rear differential 1070 Ford CS

Hi guys. I need to replace my rear differential. It is creaking and cracking when driving. I need to order a new center piece (third member) and I have found a source on ebay (powerseller with 100 per cent excellent feedback). I do not know the ratio on my original differential. I do not want to open the case or else I will not be able to drive the car anymore (unless of course I pour new oil in). Would anybody know what is the normal ratio on the rear differential on a 1970 CS? Do the serial numbers etched on the outer casing and the little metal tab indicate the ratio ? I still have the original diff in. I have found a new diff for 840 USD. Is that reasonable ? Which oil do I need to use with the new diff ? Thanks for your advice. Cheers. Patrick

 

Hi Patrick! The small tag on the diff should tell you the size of the ring and pinion and a standard ratio to that size should apply. Depending on engine options it appears the ring and pinion size ranges from 8.5", 8.7",9.0" and a rare 9.375". The ratio of the ring and pinion should be preset and match it by size.

 

1970 Ford CS differential

Thks. That is very helpful. I will go take the numbers from the tag. Does it have to be that exact ratio or could one go for one or the other (I hope this does not sound stupid) ? Patrick

 

Not stupid at all Patrick, most driveline or axle specialty shops can get you a "custom" ratio depending on what you do with your car, like high speed or highway driving or if you want an off the line ratio that gets up fast but runs higher RPM's at higher speeds. It can be custom designed to what you want. A good shop should take it from there.

Most axle housing only have a certain amount of room to accommidate the ring gear so the ring gear size generally won't interchange from housing to housing.

 

yes the tag has your ratio on it if you cannot get it take it off and wire brush it and take a pic and post it

 

You can also jack your rear wheels off the ground and put the car in neutral. Put reference marks on the drive wheel and fender with chalk. Do the same with the rear yoke and the differential case. Turn the driveshaft by hand while having someone observe the drive wheel. Count the number of revolutions it takes until the drive wheel makes one complete turn. You can extrapolate the decimal portion of the ratio using a clock system.

Example: It takes 3-1/4 turns to make a complete revolution. Your ratio would be 3.25. Or, it takes 2-3/4 turns....your ratio would be 2.75.

This can then be compared to factory offerings.

 

I am not sure about Fords that old, but my 78 and up Fords have had a data tag on the driver's door frame. The bottom right number/letter is the ratio for the back axle and whether it has limited slip or not. Chances are you are looking at a Ford 9 inch axle, and that is a very simple one to get parts for. During the off season, a lot of off road shops can and will rebuild a Ford 9 inch for very little cash.

 

66 sad to see you have a problem just a thought but what's the condition of your wheel bearings/universal joints may give a similar sound or feeling when driving from Downunder

 

1970 differential

Thanks guys. Very helpful indeed. The diff case has numbers etched in it and I have the metal tag. So I will go take pictures and post them. You guys are really helpful. Thanks bunches. I know nothing about mechanics so I kind of depend on my mechanic who is a good guy though. But he speaks no English so I am kind of lost with the technical terminology I need to look up when I look for and order parts in the USA over the www. That is why I chose to sign up. Plus I love the wagon talk here !! :2_thumbs_up_-_anima Patrick

 

I think you should have an accurate diagnosis made before you spend a lot of money on a whole 3rd member. It sounds like you have bad universal joints rather than a bad differential.

There really isn't much to go bad inside of a differential other than the bearings go bad and it is far cheaper to replace them than to buy a whole unit.

Spending money on a bit of gear oil far outweighs buying a new unit. Drop the driveshaft and inspect the universal joints. If they are good, then pull the pig, inspect it, and inspect the axle bearings at the same time.

 

thought about that after my last post too

 

Creaking is almost always U-joints. They are about $60 for the pair, plus shipping in your case. They are not difficult at all to do. If they haven't been done in living memory, it would be a good idea to do them then the driveshaft is out anyway. It sounds like that is the first thing to check carefully, anyway. Some U-joints have grease nipples on them. If they have not been greased in a long time, they will creak and groan to beat the band. The heavy duty units do not have grease fittings normally as the hole for the nipple weakens the body of the unit.

 

1970 Ford CS differential or not ?

U joints ? Hmm, seems I have to take the car back in and strat anew. When the car drives the noise comes from the rear and it makes a rotary motion. So it seems to have to do with a part that is in a rotary motion, grinding, creaking and every once in a while, after so many wheel rotations, it seems to be making a cracking noise. Definitely a metal like noise. We thought it had to to with the rear drum brakes but we checked the brakes and the wheel bearings. All that checked out ok. We have not opened the diff case fearing then that the car would remain immobilized. Right now it sits in my garage while I determine the next step. Maybe I will take it to another mechanic for a second opinion. We are talking indeed at least 1200 USD, shipping to Europe and taxes included. I know I am asking the impossible here with you guys sitting thousands of miles away and scratching your heads. Thanks again. But I like this car. It has grown on me. I like its agressive front (continued from the 68 model) and the body, the interior and the engine are in such nice condition that I will put money in it to take care of it but if I can avoid spending my money on the wrong things that would be really stupid. Patrick

 

actually checking the U-joints is easy simply jack the rear up and support with jack stands. leave it in neutral and have someone move the tire back and forth. it they are making THAT much noise it will be easy to see it. put in park and grab the shaft and move back and forth and watch for play between the yokes and the shaft and side to side movement. any of the above the u-joints are bad

 

It is not uncommon for the u-joints on these old Fords to last a LONG time. I've had them well over 30 years old, but then again, they have started the squeaking, creaking, crackling sounds after a few years of normal use. There is every chance that yours are the original 40-some year old units. Since the factory ones do not have grease fittings, it is a certain bet that the factory grease in them has dried out by now. If they are not gone now, they will be very soon. Since the drive shaft has to come out anyway, I would plan on replacing both the front and rear u-joints whether they are the issue now or not. My guess is that they are. I am betting that it started with a high pitched squeak as you were reversing, and it has progressed from there. If you have long arms, or are fairly slim, you can slide under there and grab the driveshaft without even lifting the car. With the car in park, and the wheels on the ground, any movement, in any direction, means your u-joints are done. At least they are inexpensive and easy to do.