Recently overhauling bike, put on new chain and somehow just notice that the lower sprocket of the rear derailleur is not fully engaging with the chain.

It is catching the end of the links and not the middle resulting in variations in the chain and a grinding noise.

I thought my chain was the right length but I dont know now. Its like it needs to be 2mm shorter if that makes any sense. or my derailleur is screwed up.

what should i do, thanks.

also note, cassette is new, chainrings are not, derailleur is also not but no broken teeth or anything like that.

Have you ensured that your RD is tuned-in since replacing all these components?

It might be a question of cable tension and/or position of the High stop screw...

Have a read of this.

2mm wouldn't make any difference to a chain that's in the right length ballpark.

The chain should be able to loop around the large chainring, and largest rear sprocket (missing out the RD) with 2 full links overlap.

The RD pulleys also wear, and become smaller in diameter. That will mean that eventually the pitch won't match, same as with sprockets, and you'll possibly feel some roughness, especially if the pulley bearing is at all sticky.

Start by pulling the chain away from the pulley and seeing if it spins free. If not, remove it (easy enough with an allen wrench) then clean and oil it and also check it's overall condition, If it's visibly worn expect that it still may not run smoothly.

There's an entirely different possibility, which is that the RD is out of line, and the chain coming from too much angle. This makes it climb up the side then slip back in, much the same way a clothesline does if the pulley isn't on line.

thanks, I spent a long time getting it in line, Its not climbing up the side and I dont think it is that worn. I will try adjusting the hi setting, if not possibly a new g pulley,