I recently was given a job where I need to bore a 1.25" diamter hole 5" deep in 6061 aluminum. There is a pre-existing 1" hole drilled in the material. Work is being done on a Femco Durga lathe with a 3/4" carbide boring bar sticking out 5.25".

I have been using an A-MWLNR 12-3W Iscar boring bar with WNMG 3-1-PP IC3028 inserts (Iscar recommended insert) and there is no problem going 3" deep with this setup. I was "gifted" an Ultra-Dex E12S MWLNR3 boring bar so I figured it was pretty much the same insert. But I can't even cut .1" deep without serious chatter. I have tried all sorts of feed and speed settings but nothing seems to work. I went to look up the manufacturer's specs on this bar and found they recommend a WNMG332 insert. Could the .031" difference in insert thickness be the culprit? Could this be putting the inseret tip below centerline?

Also, any speed/feed help would also be appreciated since I have never used a carbide bar before.