4.3 mm holes, 20 mm deep

[Lee Roberts]

So your going to be doing some tapping?

I would just spot drill (good them osborn/europa), then open them up as normal i.e jobber/twist drill bit, I've been using these Heller HSS German made bits pretty much exclusively recently without fail.

They do HSS-R and HSS-G versions, R is for non alloyed steels and G is aimed at alloyed, the G have a 135 degree split point for optimum centering and dimensional accuracy.

You can find them all over eBay if you search, I've even got their 15 and 20mm "black smith" type to open up clearance holes for my leadscrews...they worked sweet as a nut!

I wouldn't go anything less than the 4.3 you need, use the 4.3 for the 5.3 as well so you've got a nice pilot hole you can then open up, use plenty of cutting oil and take your time, dont forget to give the holes a small chamfer to help the tap locate :)