'83 300SD Diff filler plug seized
 

So I'm putting on new flexx boots on my '83 300SD, and for the life of me, I cannot get that !@$% filler plug off the differential without the car coming off the jack stands, so I decided to just get the outer end of the axle off and get the boots in that way using a rather large funnel and some grease. However, since the axles cannot come off at this moment, I cannot drain all the oil that's inside completely; is it going to harm things if I mix the Moly grease I got with the oil that stays in the cans? Or what could be a method to drain the cans more effectively?
A million thanks for any help :)

This is a very common problem. I couldn't get mine off with an impact wrench and tons of PB, I'd be interested in hearing how some folks get it loose. I just left mine and let it whine away.

Have you tried soaking the filler bolt with penetrating oil, and/or heating it with a torch?

Also I always make sure I can take the filler bolt out before removing the drain bolt.

There should be some threads on you situation.

Most of the ways I have read are to Heat the Area with a Propane Torch and a Breaker Bar with a long Pipe on it.

Some have arranged it so that a Jack pushes up on the Pipe/breaker Bar and one person Jacked the Car up and arranged the Pipe/Breaker Bar so that when the let the Car Down the wight of the Car applied pressure on the Pipe/Bar.

If you have been using a big Allen Wrench and want to get a socket on it you go to the Hardwars Store and ge a Rod Coupling Nut/Coupling Nut. I believe the size you need takes a 9/16" (14mm) wrench on it.

What ever bit of grease stays on the Boot will hot hurt anything if it mixes with the Axle Oil you put in..

I took a cheap socket wrench i didn't mind breaking and used my floor jack to push up on the wrench.

Doing this used the weight of the car against the wrench to crack open the filler plug. Definitely worked in my case. It didn't take a lot of pumping of the jack to open the bolt, so be careful to not do anything stupid.

If this doesn't work for you (highly doubt it) than you can unbolt the differential cover and remove it entirely. Put it onto a bench and work against it standing up.

But try my suggestion first. That's worked for me every time.

I like the idea of using the jack, didn't thought of that... Thanks guys :D And I'll try to spray some Coca-cola and see if that helps...

The cover is aluminum.

A bit of heat with a propane torch, ON THE HOUSING AROUND THE PLUG, will expand the housing and break the threads loose.

If you lean into a wrench after you heat the housing, you'll probably fall on your face because the torque required has been reduced significantly.

You only need heat for about two minutes...........don't want to catch the oil on fire...........

Tight is not the word for that differential drain fitting sometimes. A six foot bar is not out of the question. Although some heat does help to eliminate the feeling that a piece is going to break out of the cover before the plug lets go.

I always use Kroil for stubborn bolts.

Also, it's worth restating:
remove the fill plug BEFORE removing the drain plug.

Jeff

That is exactly what I used: a 6' piece of pipe over a breaker bar. No kidding!

Yep, The way I do it is to leave the car on the ground (Ive neer done the diff on a w123 on jackstands), and use a really long extension on a breaker bar.

It is a bad design. Even using antisieze did not help...

Five Words
 

Zinc Anti-Seize.Torque Wrench.

[LOCTITE® Zinc Anti-Seize is a smooth, homogenous mixture
of zinc dust and petrolatum. It prevents seizing during
assembly or disassembly of threaded or unthreaded metals
joints, particularly if these metals joints involve aluminum or
aluminum alloys. LOCTITE® Zinc Anti-Seize also provides
corrosion protection for both aluminum and ferrous metal
components. Within a metal threaded connection, it acts like an
"internal galvanize" using the electrochemical properties of zinc
dust to protect iron surfaces. Typical applications include
aluminum and other soft metal bolts, screws, studs, pipe joints,
nuts, and plugs. It is an excellent choice for rust preventative
anti-seize applications. This product is typically
used in applications with an operating range of -29 °C to
+398 °C.]

didnt work for me...

use a 3/4 breaker bar, and a 3/4-1/2" adapter with a 14MM 1/2" drive socket and a 4' extension on that, and you will not have the flex issues a 1/2" breaker bar has. oh, and use propane, not acetylene... don't ask...

Why wouldn't you simply drop the differential, take the cover completely off, let it drain, pull the clips and completely remove the axles and reboot them on the bench. Then you can work on the filler plug on the bench also.

Sometimes it can be very difficult to get the axles out of the hub if you don't have the differential jacked up even higher than it usually is positioned on it's mount!

It would seem that working on the axles while they are still attached to the differential is a real ball breaker!