Pleeeeease tell me....

...this is not a s bad as I think it might be. I've been working on the A and here's the story, for recap:

Because the manifold was left exposed for years, I pulled the head, and cleaned mud-dauber mud, bug pieces, and other junk out of cylinders 1 & 4.

Pulled the oil pan too, cleaned it out (lots of junk), and visually/grab inspected the crank & rods. Everything APPEARED to be tight.

The engine was rebuilt at some point, and the oversize pistons & sleeves looked great. Lapped valves & seats, replaced gasket, cleaned & replaced head.

Pulled crank pulley and front cover, cleaned mud dauber nests out of timing gears (tractor sat with magneto and governor off), replaced front seal, replace gasket and cover.

Replaced oil pan and gasket.

So, here's the sucker-punch to the solar plexus: Yesterday, remembering that I had left the cylinders dry when I cleaned them out, and worrying about rust, I removed the spark plugs and squirted some clean motor oil into each cylinder, and then, putting a wrench on the bolt I have threaded into the end of the crank, cranked it over to splash the oil around. That's when I heard the soft clanking sound. It seems to be coming from inside the front area of the crankcase. I'm worried it's a shot bearing, but having no previous experience with engine internals, I'm hoping it's not. I did turn the engine over quite a bit while I had the head off, and never noticed the noise- which seems strange. Advice on how to proceed?

Governor weights flipping around
maybe?

is it by chance the impulse coupling on the magneto making noise???

It could also be the impulse from the magneto. It will click as you turn the engine over.

I wish it was. It makes the noise with neither the governor nor the magneto installed.

Not there to hear, but one thing could be when hand cranking the camshaft gets to a certain spot and valve spring pressure pushes the cam and rocks timing gear ahead and whatever backlash is in the gears makes noise. Also make sure a valve isn't sticking open and then spring snapping it closed later than intended.

Bingo. Do I feel stupid- and relieved. I had checked the valves again already, and saw nothing amiss- I checked them again, and removed the valve train and push rods, and voila! No noise. I did discover a little item I had forgotten, which is a really good thing. Apparently, I had not tightened the nuts on the valvetrain when I replaced the head. Don't remember if that was on purpose or not, as it was over a year ago, but that coulda been bad.

All the teeth looked good 360 degrees around on the timing gears? Did you have the front cover
off?

(quoted from post at 12:43:22 04/18/16)
Pulled crank pulley and front cover, cleaned mud dauber nests out of timing gears (tractor sat with magneto and governor off), replaced front seal, replace gasket and cover.

Click to expand...

Yes, per OP. everything checks out visually. Glad to have found the (benign) source of the noise.