I shortened my MT4 tool holders 40 mm on the MT4 end. But because they had a 10mm depth thread relief , I only needed to make a draw bar 30mm longer. This will let me change the tooling out with the need in most cases of raising and lowering the head or changing position to change a tool Im very happy it. Hindsight say's I should have as not had the relief at the top of the holders like I copied. The only holder I did not change was the precision drill chuck with an integrated MT4 to the chuck body.

Its the angle of the photo and the lens making an illusion. They are MT4 taper.

I used these inserts, picure in my album and on the recent insert thread. TNGG160402R-S PR1125 (Wonder Insert) One hard steel Iuse it at 25 to 30 M/min surface speed,slow feed like 2 thou per rev or so. For the Drill chuck, I put a 10mm dowel in the chuck, and did the chuck up tight with the C spanner that comes with them, normally for undoing the chucks. Anyway I held onto the dowel, and used the centre to support it. Then just nibbled away at them at about 200 rpm and taking about 0.4mm cuts(0.8mm diameter) till I got down to about 17mm diameter. Then used a hacksaw to cut off the excess, and cleaned up the end face with small cuts. Then re tapped the M16 deeper. The other holders we held in the reverse jaws. They were hardish on the outside, but soft under the 3mm or so skin thickness. Once through the hard skin, could up the rpm a bit. I should not have had the parallel part where the M16 thread is, I should have just round off the end. The Drill arbour did have already the M16 thread in it. It was not a tanged MT4.