Rebuilding a basket case 250 Chev.
Fired up in stand, sounds like a rod knock.
So I tear it down, rods about .004 to .006, journals all look good just worn down to the copper.
So I tighten the bearings up to .002 to .003, figure it's good.
Fire it up, SAME NOISE. Bang-bang-bang one per crank rev.
Pull the fuel pump & belts, same noise.

Pull all the rods, piston pins all tight, all looks good.
Soin the crank all quiet. Spins easy.
Add the cam & spin it over with spinner wrench: THERE IT IS.
Either the cam gear or retainer plate or crank-cam gear mesh is making the noise, once per crank revolution.
Aluminum cam gear, steel crank gear, looks all factory.

Anybody ever hear a noise like that? What is it?

How do the cam bearings look?
I have heard noise when spinning engines over by hand on stand. Usually it is the lifters making the noise. Probably not in your case.
On another note, in later engines, GM put a type of wobble shaft in on the lower distributor shaft. They found the blocks were not getting machined perfectly and the wobble shaft corrected the misalignment.

It literally can be any moving part inside your engine. Considering you had such excessive wear on your rod bearings, you will be better off rebuilding the engine or buying one thats already rebuilt instead of trying to diagnose one that's likely worn out. The noise could be excessive piston to cylinder wall clearance, broken piston skirts, excessive crank thrust, excessive cam thrust as you mentioned, worn wrist pins, etc....

Another thing to look at is the flywheel Bolts. Also look to see if Maybe the crank is hitting the Oil pick up tube..

Thanks for the ideas. It's been a while since I tore into one of these.

Tom: I've got everything out except the crank: Spinning by hand, no noise.
Then add the cam & gear (no lifters) with the 2 plate screws snug: Spin by hand, thump thump thump just like when it's running

CNC: Pistons show very little wear, about .003-.004 piston-to-wall, skirts ok, no noise from crank, cam thrust is very tight (maybe even TOO tight, the plate doesnt rotate as smoothly as I'd like to feel). So I'm convinced its the cam or cam gear setup, only noise I can find.

The cam IS worn, some lifters concave, 4 wouldnt even come out (had to pull em from the bottom then file a ridge off the edge). I've seen & run worse cams though. One pushrod wasnt spinning so I pulled that rocker & pushrod, but the same noise while running. It just doesnt SOUND like lifter noise.

I had something similar once on a big Ford, turned out to be a high spot on the damper "ticking" against the timing tab. REALLY sounded internal, disgusting it was something that simple. That's why I stripped this one all the way down til I found a possible culprit.

I'll try to post a movie.

Sure, I could re-do it all but at this point this is a matter of PRINCIPLE, I'm determined to find this noise & fix it for free. This engine was supposed to be a free project (gaskets only) & now I'm obsessed with that. Hope you get the idea - just determined to find it!

Ok here it is on You Tube. I know it almost sounds like a loose bolt or the spinner wrench rattling but it's not.
This gear is aluminum, are the fiber gears quieter or less prone to this noise?
Then again my 292 has an aluminum gear and no such noise:

Yup looked at that too. It starts on the stand with no trans, just the flexplate. Can definitely hear the noise is from near the front even while running. It's a rear sump so I did check the oil tube, plenty of clearance there.

The hand spinning in the movie is with flywheel off, oil pump & tube out, literally only the crank & cam in the bare block.
I could understand gear noise or even grumbling but as you can hear, this is definitely a clunk once per crank revolution.

The crank gear is the same whether the cam gear is aluminum or fiber, right?

Interesting sound. It has to be the crank gear. You should be able to see if it has a bad tooth.

When watching the Vid I could hear the hit/noise at the same time I see the Keyway on the crank.SO with that being said The spot on the crank gear Hit the cam into different Places, So would look closely at the crank gear having a Miss aligned tooth maybe. And with that much Hit/noise I would think you could feel that Hit when you turned it over slow. Yes the crank gear is the same. But they are made as sets or to be as a sets I should say.

Piece of something stuck in the root between teeth on your crank gear? Might be a witness mark on the cam gear in two spots if this is the case. Or the thrust plate or screws interfere with cam gear?

Some one has put the cam gear on with a hammer or press instead of heating it. This will shave some aluminum out of the gear so it will not be tight enough. The gear will slip forward just enough to let the cam move back to let the crank journal or rod to hit a cam lobe. Sounds just like a rod. Always heat the aluminum gear enough to not have to press it on,just light taps with a hammer will seat it. Hope this helps.