Ok. It may be buried somewhere within this thread (pages 20 and 21 ) or even repeated somewhere else on the forum.
So doing this from memory, I'll may have to or someone else can correct me if my sizes are wrong.
I bought a set of taps and drills for the Aristo screw thread size 2mm with a 0.4 pitch and the drill was a 1.75mm. I would need to look back to confirm my supplier source, but I bought 3 sets of drill and taps.
I still have two, one was given to a fellow member of this forum who was looking to do the same as me.
So my method is to set up a crude jig for the hole drilling, and it is as basic as this...
...by aiming for the ctr of the oval with the drill bit and going for it, up to now it has worked out quite well, but you may need to change the fishplate after a while. I also had/have a reverse jig for the other hole as it speeds up operations when doing a larger batch. Otherwise you would keep having to turn the rail over to get the other end as it were.
The tapping is done by slow speed drilling, as my battery drill has a slow speed setting, and with a bit of forward and reverse to get to the full tapping, as you would by hand, but just taking it slow I can get away without that.
One thing to watch out for though not all fishplates of this type are the same, Aristo (Difficult to get hold of), USA Trains and Bachmann all have their variances in size and the expansion oval slot, and indeed some of the same makers fishplates can be different lengths, so in essence one jig does not fit all.
For me it started out as a belt and braces fix for lecy continuity, and up to now it has worked. I only have three feeds to the line, one in the shed and the other two are in the station area, yes I have jumper connections between the tracks as it is DCC, in the shed, the station area, the phase one passing loop and at the start of the phase two loop, which is yet to be finished, so is in effect two dead end spurs for now.
Still use copper grease when fishplating up and I have also invested in screws, whilst they have a different size key head they are the same dia and pitch.
I also use rail clamps on pointwork and the lift out bridges for ease of access for maintenance and access.
Edit, remembered my supplier...
Full range of left hand, special thread and insert tapsDrills, reamers, endmills, chasers, saws, milling cutters, carbide tools and more…
www.tracytools.com