The first thing to do is to check for
cracks in the cases by chasing down the source of fluid leaks.
Unless you're doing a complete teardown of the motor, avoid the temptation to fuck (in any way, shape, or form) with
ANY of the drain plugs in the cases.
Factory tooling was badly worn by the end of the ironhead Sportster production run & the metallurgy of the period wasn't that great to begin with.
Screwing & unscrewing steel drain plugs in & out of much-softer aluminum is an exercise in masochism that will cost you dearly when one of those drain-plug holes strips it's threads out & you're forced to Mickey Mouse it by going to an over-size plug, which is guaranteed to never seal properly & eternally mock you with a constant drip-drip-drip of 60-weight.
At that point, there won't be a large-enough hammer in the world to beat yourself in the head with for having gone down that particular route.
Instead --- drain the oil out of your tank & primary,
thoroughly de-grease the surrounding areas as much as possible & then seal those fuckers shut by smearing a good-sized bead of Honda/Yama/Kawa/Suzuki-Bond all around the plugs.
It'll be completely unnecessary to pull any of those plugs to drain & change either the motor oil or trans/primary fluid ever again.
Also check your rear motor mount, it's cracked on the majority of IHs out there.
Invest in a solid Pingel unit & watch your vibrations go
WAY down.
My front end used to dance
The Macarena at stoplights before I made the changeover.
Want to really make a major upgrade & eliminate one of the biggest pain-in-the-ass maintenance tasks?
Convert to a hydraulic clutch (
http://rjarrell77.wix.com/hyrdroclutch ) so you can shitcan the clutch cable and the dreaded/hated/despised balls & ramp clutch-release mechanism.
Since it screws into your primary cover in place of the clutch-adjustment plug, you can now yank the primary cover on a more frequent basis (this is a
GOOD thing) to check the primary chain condition, chain-adjustment shoe, etc. without dreading the clutch re-adjustment afterwards.
It's a set it & forget it deal.