See also in the Sportsterpedia;
….. *57-85 Oil Line Routing
….. *Pics and Information on stock IH Battery Trays and Parts

Click on any pic below to enlarge:

1957-1966 tanks had a separate bracket (47371-52) to attach it to the frame.

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From 1967-1978, the bracket was spot welded to the tank.

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There have been negative comments about in-tank oil filters in the past, but these all refer to an old, original “horse hair” filter. ¹⁰⁾
The modern replacements are very good indeed.
But you'll need to shorten your dipstick to use the newer in-tank filters unless you find a “Horsehair” type. ¹¹⁾
The newer pleated paper ones are a better choice though.
See the example below of shortening the dipstick.

Uncut dipstick. 12)	After cutting. 13)	Roll pin goes thru the “top” (smaller) hole. 14)

These pics show how the oil cap comes apart.
Here is what you have when you flip it over and clean it up.

With the C-clip removed, hold the round end that the dipstick comes out of.
Turn the wing nut counter clockwise to unscrew it from the rest of the assembly.
The lower section will slide to the side and then lift out one side at a time.
This is not necessary to replace the gasket or the O-ring but it may be for something more in depth.

Right against the cap you have a C-clip. Pull it out.	Unscrew the dipstick from the cap.

There is an O-ring at the top of the shaft between the cap and the wing nut.
If it's flattened, it may cause an oil leak on the top of my tank.

Check the O-ring.	This one was replaced with one from an Evo head rebuild (bigger O-ring). The inside diameter was a little big but it fit inside the groove and was thick enough to seat against the cap.