I think we may be on to something by focusing on the counterweights to the crank as it would explain the on again, off again pattern within a single revolution. Unfortunately, it is not the dipstick. Damn. It is a new dipstick, but it is identical to the old, without all the rust. And yes, I did take it out and turn the crank to check. My manual says it takes 25 pints of oil, so 12 quarts into a dry engine should not overfill it.
David, You misunderstood my previous post. I was not claiming that the crank and intermediate sprockets run at different speeds. My point was that the crank will turn completely over before the chain makes a complete revolution. So any defect in the chain, such as bad links rubbing against something, would not be synced to one turn of the crank.
This was sorta my feeling too. A particularly rough patch or nasty scratch when honing a cylinder? Normal 10W-40 in each spark plug hole would confirm this by temporarily clearing the problem.
Iām out of my depth here Michael. Are you and Peter suggesting that the oil in the cylinders can cause the noise or cure it? If oil is a cure, I would think the Miracle Oil would have taken care of it. I very much doubt that there is a rough patch. The cylinders were rebored and there are new pistons and rings. And yes, the sump may need to come off.
I canāt see any scenario where a rough patch in a cylinder will produce a cyclical noise where the cylinders have been honed, somebody would have to get in there and use a scraper to scrape a groove around the bore. And that would probably only last a short while. Iām still going with my counterweight(s) rubbing on the baffle.
I think it has something to do with the chain. The variance in the noise seems to match the links of the chain. This is based on putting my finger on the chain as it passed over the intermediate sprocket while turning the crank slowly. Not very precise, but it has the same rhythm. This doesnāt explain why the noise only appears for about 240 degrees of rotation (which I measured more precisely), and is quiet the other 120.
I am with Robin here regarding the likelyhood of the crank scraping on the baffles. Try removing the dipstick and bend a bit of thick wire so that when it drops into the dipstick hole it rests on the top of the baffle. Have someone rotate the engine slowly to replicate the noise and see if you can feel the vibration through the wire.
Mystery solved! Thanks for all your ideas. The breakthrough came from Mattās suggestion that I put a wire down the dipstick hole, but for a different reason. This forced me to find a way to stand at the back of the engine while turning it over. While I had been convinced the sound came from the front, it now seemed to be coming from the back. Sound transmit very efficiently down the crankshaft as several of you pointed out. Now the sound seemed to come from the flywheel. And what engages with the flywheel? The starter. So while I represented that all the accessories were off, I was so front of engine oriented that I never considered the starter. My bad. Off it came, I eagerly turned the crank, and voila near silence. No ugly noises whatsoever.
Now I have to figure out why the starter did not fully retract, but it may just be where it came to rest during installation since I have yet to energize it. Thanks again for all the input. It really helped me think through this and I am so glad I didnāt have to tear into the engine again.