Help with timing belt, this is interesting.
 

I've timed my engine now more times than I'd like to admit while chasing after leaks and the crank and cams. Last night while putting the timing belt on and going through the tensioning procedures I noticed my timing marks on my belt and sprockets didnt line up. (i put marks on the belt ans sprocket to help with quickly reinstalling the belt.

The belt was slipping. This is the first time this has happened. I figure that it had too much oil soak into it,a nd it must have stretched. probably a good idea to replace it. I went and got a new belt today at Napa. I put it on and the same thing happens.

I've got 19 teeth between my cam sprocket alignment marks, so I can't magically make the belt any tighter. I am going to check to see if I have a bent sprocket (don't really know how to check, but I'll figure it out. Idler pulley and tensioner pulleys are practically new.

So what could cause the same problem with two different belts, that wasn't there a few days ago?

Did you try turning the motor over several times? Those marks might only line up every 20 rotations FYI. Are your E and I marks lined up correctly when at TDC?

A tooth is 3* and that's a large enough angle that you can see it on the cam lobes in relation to the lifters. Line it up, turn the crank by hand 1 4/5 turns and tighten the tensionsers and drive.

Jacob 300zaids is right, 1-rotation on the engine =! one belt rotation.

I feel so dumb. I wish I could document my frustration with this project. I'm to the point that I'm just not thinking clearly. I made those marks so that I could quickly reinstall the belt. The cam marks always line up so the belt is not slipping, my brain is. Thank you for bringing clarity back to my situation. If we cross paths, I owe you a beer!