How about replacing the plug itself. It's a tapered plug and inserting it too far can damage the threads and even crack the cover not to mention contacting the clutch basket. It's not a major job to retap it and use the newer type plug with the "O" ring on it either. Many choices here for this including a thread sealant on the plug itself.
Take a look here.
Primary Chain Case Servicing - Harley Davidson Community