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Old 03-01-2003, 03:14 PM
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Join Date: Apr 2002
Location: Dallas Area
Posts: 369
w124 diff oil change - help please


I set out the change my diff. oil ('86 300E, 77,000 miles) and as mentioned in an earlier post tried to remove the filler plug first.

My friend and I have been trying to remove it for the past couple of hours and can't do so.

One of us held the allen wrench and the other hit it with a mallet (rubber) and the plug doesn't seem to budge. I have soaked it with WD 40 and am waiting for a while to tackle it again.

I am looking for any and all inputs as to how to get the filler plug out.

By any chance is it a left hand thread. Or if the previous owner/mechanic have cross threaded it in, would it be really hard to get it out. Or if the diff oil hasn't been changed at all, since the car left the factory, would it be hard to get the filler plug out.

Thanks to all in advance.
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Old 03-01-2003, 04:22 PM
J.HIDALGO's Avatar
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Join Date: Jul 1999
Location: Jax, FL
Posts: 1,785
I had to use a big breaker bar and a lot of sweat!

Regular thread.
Good luck!
'86 300E
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Old 03-01-2003, 04:36 PM
dmorrison's Avatar
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Join Date: Dec 2002
Location: Colleyville, Texas
Posts: 2,694
Go to Harbor Freight

And get the breaker bar

Get the allen sockets

Check to see if they have the size you need. And really get into the breaker bar. They can really be stuck. MAKE SURE YOU RELEASE THE FILL PLUG FIRST. You don't want to drain everthing and then find out you can't fill it.

I could also recommend the 500 FtLb impact wrench they have, but do you have a compressor and would the impact wrench fit into the available space???

Also consider the breaker bar ( or allen wrench) and a piece of black pipe that will go over the BB/allen wrench that would extend the leverage and help you.

Again loosen the fill plug first

1970 220D, owned 1980-1990
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1986 300SDL, owned 1993-2004
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1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
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1965 Mustang Fastback Mileage Unknown(My sons)
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Old 03-01-2003, 08:52 PM
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Join Date: Dec 2001
Posts: 565
Good advise in previous posts about using a LONG breaker bar.

One other thing that may help would be to back the car up on ramps if you haven't already done so. Now you'll have more clearance to get an even longer bar on it or attach a length of pipe to the end of your existing breaker bar to create this longer bar. The longer the bar, the better.
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Old 03-01-2003, 10:00 PM
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Join Date: Jul 2002
Location: S. Texas
Posts: 1,237
Before you hurt your self try this. Get a 1/2 in. drive socket that is the diameter of the drain plug. Not bigger since you don't want to hit the edge of the diff. housing. Grab the socket with a pair of Channel locks or similar pliers. Get a 2 lb. steel shop hammer. Get in a position where you can get a good, direct hit on the diff. housing, i.e., car on jack stands. Make sure that the hammer face can hit the back of the socket squarely. Get your friend to hold the socket with the pliers so that it is flat against the plug. Hit the socket against the plug one or two good hits, no more. I live near the water in S. Texas and deal with frozen fastners of all kinds on a daily basis in my job. This will compress the plug slightly causing a sort of shock wave that usually will loosen any fastner that is going to come off. Next try the allen wrench. Make sure the wrench is the correct size. A millimeter too small and you could strip out the hole and then you are in for big money. For a cheater pipe go to yor local hardware or plumbing store and get them to cut you a 12 in. piece of 3/4 to 1 in. pipe (black iron is best, but galanized will do). Attn. Be careful when you pull on the pipe to brace your foot against the frame of the car. You can pull a car off the stands easier than you think. Falling all this, you may have to dynamite.
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Old 03-01-2003, 11:21 PM
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Join Date: Dec 2001
Posts: 565
One may pull a car off stands, but not off ramps as I suggested. There's a difference.

The reason so many people have trouble with the 14mm rear diff plug is because they do not create any working room. That's what a pair of ramps will give you.

Shop techs have the car up in the air. Lots of room. Ramps simulate that situation.
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Old 03-02-2003, 01:31 AM
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Join Date: Apr 2002
Location: Dallas Area
Posts: 369
Thank you all for the feedback. I'll give it a shot tomorrow and let you know how it goes.

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Old 03-03-2003, 12:39 PM
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Join Date: May 2002
Location: Mass
Posts: 1,127
If you are using an L-shaped hex key, I second the suggestion of using an appropriate length of steel pipe for more leverage.

If I remember correctly, there may be clearance issues using a hex-key socket.

Teflon tape will prevent this problem the next time.
1986 300E 5-Speed 240k mi.
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Old 03-03-2003, 01:26 PM
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Join Date: Apr 2002
Location: Dallas Area
Posts: 369

I sucessfully changed the oil in my differential ('86 300 E).

Thank you all for your suggestions in helping me change my differential fluid. I sincerely appreciate it. This forum rocks.

This is what my friend & I did:

1. Used a 14 mm (regular) allen wrench. One of us held it in the hex hole on the filler plug while the other used a mallet to strike the free end of the allen wrench. The after a few blows, the plug loosened & came out.

2. We repeated the procedure for the drain plug too.

3. Drained the oil and installed the drain plug.

4. Filled the differential with Redline synthetic gear oil (75W90) using a pump. We inserted the tube that comes out of the pump into the fill hole and placed the the pump behind the rear axle (the tube that came with the pump was long enough). There was enough clearence there and in less than 5 minutes, we had the differential filled. (After pumping some of the fluid in, I cut the neck portion of the bottle and so the opening was wide enough for the pump's stem to go all the way in. After the first bottle was empty, I poured some oil from the second bottle into the cut bottle and pumped away).

5. Installed the filler plug.

6. Then, we drove the vehicle's rear wheels onto ramps. This gave us the required clearence and we were able to torque the plugs in.

Once again, thank you all.
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