Where is the oil drain plug on the bottom of engine for 1972 Harley Davidson Sportster?

Ironhead drain plug

The drain plug for the engine oil is on the right side (when sitting in the saddle) underneath and close to the front.

You should drain the engine oil from the oil tank which is located on the right side of bike. you should never remove any plugs from your engine cases except for the transmission. How ever in a bike this age I find removing the primary the best way to drain trans and primary oil, because the primary chain will probably need adjusting...