You could also look up some of the threads on this forum by David George who has made a really good job of restoring and using an ML4.

I’m going to disagree about big shanks in this case.

With boring bars a big shank is often an advantage as they tend to be used with a lot more overhang than an external turning tool. Having said that you will also need a small one if boring a small hole where the tool won’t fit if the shank is too large. Over the years I have built up a range from 3mm to 28mm shank.

I certainly would not want to bore a 2″ dia hole 6″ deep with a 1/4″ shank boring bar.

External turning generally has little overhang so smaller is OK, easier to grind small section HSS or small shank Insert holders use smallerand therefor cheaper inserts. 6mm or 1/4″ would be OK for most things external. Parting tools I would go for a larger shank on as rigidity will help with those sort of cuts.

You need a selection of boring bars to cover the range of hole sizes, but they are easy to make in larger sizes.  I suggest you them as you need them for a project.

The tool height on a boring bar for exchangeable tips are on the centre height i.e. centre of the shank diameter so if you drill and ream a holder using the drill in the main spindle the tip on a boring or screw cutting bar will automatically be on centre height. Recently I needed to bore and internal thread a 1.250″ thread and the thread insert and the smallest shank diameter is 20mm  so I made a solid block to replace the tool post drilled and reamed a hole using the spindle which makes it perfect for larger jobs as well with a larger boring bar as well the clamp screws on top hold the top clamp the boring bar strait and flat. It only takes a couple of minutes to swap the tool posts on my lathe or if you like put it further along the top slide to use it.

David

As a footnote to this topic its worth keeping an eye put for a couple or four “suitable” sizes of large MT shank drills at an affordable price on E-Bay, Facebook Marketplace et al to make starter holes. Drilling is the quickest way to remove material so getting somewhere close with a drill before finishing off with the boring tool is worthwhile saving in time and tool wear. Working your way out from the largest box set drill that can be held in the usual tailstock chuck  (1/2″ or 13.5 mm  Ø) to an inch or two can be a miserable experience.

Exact size of your big drills doesn’t matter so frequently bargains can be found in odd sizes that won’t fit anything specific. Fractional inch in by 1/64 th sizes for example.

In smaller sizes a broken slot drill (2 flutes) that still has one good edge can quickly be turned into a decent small boring tool with a bit of attention from the grinder to make clearance on the damaged side. One day i shall take my own advice and make a holder or two for the broken slot drills I’m religiously saving!

Clive

The easy solution is to make your own

The advantage of using the largest possible size shank boring bar is rigidity.

But the larger the shank the larger minimum size hole into which the bar can be inserted for machining.

So a 6 mm shank bar, will probably need a 9 mm drilling into which it will fit to start boring operations.

The heavier the cut, the greater the deflections, so more “spring cuts” will be needed to reach finished size.

Probably, for a ML4, a 6 mm bar will be adequate. A 8 mm largest, and 10 mm might even be too large

Smaller that 6 mm, will probably call for a lot of grinding of a HSS toolbit, to thin one side to allow entry into a smaller hole, and greater need for small cuts and spring cuts.