Found this:

1353031011 Genuine Toyota SPROCKET ASSY, IDLE 13530-31011 | eBay

Same “crack” appears in brand new one if you zoom in. I assume the reason why the inner collar looks perfectly smooth on mine is 15 years of heat and rotation which has worn down and welded that crack to a perfect seal. So. I will leave it and hope for the best Like always though, thanks for the help. This forum is full of very helpful people!

Good research and great find. Back to your question on the timing marks, the attached diagram shows the intake sprocket timing mark positions with the crank at zero mark. Also attached diagrams for the crank marks showing the position of the chain marked link on the sprocket, and the block casting timing marks. And finally, a pic of my bank 1 showing the staggered positions of the chain marked links on the main chain (offset to 1 o'clock) and the exhaust chain (12 o'clock). Hope these help.

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I am going through this as I complete my engine build.

I don't think its a crack. I think its a bronze split bushing. Same type of bushing type used for the wrist pins. I also bought the Cloyes kit that came with the idler sprocket but not the sprocket assembly. which sucks because I cant figure out how to separate the assembly. You guts posted some good pictures, take a look at how its assembled it doesn't appear it can be disassembled. So you end up with a new sprocket that cant be used.

Also had a problem with one of the Cloyes tensioners. The housing appear to be slightly miss aligned. The housing has a bolt hole and a alignment pin hole. My assembly is slightly cocked where the pads don't align with the chain.
Fun project, good luck.
I have the FSM if you need some specific instructions on the timing mark.

waypoint has some good info. This is my view today.

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Thanks again for the replies, good to know I'm on the right track.

Next problem/question... I have all the chains/sprockets back on and have been turning the crank to make sure it stays in time. Good news: It does. Probably 10+ rotations now, all the dots stay in alignment. HOWEVER, part way through each turn, at the same point on the turn, slack is introduced around the idler pulley. It then tightens up again. It's not a stretched part of the chain as the chain is gradually shifting its position on each rotation obviously, and the slack introduces itself at the exact same point of the crank/cams turning each time.

Video below. Any thoughts as to if this is normal or if it's an issue? I didn't video this but the tension on tensioner 1 is variable - it goes in and out a little as the chain is going around.

Video of slack in chain

If you look at this video:

YouTube

At 1:27:47 he starts rotating the crank and you can see the slack suddenly introduce in the idler sprocket. Can also see the no. 1 tensioner being quite variable and coming and going on the chain guide there as he turns it, which is what I also see. So I'm HOPING I'm ok? It's just a lot more slack than I imagined around that idler sprocket.

EDIT: Just did more experimenting. It seems like, at the point slack is introduced, I can slightly turn the intake cam on each side. Not much, but enough to move the slack to either side of the bank in question (tensioner side or idler sprocket side in this video). When the chain is tight, that movement doesn't exist. Is it just that, at some point, the intake and exhaust cams are "balanced" and so they get a tiny bit of slack which introduces slack into the main chain? Is this an issue? Video below:

Video of minor cam movement when slack appears

EDIT 2: Seems to happen in this video too at about 18:32. Must be normal!

YouTube

Another question... are these DEFINITELY threaded studs?

Studs - Album on Imgur

Just put a 6mm socket on the end and have a few gentle twists and didn’t budge. Don’t want to snap them out, that would be a horrible end to the project. Definitely feel pretty lightweight though. I like your idea of having them out the way to plonk the timing cover straight on the sump though, rather than slide it on with the sealant squishing everywhere while I do it.

They are threaded studs, but are Torx not hex...you'll want a proper Torx socket to remove and reinstall them.

Removing these and leaving the crank main seal out of the timing cover helps during re-installation of the timing cover. You don't want to damage or dislodge the oil pump pickup o-ring, and aligning the oil pump to the crank gear is tough to do even following the shop manual.

Don't take my word on the slack, but the tensioner pin is oil pressure based, not a high pressure charged cylinder like many other cars might have for a rubber belt and won't be pushing onto the chain very hard unless the engine is running.
I used the AutoZone guides when installing mine and checked it 10 times before bolting it all back together.
That first startup when it is together is always unnerving.

Thanks, yeah I’ve gathered the oil pressure will fix the slack from a few things I’ve found. And yes. I am crapping myself about first startup. Afraid I’ve dropped something in the sump or will screw up a seal or blah blah blah, imagining all the things that could go wrong. Oh well. Only a car right??

Remove the fuel pump relay or fuse before that first startup. Spin the engine on the starter until the oil pressure light goes out, and leave it spinning for a few seconds after this. Pressurizes the chain tensioners nicely and makes for a quiet initial startup.

Pre-lube the key areas before you install the timing cover and valve covers. Apply break-in lube to the chain contact surfaces...sprocket teeth, tensioner pads, guides. Dribble oil onto the camshaft lobes so they're not dry during startup. Dribble oil onto the chains.

Thanks waypoint. Is break-in lube the same as assembly lube? A moly based, oil-soluble grease? Just lightly apply to sprockets, chain, cam lobes, etc I guess? Will be changing the oil pretty quickly after applying that, I reckon!

Well, I got the studs out. One of them didn’t come happily:

Studs - Album on Imgur

I seriously need to get an impact gun. Anyway. Would threadlocker and a small amount of torque when I put this back in be sufficient? I need a welder too. Put a friggin nut on there for next time I need to take the stud out. Or is there some sort of standard thread stud I should use and replace this one?

I'd be OK using what you're describing. No problem leaving that in the oil for a reasonable break-in followed by an oil change.

Of note, there's decent body of evidence that molybdenum disulfide ("moly") is a good anti-wear additive for minimizing timing chain wear:

https://www.stle.org/images/pdf/STLE...ry%20Plays.pdf

Your chain is in very good condition as indicated by the tensioner plunger extended length in your video. Keep doing what you've been doing in terms of oil change intervals and selection.

New studs, amazingly found in my city at a Toyota dealership (amazing what common parts sometimes take weeks to get in and what random parts they have just laying around):

New studs - Album on Imgur

Timing cover is back on. I am crapping myself that the oil pickup o ring has caught when putting it on and come out of its groove. I guess there’s no way of knowing until I start it. If the o ring didn’t stay in place properly, what would I get? Oil pressure warning that won’t go away? I can see now why Toyota recommend dumping the oil pans but that looks like a helluva job too.