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Discussion Starter #1
I could use an educated eye here. I recently got a '66 coupe, 289, Carter AFB on an Edelbrock Performance RPM, T10 4-speed with Hurst shifter. A lot of work on this so far but she runs good. I'm no expert but am pretty good mechanically (58 yrs old, grew up tinkering) I want to replace the gear oil in the rear end but I don't know exactly how. I've seen videos and descriptions for unbolting bolts around the casing and cracking it to let the fluid drain out, etc. But this rear end doesn't seem to have that. And I have no idea what size it is. I live in a small town area and there are no genuine Mustang "experts" locally who I'd trust. Any "serious" thoughts/advice? And what on earth - does that look like a modified leaf spring?
 

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You have 2 choices to drain the gear oil. 1) Get a suction gun or pump and stick a hose in through that screw-in plug and suck it out. 2) Remove the entire differential from the front of the housing and let the oil run out. You will need to remove the driveshaft, both axles and about 10 or 12 retaining nuts to pull the differential. You will need a new gasket to put it back together.

That leaf spring is a "helper" spring to help the original worn out leaf springs to keep the car from sagging in the rear.
 

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Dude, have your wife or a doctor do it.
 

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I'm not a proctologist but I'll have a peek. ?
Easiest way as mentioned, get a fluid pump from your local auto parts store and just suck the old fluid out through the fill hole.
 
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You can either suck it out or drill a hole at the bottom and tap it/install a drain plug, that way it will be easier next time.
 

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If you do not have the standard drain plug on the side,remove the one on the pumpkin 3/8 or 1/2 drive, buy you a cheap hand pump at auto store or on line, and pump out.
 

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I've seen videos and descriptions for unbolting bolts around the casing and cracking it to let the fluid drain out, etc. But this rear end doesn't seem to have that. And I have no idea what size it is. I live in a small town area and there are no genuine Mustang "experts" locally who I'd trust. Any "serious" thoughts/advice? And what on earth - does that look like a modified leaf spring?
OK,

a) Anyone advising your to crack open you axle to drain the fluid is an idiot, and should be ignored. Apart from risking serious damage, it doesn't get all the oil out. Professionals use a suction device. I've use cheap ones and expensive ones, both work.

b) Those helper leafs were popular in the 70's for sagging springs. I was throwing away a set once, and just to find out I bolted them onto my car and measured the difference. There wasn't any. In a word, they don't work.
 

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Wow- harsh... an idiot?!? Alan merely pointed out that there are 2 options- suck it out, or disassemble it. Since he said suck it out first, I'd take it that's his recommended route. Either way, he's correct- right? Technically there is a third he failed to mention- jack up the front end high enough to let the fluid drain out...….
 

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OK, maybe "idiot" was a bit harsh (I was thinking of the videos he referred to), but seriously, loosening the differential from the housing, risking getting debris from the flange gasket inside, plus the fact that you can't get the last inch of oil out that way. Of course, to do that you need to remove the axles, and the differential, and replace the differential gasket, and the copper stud washers.

Or you could just spend $14 at Harbor Freight and do the whole job in an hour.

752384
 
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(I was thinking of the videos he referred to), but seriously, loosening the differential from the housing, risking getting debris from the flange gasket inside, plus the fact that you can't get the last inch of oil out that way.

View attachment 752384
Which videos did I refer to?
And if the OP removes the entire differential as I explained he can easily "mop" the last bit of gear oil out of the housing with a rag or paper towels.

OK, I reread the OP's post and HE referred to videos that he watched where somebody "cracked" the rear end housing to drain the fluid. Obviously he was watching videos that showed a Mustang with a 7.25" or 8.8" rear end gear oil change.
 

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The least could at least buy a guy a drink first...

Or you could just spend $14 at Harbor Freight and do the whole job in an hour.
View attachment 752384
Once you have one of these pumps in your garage, you will be shocked at all the jobs that get easy. That said, I personally use a drill pump that fits on a drill (I have a SHUFflo I think) so you don't have to pull to inhale, pull it out and then exhale into something all while under the car making a mess. With the drill pump, You can just stick the tube in and run the drill and Bob's your Uncle. Get the pump then head to lowes and get all the hardware: 2 garden hose to nipples and several sizes of tubes for the intake side. It takes a rather stiff plastic tube to really get to teh bottom. I use black tape around one end of the smaller tube and stuff it into the larger tube to go smaller diameter. Hi Tech but it works... BTW, hook the output hose to the input for storage, they leak in the tool box.

Run the car and got the diff good and hot so the oil thins, it will pump easier. OR take it to a garage and let them do it, they have a good machine. Sometimes its just worth it to pay the man. Use good oil as it will be there for 10 years.

Don't forget the additive if you have clutches.
 

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Discussion Starter #18
Folks, I really do appreciate your advice; that's very helpful. Bottom Line Up Front (BLUF): Can y'all recommend the proper gear oil for the rear end in this case? It's not easy, around here, to find a good mechanic who won't break something while fixing the thing you needed fixed (the good ones retired last year). So, I rely on myself for changing oil and other normal things that don't require major repair. And this car is relatively easy for me to work on. I replaced the oil in my T-10 transmission with Sta-Lube SAE 85W90 Hypoid GL-4 gear oil (someone in another string or forum recommended it and others seconded his recommendation, so I went with that). When I drained the tranny, the old oil came out opaque green. But got-r done and all works fine.
 

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Discussion Starter #19
The least could at least buy a guy a drink first...



Once you have one of these pumps in your garage, you will be shocked at all the jobs that get easy. That said, I personally use a drill pump that fits on a drill (I have a SHUFflo I think) so you don't have to pull to inhale, pull it out and then exhale into something all while under the car making a mess. With the drill pump, You can just stick the tube in and run the drill and Bob's your Uncle. Get the pump then head to lowes and get all the hardware: 2 garden hose to nipples and several sizes of tubes for the intake side. It takes a rather stiff plastic tube to really get to teh bottom. I use black tape around one end of the smaller tube and stuff it into the larger tube to go smaller diameter. Hi Tech but it works... BTW, hook the output hose to the input for storage, they leak in the tool box.

Run the car and got the diff good and hot so the oil thins, it will pump easier. OR take it to a garage and let them do it, they have a good machine. Sometimes its just worth it to pay the man. Use good oil as it will be there for 10 years.

Don't forget the additive if you have clutches.
Love the video! I have a bit of suspension work to do on mine before I can ever do something like that... She wanders a bit, and the steering isn't tight, so I have work to do.
 

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Discussion Starter #20
I was wondering why he wanted to replace the gear oil.
Tom991, I don't know when the gear oil was replaced before I got the car. And, I just don't know how often, or not, it should be changed, so I figured I'd be on the "safe" side. Don't know if that's right or wrong, so that's why I'm asking. Not sure the previous owner was just good a painting only - the car had coolant and oil leaks, so I replaced the timing cover, water pump, valve cover gaskets, plugs, radiator, hoses. All leaks gone, except for the oil pan (parts on the way - and I gotta figure out how, exactly, to remove the appropriate linkages to get it out)... lots of fun...
 
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