Hi Julian...I can't remember the thread on that one but even if you could source a tap at that sort of size it would be very expensive..probably over £100...If you carefully apply some paint stripper to the threaded portion it will lift the powder coat and you should be able to remove it with a small wire brush...Ian
email (option): ian@wright52.plus.com
There's a list of threads in the Technical Section, provided by one Mr Ian Wright (?) that says: "1 25/32"x 19 Non-Standard thread" 
email (option): viaconsu [at] planet [dot] nl
Hi Hans...I knew I'd written it down somewhere...
...Ian
email (option): ian@wright52.plus.com
Paint stripper it is, as that is another peculiar thread on this machine, I'm starting to understand why they had to standardise on threads over time as this bike is a right mish mash of all and any thread form available.
email (option): 79aust@sky.com
As a matter of fact, I made a tap to make the hole for the cap in the timing gear cover, which is identical with the one in the primary cover. Just ordinary mild steel, surface hardened with Kasenit, I tapped several holes with it using methylated spirits as tapping fluid. I had to correct the thread on the plug a bit with a thread file to get a good fit; an O-ring provides enough friction to prevent loosing the plug
email (option): viaconsu [at] planet [dot] nl
Hi Hans..Paraffin (Kerosine) would be the cutting fluid of choice for that job...Ian
email (option): ian@wright52.plus.com
Thanks Hans,
You made me realise that it would be far easier to turn and mill a tap up.
I have a couple of lovely little Harrison Alpha NC lathes and a few conventional mills that have been a joy to play with on this bike so far, as I'm only cleaning out powder coating I didn't really relish having to go down the paint stripper route as I'm a little worried it won't clean up all the area sufficiently without causing damage elsewhere.
A nice clean cut thread on a bit of 2" bar with a few slots on the mill will do the job proud and hopefully leave a lovely clean cut form as good as new.
Just need to confirm all sizes first and is the thread a UN or Whitworth form?
email (option): 79aust@sky.com
Thanks Hans and Ian for your support and assistance.
Managed to turn and mill my tap tonight and have just had the biggest smile on my face once I got the thread to clean out the Inspection Cap location.
I would like to advise that I think the thread for this inspection cap should be 1.13/16" not 1.25/32" as I found that 1.25/32" was a little sloppy in the hole and the cap wouldn't fit without excessive force.
The thread form is 19 TPI and 60 degrees.
The end result makes the evening of machining worth while, I had no idea this restoration could be so much fun and challenging, but so great when you get it right.
Thanks again Hans and Ian
email (option): 79aust@sky.com
My tap is not that sophisticated...
email (option): viaconsu [at] planet [dot] nl
Hans,
Your more than welcome to borrow mine anytime as it may be a little redundant now?
Sprayed it with chain oil and stored away for the next project.
Your tap has been hardened and mine is just plain mild steel, so its swings and roundabouts
LOL
email (option): 79aust@sky.com