Are you expectin the marks to line up perfectly before the belt is tensioned? That is most unlikely as the belt will very slightly stretch and settle after it has been tensioned and turned several times.
The other thing is the balancer ing may have shifted slightly and/or tolerances may be stacking slightly, if the head or block have been skimmed, that will also move things slightly.
What I would suggest doing is getting them as close as possible, setting the tensioner, turning it over several times by hand (if it suddenly gets very hard, stop - it may be out far enough for valve-piston contact), resetting the tension and then checking the timing marks.

 

Absolutely not! That's why I'm wondering if anyone can tell me how I need to arrange everything before applying tension. The belt as been tensioned, released, repositioned, and retensioned and turned twice by hand several times now, yet depending how we do it to try and line it up, it either comes out like the link above with the crank in line, or like these pics, and confirmed by the bottom cam pulley

 

Ah, got a picture of the camshaft position with the crank at TDC? Half a tooth there equates to a tooth at the camshafts.
What I would do, is confirm the crankshaft marks are correct using a DTI or, at least, a long probe down #1 plug hole, then work from there.
As you seem to be asking, yes, because you may find it very difficult to fit the belt, you may need to back the crank off slightly - the important thing is the final timing with the crank' at true TDC.
It isn't unusual for them to be slightly out, so sometimes you need to take the cam' sprockets to the closest tooth. If you are really concerned, pick up a couple of adjusteble sprockets for exact timing.

 

I put it in gear and pushed it forward a little to get TDC and this is what happened

As you say, half a tooth, but only on the inlet?
As I said, I don't do engines, mate is a mechanic of many years, last belt he did was on a Sierra when they were popular taxis, so presumably never one of these.
Worrying too much? Nothing to worry about? With it being so closed couldn't be closer, he had to leave to see to his kid, I put the rest back together and I did start it on his suggestion, but it idled quite rough and there was a few backfires out the exhaust. Switched off quick.

 

That is about as good as you'll ever get.
Can't comment of the mis-fire reason, but double check all hoses, sensors, etc, are correctly hooked up. Should you be concerned you bent a valve turning it over with incorrect cam' timing, do a compression test - most unlikely, though, as it is quite noticeable and I think you would have stopped in time, if so.

 

That is about as good as you'll ever get.
Can't comment of the mis-fire reason, but double check all hoses, sensors, etc, are correctly hooked up. Should you be concerned you bent a valve turning it over with incorrect cam' timing, do a compression test - most unlikely, though, as it is quite noticeable and I think you would have stopped in time, if so.

 

I'm probably worrying about nothing then, thank you for confirming that lol. I doubt a valve is bent, might do a compression test anyway. It is a bit of a mash up though, Ford ignition, Weber carbs, it's only really hot a Frank sensor. Thing is, last time I checked it, I'm sure everything was bang on. Maybe I need to turn it over a few times, or the belt needs to relax in tension or something.
I'll need to stick the timing light on and check all of that.
Seeing as I don't do engines, this is all new to me, so no idea of the tolerances and stuff, it's not been in a position where it could have hit a valve, so who knows. I'll recheck my sensors and stuff (there's only 1 lol)

Thank you for your help! Very much appreciated

Might look for a spare engine as a rebuild project... to learn them and get more comfortable with them

 

You won't ever get it bang on. As long as the mark are within touching of the whitespace of each other. If it still bothers you, fix the crank in place by locking it at the flywheel end, lock the cams with the tool, and slide the belt on, don't try to do it tight, keep the tensioner on the loose position, then let it snap back the tension, remove the cam lock 1st, then the flywheel and rotate it clockwise, or rotate the engine so the belt pulls tight on the non tension side, do it very slowly 1st, plugs out (no compression) and rotate it two rotations at a time for several rotations, by hand, by the 8 or 9th rotation by hand if the marks haven't moved, then you just did it textbook.


"I put it in gear and pushed it forward a little to get TDC and this is what happened "

Your mistake is rotating the engine by moving it in gear, the timing marks are off due to the backlash in the gearbox you must do it by hand, no spark plugs, using a wrench on the pulley..

 

Thanks for that, I noticed the aux pulley didn't quite seat as well as I expected, so removed it again, reset it, and the marks are better, still not ideal, but I'll take it? Only a tiny bit out.

However, the reason for changing the belt was the belt was really slack after switching the engine off. Changed the belt, idler, tensioner, tensioned it all correctly as per the instructions in the gates kit. Ran the engine yesterday, switched off, guess what, belt so slack you could actually slide it off the idler withoutnslackeningnthe tensioner. I don't understand it

 

Okay.

You need to stop. And fully understand how the tensioner works.

 

It's very easy to incorrectly set the tensioner.

Check video from 9 mins to 10 mins 52 seconds. It's easier than me trying to explain it.

https://youtu.be/UDc2EHgy7z8

 

If after looking at the video, you follow it and your still getting problems with tension, you either have the incorrect kit, or it's defective.

Also just realised the x20xev has a cam locking PIN not a cam locking block. If you are using the block you won't be able to set the marks correctly, which might be why your having a problem.

The correct locking PIN for the x20xev is
GM part number 90501111

 

When I changed my belt on my Calibra Ecotec, I never had alignment problem, other than I could not get the engine pulley off, it had welded itself to the crankshaft flange, and in the end needed a massive whack with a 20Lb sledge hammer swinging from 3 feet away and smack on the side of the pulley, all other methods failed like leaving WD40 to penetrate overnight etc all failed, whacking it with a massive hammer striking a hard wooden block also failed, the hard wood disintegrated, so in the end anger piling up paid, one great big swinging of a sledge hammer and the pulley moved about a mili meter, so after that it was a matter of few more blows.

I used malgrip wrench to hold the two pulleys together at their relevant timing marks, If I am not wrong the marks on pulleys never do align 100% but close enough like 99%