Hi, T20 1963 new to me, removed outer engine casing on both sides but two bolts won’t refit on the right hand cover side. Would like to keep with the original bolts, I would be very grateful if anyone can recommend the correct helicoils I should be using?

Look on Ebay for the replacement bolt sets.  That should tell you the thread size.  Failing that, phone Meriden Off Road, UPB or Greystone.

 

I had a 'well used' set of Crank Cases which I tapped the small bolts to M5 and Heli-coiled the larger ones to M8 as most were stripped and I already had metric inserts and bolts bolts in my shed. 

 

However, if only a couple are stripped I can understand your desire to keep original.

 

Hope this helps.

 

Bruce.