I took mine to Autozone as I OVERTIGHTENED it (primary) and stripped it. So they had a slightly oversized plug and it fit fine. No leak (I used a bit of heat resistant thread locker and an O ring like my drain on the oil plug) for 6000 miles now.

 

The Op's primary plug doesn't have a "head" to the bolt nor a machined surface for an o-ring..... If overtightened, it won't strip threads, it might crack the case...

From about 1980 to 2003... the primary plugs were just pipe plugs, most with a magnet, like the one pic below. It's an expanding plug with a pressure fit... it may/can/will crack the case if screwed too far into the threaded hole... That's why the bikes in this age range don't have a torque spec for that plug... they have a depth spec ("tighten until it projects .160-.180" from the case").




Then in 2004ish.... Harley changed the primary plug to match the trans and oil plugs... The primary case now had a flat, machined surface around the threads so a plug with a head and an o-ring would seal against the flat surface.... The primary plug then got a torque spec for the new style plug, just like the trans & oil plugs..