I forgot to mention that I had new rods poured. The crank main bearing journals were within half a thousandth of specs, so we did not replace the main bearings.
Just now removed the oil pan, and found the noise: Too much end play on the new connecting rods. I can grab the rod cap and move it back and forth on three of them, and I get the metallic slapping noise that comes and goes. What should I do? A reputable company in South Dakota poured the rods. The owner has now retired and shut the place down. I may just drive it and ignore the noise. It's only at certain RPMs. I'm already upside down in the car as far a $$$ I have put into it (common story, eh?).
Pete Phillips

Maybe a forgotten camshaft thrust washer/button/spring?? Miss matched camshaft gears? Have had this problem when
a new alum cam gear was used with the earlier pitch steel crank gear. Just a couple of thoughts.

"Ric"

I wonder if you could merely tighten the caps to take out the play? In order to do this, you would have to remove the caps and grind the metal slightly to allow them to be tightened. You could get some shims from someone who's rebuilt old 3x5's, and that will help you adjust them to an accurate clearance. (The older rods had the ability to be shimmed but I think that ended in 1938). I'm not really an engine expert so I would defer to the good advice of an engine expert, on this suggestion!

Geoff Clark in NZ would know.

He probably forgot to put the brass shims back on between the rods/rod caps. There are little brass shims that are installed between cap and rod on each side where the bolts are. Usually a couple of different thicknesses are installed -- depending on tolerance of new bearing poured, you may have to remove 1 or 2 of those to get the right clearance for rotation, but I assume those were all lost -- many are careless and discard those during teardown. So it can be fixed. You have to either locate some shims or cut your own from brass shim stock (had to do this when I rebuilt one of these).

Have you replaced the timing gears? If the gears are fiber, then my guess is the centre is turning slightly, causing a collision between the cams and the connecting rods.

By the way, shims were eliminated from 1938 onward in the connecting rods. .

Is the tapping load sensitive or strictly rpm related? In other words, if it occurs at 700-1400 rpm at no-load, does it still do it through the same rpm span under acceleration or pulling a hill?
I somehow don't expect the problem to be rod end play. How were the wrist-pin bushings done? It's a detail that is often overlooked or accepted "as-is" when they shouldn't be.
If the cam thrust washer is broken/missing the issue would be present at all times and be much worse, probably destructive. A cam thrust button and/or spring omitted during assembly could be the culprit. If you still have the pan off, pry the camshaft fore and aft. When pried forward it should spring back by itself.

Frank

Oh.....one more thing. Were the rods straightened after all other operations were performed? You'd be surprised at how far off they can be.
F

How much is the end play side to side in the rods? Take a feeler gauge and check to see what it takes to tighten the side play up. That end play usually is not audible on accel, only when cruise with very little thrust applied. Geoff and Frank have mentioned several key points. Was a new thrust washer used on re-assembly?. Verify the thrust button and spring were used. The cam gears on these cannot be mis-matched. Even though all the cam gears look the same, there are different pitches, and on earlier engines, the width is narrower . I believe 14, 16 and 20 degree pitches. 20 can be a fiber gear, but used in 40-42's, not sure without a book. All aluminum gears are 20 degree. If the cam gear got replaced with aluminum and the provision was made using the new countersunk bolts, and the crank gear did NOT get changed, it will cause this concern. If they are not matched, when given a little throttle, the cam tries to walk forward, and on 3x5's, they WILL HAVE interference with the crankshaft. If you still have the oil pan down, check closely at # 2 and # 3 connecting rod where the bolts are pressed into them. It will have a slight groove cut in them from the interference with the cam. Frank also mentioned a good point in piston pin bushings. These engines have tight tolerances and if the piston pin bushings are loose, and/or fit loosely, will sound sort of like a diesel, especially on light accel. Also as mentioned, these rods bend and is very common to find the beams of the rods bent. [5" stroke ] You can visibly look up the beam of the rod, while installed in the block, and see them curve.. Interested in knowing what your findings are ?

Also, did the deck of the block get cut?, if it did, you could also have pistons hitting the head. How much did the engine get over-bored also comes into play. If the pistons don't have a bigger chamfer cut around the edge, it can allow the gasket and piston to make contact when it comes to top dead center, not noticeable when rolling the engine over by hand .

The deck of the block did not get cut. The cam/timing gears and chain are the originals--they showed virtually no wear, so we re-used them. This is a 38,000-mile engine that sat for 50 years. The camshaft showed wear so I had that built up and reground. So I am using the original camshaft and timing gears. The looseness and noise are definitely from side play in a couple of the connecting rods. I can move #4 and #5 in a direction from the front of the engine to the back of the engine, about 1/16th of an inch, and they make the metallic sound that I hear when the engine is running. I have not tried to put a load on the engine nor drive it yet. The metallic tapping comes when engine speed is raised, then goes away as it is raised more. It comes back when the engine speed slows back down, then almost completely disappears at idle. The tightness of the rods and caps against the crankshaft I do not think is the problem. We plasti-guaged that and the plastic was completely mashed to specs. We did not think about trying to straighten any rods--perhaps that is the problem. I appreciate all of the suggestions and advice. Yes, the engine had to be re-bored, I think it was .040 over-sized (the machine shop did that, not me). The reboring was necessary due to rust on the sides of the cylinder walls, not due to wear. I need to pry against the camshaft and see if there is some play there, as was suggested.
Pete

Oh....just one more thing (I'm starting to sound like Columbo), The alignment of the lifter guides is very important and easily left in a bad position during assembly if the assembler isn't familiar with these engines. There are "flats" on the guides ([i]lifter[/i] guides, not valve guides) where they are held down into their bores in the block. Those flats must be aligned parallel to the engine/cam/crank centerline to avoid collision with the camshaft lobes since the lobes pass through a slot in the guides while running. The guide clamp is used to align and clamp them in place.

Still, I wouldn't expect that to be your issue since speed would have little or no effect on the sound. Cams are thrust rearward at all times when running due to the helical cut of the timing, distributor and oil pump gear-sets. This is why the cam thrust washer, or "cat-head" washer is so important. I use a roller thrust bearing at that point. The fiber washer just isn't quite adequate in my opinion.

1/16th end clearance on the rods is quite extreme and I can't say that such clearance wouldn't be the culprit. Honestly, I've never seen any [i]that[/i] bad and I don't know what the solution would be for such a deficiency. The assembler should have taken note of that at the time it was done.

I can't imagine where "tightening" of the rods would have any effect on an excessive end-play issue. My recommendation there is [i]don't try it[/i]. Rod straightening at assembly time is only accomplished correctly with proper equipment....."eye-balling" them won't do it. One should be concerned with half-thousandth accuracy when doing that job. A rod, even slightly bent, may cause the rod to snap back and forth between intake and exhaust strokes. Compression and power strokes result in thrust in the same direction, thus, no snap.......probably, but we're shootin' in the dark here.
F

As Frank replied, "shoot'in in the dark" ? .040 over ?, it can really be there is not enough bevel on the pistons, and after the head is torqued down, the gasket squeezes out into the bore, and if there isn't enough bevel/clearence, the pistons hit the head gasket and hammer it on the cylinder head. If you have a cylinder bore scope, remove the spark plugs and look at the gasket. Bore scopes are a very common tool shops have these days, so that shouldn't be hard to come up with. Keep us posted. Also, did you inspect the top edge of the connecting rods to and see if there was a groove where it had possibly been hitting?.

+40 is considered the last overbore and Doug & I question the wisdom of going that far (that was an assumption on my part).
There are so many things that it could be that I (we) would need to go to your house and look for ourselves. Got beer?
Another idea is the length of the rods, although I'd expect the noise to occur all the time if that was it.
After the rod Babbitt is repoured, they must be bored/reamed to the finish size. If the specified center-to-center dimension (wrist pin to big end) isn't closely controlled, any number of pistons from one to six could be striking the head or head gasket as Doug says.
Again, I'd expect that to be consistent and not varying by engine speed.
Frank