timing setting opening thread

Advice please.
In order to remove the green plastic screw-in cover for adjusting the timing on my car I had to chisel the plastic cover out as it was impossible to remove it otherwise.
In doing so, I have scored the metal threads on the gearbox and the new green plastic cover is reluctant to screw in.
Any advice on how to re-tap the thread and where i can find a tap the correct size, or any tips on how to get the new green plastic cover to fit would be appreciated.
Thank you,
Jim.

You should of posted the question before going Nuclear on it.

I would order a new green gear then take it to a machine shop to measure it to purchase a Die to chase it out.

What I did was to use a Dull Straight blade screw driver and tap the edge of the nut next to the tranny case, I onlt used my 4oz Ball peen hammer and it started to spin.
This was way before the interweb was invented for the masses.  

On my 90 5 speed I used a piece of PVC from a home store.  glued in a length of pipe  so I could get a pipe wrench on it.  Worked like a champ



Can't remember the exact size but IIRC it is @ 27mm

It's a very big thread to tap out. I think you might have to mess about with some needle files to tidy it up. When you fit the new one, use a lot of copper grease.

I bought one of these 27mm hex key sockets, arguably a bit over the top for a low torque plastic plug, but it made a seemingly impossible job possible. The plug was still very reluctant to come out first time, and finding the thread to reinsert without cross-threading is still fiddly:

https://www.ebay.co.uk/itm/181184985003

Are you sure the thread is damaged? Given it is aluminium, you might be able to get rid of minor nicks in the thread with a fine pointed tool, as paceman suggests. Rotating the plug slowly anti-clockwise to the exact point at which the thread ends "click" past each other before slowly rotating clockwise should help to reduce the risk of cross-threading. Because the thread is so fine, a constant slight downward pressure on the plug while you are rotating it will also help.

many thanks for the advice folks, I will try to refresh the threads and hope that works as I already have the new green replacement gear cover.
regards,
Jim.

Careful with filing, easy to take off too much if you're not careful.

Many thanks I will take care,
Jim.