I changed my chain on my G35 by myself and the tool you are probably talking about slides into where the starter goes and locks it in place. I just set my #1 cylinder to TDC and did it without any problems without using the tool. I also have an Audi and I had to use special cam lock tools to change the belt on that car.

I have a post where I documented what I did when I changed my chain.

 

 

If I had to do it again, I would try to find that tool or stick a pry bar, screw driver, or something else into the teeth to stop the crank from moving. What happened with me was this: I had to change the water pump, so I started doing the job without any problems. But, with very minimal effort the engine rolled over on me, which turned it into the timing chain job. To get the tensioner and water pump out you need to take the tensioner out and then wiggle the crank to give up some slack and that's where my problem occurred. I needed to replace my radiator as well, so I took that out while I was doing the job, but otherwise I would put a thick blanket over it to protect it. I was able to do everything with the bumper still on and without too much difficulty. There is also two hidden bolts on the bottom of the cover that require you to take off the lower oil pan.

I bought all OEM parts from the Nissan dealership as well and the guide rails were updated as well.

 

 

The tool is called a crescent wrench, you take the valve covers off and hold the cams still with the large cast hex section at the front of the crank.

It's VERY important to actually line up the match marks on the cams/crank to the block and not just try to "keep everything from moving". Read the FSM and look at the diagrams so you see where everything needs to be set.

 

 

Normally I will set an engine at TDC1, remove the tensioner, chains, guides, crank sprocket. Then compare the old chains to the new ones to ensure you were sent the correct pieces, line up cams/crank on the factory marks if they're a tiny bit off, then install chains with the match-links in the correct orientation and keep slack towards the tensioner. It's all spelled out pretty clearly with pictures in the FSM if you have questions, section EM.

I've never even tried to replace chains by just eyeballing it to "make sure nothing moves" because the old chains likely HAD SLACK and the new chains will require you to make a slight correction to get everything back to OEM spec anyways. There is a high likelihood that you will end up being off by a tooth, just follow the directions and do it right the first time.

 

I think you're talking about the "spacers" that the FSM mentions and those look like they might go between the cam sprocket teeth. If you're just replacing the primary chain like I did then you can just line everything up first and replace the primary chain. Again, I'm giving a short description, but the cams stayed in place pretty much by their own chains when I took the primary one off. Then you just line the colored links on the new chain with the marks on the crank and cam sprockets. I think if you're taking the secondary chains off then you need to worry more about the cams possibly slipping on their own, I would lock the crank in place if I had to do it again, because that is what caused mine to move in the first place (trying to free up some slack). I'd feel pretty safe leaving the secondary chains in place, doing the job, and then rotating by hand afterwards to make sure eveything is smooth.

I bought all my parts from a Nissan dealership and it was about 300 dollars. The secondary chains have guides as well, but I didn't replace those.