piston to bore clearance? It should be .0035-.004 regardless of what any piston mfg tells you. Tight clearance here will ultimately lead to scoring, shrinking, and slap. Slap can drive you crazy because it can be inconsistent and the location of the sound seemingly can move around.

There have been posters here who chose to ignore piston slap. i hope they are not planning on touring, and only going for an ice cream on Sundays

7. You can actually run an A on just one cylinder if it is not a basket case. Ground out any 3 cyls while running and see if you get any noise from the remaining cyl. Do that for all 4 cyls.

I am not sure how to answer the oil pump spring question. Theoretically if a huge amount of oil pressure builds up (it can't, this is an open system) the pump could be pushed downward, the spring acting as a sort of pressure relief deal.
But I cannot imagine anything like this happening at idle.
Maybe Purdy or Tom W or James Rogers etc can chime in about the oil pump spring.

I am not quite ready to bite on worn cam bores

Try some of the maneuvers mentioned and get back to us.
I would vote a bore scope to provide the biggest piece of info here. One way or the other.
 
My first thought was a loose two piece cam gear, but that's already ruled out.

As far as the pump spring, they are quite hard to push the pan in place with a good spring, and if any rust or pits are on the spring, it should be replaced.

At this point I can only think of piston slap, or maybe the center cam bearing is worn.
 
Tom, how often have you found a bad cam bearing in the block or the cam itself? You've been at these As more than I....

and as far as bad cam gears are concerned, I notice they are still selling the so-called macerated type gear, which is a fiasco...wonder why they don't just heave'em in the dump...rather than ply it on an unsuspecting person....
 
Greg,

Last year one of our club members had a ticking noise coming from his engine; you could only hear it at idle. The engine ran just fine at driving speeds. The engine had recently been rebuilt when he got the engine; do not know who the engine builder was.

I removed the pan, turned the engine over with the crank, each revolution you could hear a tick. To make a long story short, found # 1 rod was forward on the wrist pin instead of centered; the rod was striking the inside of the piston on the up stroke; #3 rod was almost striking the inside of the piston on the up stroke.

I determined the engine builder ? had made the thrust on the rear main cap too thick, which moved the crankshaft forward several thousands.

I had a local machine shop take .050 off the front of each rod at the wrist pin end; did this to keep the rods in balance. When reassembled in the engine and the engine run, the ticking noise was gone.

The engine has been driven about 1500 miles since the correction was made and is running like a dream.

Ron Rude
 
PC/SR said:
Did you check the cam gear and lash when you replaced the cam spring?
Click to expand...

tbirdtbird said:
Tom, how often have you found a bad cam bearing in the block or the cam itself? You've been at these As more than I....

and as far as bad cam gears are concerned, I notice they are still selling the so-called macerated type gear, which is a fiasco...wonder why they don't just heave'em in the dump...rather than ply it on an unsuspecting person....
Click to expand...
.
I'm not Tom, but it's very common I see it on both the shaft and block if the valves are out, and you have around .005" between them, you can actually grab the cam and make it click as you push up and down
 
tbirdtbird said:
Nice to have a very articulate, knowledgeable poster.

1. I am sure you checked valve lash
2. If you have not done so, remove fan belt and see if anything changes
3. Shorting out plugs one at a time can sometimes help narrow things down, sometimes not.
4. While you had the pan down I am sure you inspected the bottom of the bores for scoring and there was none.
5. Can you obtain a bore scope and inspect for scoring at the top of the bores? If the bores are scored then the pistons are also, and you may be getting piston slap from a shrunk piston. (search my recent posts for discussion on this ad nauseum)
6. Sounds like only a few miles on a recent rebuild? Can you talk to the builder and ask what he uses for piston to bore clearance? It should be .0035-.004 regardless of what any piston mfg tells you. Tight clearance here will ultimately lead to scoring, shrinking, and slap. Slap can drive you crazy because it can be inconsistent and the location of the sound seemingly can move around.

There have been posters here who chose to ignore piston slap. i hope they are not planning on touring, and only going for an ice cream on Sundays

7. You can actually run an A on just one cylinder if it is not a basket case. Ground out any 3 cyls while running and see if you get any noise from the remaining cyl. Do that for all 4 cyls.

I am not sure how to answer the oil pump spring question. Theoretically if a huge amount of oil pressure builds up (it can't, this is an open system) the pump could be pushed downward, the spring acting as a sort of pressure relief deal.
But I cannot imagine anything like this happening at idle.
Maybe Purdy or Tom W or James Rogers etc can chime in about the oil pump spring.

I am not quite ready to bite on worn cam bores

Try some of the maneuvers mentioned and get back to us.
I would vote a bore scope to provide the biggest piece of info here. One way or the other.
Click to expand...
I agree with all this except the part I highlighted. If there is some kind of obstruction in the tube leading from the feed to the front of the valve chamber. If it is the spring could act as you indicated, I have had it happen.
 
If you were only able to remove a shim here and there from the mains and rods - especially the rods - this may be a fairly low-mileage rebuild. It's possible that the noise came with the car, and just as possible that's why the previous owner sold it - he couldn't solve the problem, either. It's your problem now. The reason I stress a new rebuild is because the bore size is probably unknown to you and if it's 0.100" over 0.125" oversized, the new pistons may be hitting the head gasket in one or more cylinders. Most builders grind off the sharp edges of the new pistons, if they aren't already ground from the factory. Some older head gaskets and poorer-made ones can't handle the larger over-bores and are a bit too large in the cylinder border area. If the piston rises slightly above the deck - a very common occurrence in these old engines - then the edge of the piston will rub the portion of the head gasket hanging slightly into the cylinder. I have seen this positively in two cases with the same kind of knock you described. In both cases, the larger bore pistons were striking the head gasket in one more cylinders. Eventually the overhanging portion of the head gasket will burn away because it is so thin. But in a low mileage engine like yours, it will take a while for this to happen. The solution might just be to pull the head and install one of the newer generation head gaskets that allow for larger bores. Who knows how old or where the head gasket came from that's in your engine? If it's a hold-over from the Rick's Antique Auto or JC. Whitney Model A parts selling days, I can almost guarantee that the gasket is foreign and cheaply made. Lots of problems with that junk in the 1970'-1990's. And the stuff still turns up, especially in Model A projects that have sat unfinished for a couple decades.
Of course, continue checking all the obvious suspects for the cause of the knock. But when you've run out of possibilities, consider spending two hours installing a quality head gasket.
Marshall
 
Since we've reached the point of considering weird noise makers, let me add one more from personal experience. I wrote about this incident in my former column "What's Wrong With It Now?" in the old "Model A Trader" in the late 1990's. This happened to me on a cross-country trip from Arizona to Iowa in my 1931 Deluxe Roadster. I'm only going to give the solution here because the symptoms have already been described in the original post above. Describing the symptoms and frustration encountered while trying to track down the cause took up most of my article. I'll spare the reader the gory details. The intermittent knocking and clicking was caused by a hardened valve seat insert (something rebuilders often install these days to accommodate the cruddy unleaded gasoline we have now and which will get even worse in the future) that had come loose from its bore. The head of the valve was hitting the dislodged seat for a few minutes until the seat was pounded or simply dropped back into place. Then the noise stopped. A little later during the trip, the clicking and pounding returned when the seat dislodged itself again. Depending upon the severity of the knock, the engine also slightly missed fired because the valve couldn't do its job properly while the seat was out of its bore. Cutting to the chase, I finally pulled the head and used the sleeve locker Loctite 640 to keep the seat in place. And it worked! No more knocking or loose valve seat problems for the rest of the trip or for as long as that engine was in the car (replaced by a Ron Kelley creation).
I told you it was a weird cause, but that's what it was: a loose hardened valve insert. If you pull the head to change the gasket, double-check for signs of a loose hardened valve seat, assuming the rebuild had installed them.
Marshall "Everything Happens to Me" Daut
 
Do you know if the the noise was there before changing the dist. drive gear ? I've found that the new gear likes to argue with the old one until they get used to each other.
 
Thanks to all for taking the time to respond. Especially to Marshall for sharing his experiences. Interestingly, the car has a nice smooth idle EXCEPT for an occaisonal "pfft" like a miss. Also intermittent. I will have to see if the "run-run-run-run-run-knock-run-run" coincides with the "run-run-run-run-run-pfft-run-run" sounds. I do foresee pulling the head for a look-see quite soon and will report back.
 
Marshall, good call on the valve seat. I've seen and heard of a few seats coming loose. I wouldn't use a seperate seat unless it was absolutely needed, and then I'd make sure the installer knew how to do it right.

I checked on the valve equipment where I used to work, since I did the work, but the boss just sold it to someone else. People never seem to care when you tell them you want to buy something and please call if it ever comes up for sale. In fact I bought a 1950 Studebaker from a retired farmer 10 years ago and I looked over his mint original 1929 Studebaker Dictator with straight 8. I told him I'd like to buy it if he ever decides to sell it. I called him last night to see if he still had it, and he told me he sold it a year ago. The real crime here is that the new owner restored a mint original 1929 Studebaker. That hurts!:mad::eek:
 
A year ago was at a car display and I heard an off and on knock at idle. Called another guy to listen. The altenator was bumping on the Tension brace. Moved brace a little tighter knock gone, Tony
 
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