Believe it or not Harley Big Twins once had crankcase drain plugs. Others, though not all, still can. Fables abound regarding why H-D quit with the plugs. One is AMF was too cheap. Another that some frames came too close to the case for a plug to be used. Maybe it’s both. Maybe some other reason entirely. Whatever.
Tip #1—Check to see if yours has the boss for a plug cast into the original cases. (Any Evo that got replacement cases with no steel bearing insert won’t have the boss.) See figure 1.
Tip #2—As long as you can get at the spot on the case, you don’t have to wait until you tear the motor down to add a drain plug. Use an aircraft fuel tank drain of that size, or one for an air compressor. See figure 2.
Tip #3—Carefully drill and tap ¼” NPT (0.405 diameter) Use grease on the bit and tap, to catch shavings… clean, clean, clean as you go. Then, squirt brake cleaner in the new hole… copiously! Remove the spark plugs, use kickstand, and crank the engine with the starter. Clean again, repeat as/if necessary until satisfied you got it clean. Install all plugs, top off oil, run engine briefly, check for leaks, shut off engine, check/clean tappet screen as required. Go ride.