Why does My oil cap keep popping off when I rab my 883 sportster up

SOURCE: 2003 883 sportster, how do you drain the engine

warm the bike up ,then if you look in between the rear wheel and the engine case on the left hand side of the bike (if you are sat on it ) on the underside you will see a black plate (part of the frame ) and it will have a black hose on it secured by a jubilee clip , remove the jubilee clip place a oil drain pan underneath it and pull the hose away from the frame and the engine oil will drain out .
hope this helps
kind regards
bert3100

SOURCE: how to drain oil from 2001 harley sportster

The drain hose comes down the back of the frame and looks like it attaches to the frame.

SOURCE: I have just got a 2005 883 hd sportster please

The drain hose attaches to the frame at the bottom rear. Make sure the bike is at running temp before draining the oil. I use 20-50 mobile 1 full syn, 3 quarts. You must put 4 ozs in the filter to prime the system. After you drain the oil make sure you look in the oil tank before you refill it there is a tendency not to drain all of the old oil. If it dosen't all drain use a turkey baster to **** out as much of the old oil as you can. When you refill the tank you want the oil to come about midway between the lines on the dip stick. If you fill it to the top line you will get a lot of blow by out of your air filter.

SOURCE: where is oil drain plug on Harlry Davidson 1995

Bottom infron of centre kick stand

SOURCE: Not just changing oil, but is there gear oil to

Yes, there is oil in primary and transmission of your Sporty. To drain the oil, look for the drain plug on bottom of rear section of the primary area, back near the large round section of the primary. Drain the oil here and replace the plug. Add the oil at the top primary chain inspection port, large plug with a large "screwdriver" slot or two small bolts in it. Add 32-36 ounces of H-D "primary lubricant for Sportsters".