Differential in my 75 what is it?

[Devildog]

Hi all,

I have a loud whine in my differential between 40-60 mph, also some slack in the gears when it shifts. Car has 160,000 miles and I believe this is original differential. Not unuseable, but I want to rebuild. To have my shop order some parts, any comments about what this unit is.

Dana 44...would that be correct?

I think it is a 3.7 : ratio...sound right? (I tried rotation wheel vs shaft revolution, but the limit slip is worn out).

Build sheet says it was originally a limited slip.

Any thing unusual about the bearing at the wheels, etc.?

Thank again

Joe

[Avanti83]

Joe

Your diff has been opened at on time I'll bet as the tags are missing from the bolts. Based on that, We can't be sure what's inside.

My 74 came with a 3:23 ration although it is a four-speed car. In the emissions era 3:70 was unusual. To find out you could either pull the cover and count the teeth or just drive it at 60 MPH and note the RPM. Be sure to use a GPS for speed. Just Google calculate rear ratio, you'll find a lot of calculators that will ask for that data and rear tire diameter and give the ratio.

In any case, the gears are the cheapest thing to change. If you like the ratio you have, just replace it. A set of Richmond gears is $150 or so. The posi is about $350 - 400 if you use an Auburn unit like I did in my 83. The bearing and seals are in the $125 range so that will give you a parts cost for the differential. All pretty standard 44 parts. So you either use aftermarket parts Like I did or go to Avanti guys like Nostalgic Motors.

The axle bearing and seals at the backing plates get interesting. When I tore my 74 rear end apart it was a 6-bolt configuration not the expected 4-bolt setup at the axle flanges. It's not a big deal but I had to order a 1982 or so IH Scout II set of bearings and seals. Less than $100. They worked perfect.

That should get you started on the correct road but another option I would suggest that you can do is just drop the Avanti off and let them tear it apart. The parts are only a day or two away and they will have the bearing and seal numbers directly of the parts.

Lastly, if you don't like the rear ratio just talk to the shop about changing it to a better number.

Bob

Edited January 7, 2013 by Avanti83

[Devildog]

Joe

Your diff has been opened at on time I'll bet as the tags are missing from the bolts. Based on that, We can't be sure what's inside.

My 74 came with a 3:23 ration although it is a four-speed car. In the emissions era 3:70 was unusual. To find out you could either pull the cover and count the teeth or just drive it at 60 MPH and note the RPM. Be sure to use a GPS for speed. Just Google calculate rear ratio, you'll find a lot of calculators that will ask for that data and rear tire diameter and give the ratio.

In any case, the gears are the cheapest thing to change. If you like the ratio you have, just replace it. A set of Richmond gears is $150 or so. The posi is about $350 - 400 if you use an Auburn unit like I did in my 83. The bearing and seals are in the $125 range so that will give you a parts cost for the differential. All pretty standard 44 parts. So you either use aftermarket parts Like I did or go to Avanti guys like Nostalgic Motors.

The axle bearing and seals at the backing plates get interesting. When I tore my 74 rear end apart it was a 6-bolt configuration not the expected 4-bolt setup at the axle flanges. It's not a big deal but I had to order a 1982 or so IH Scout II set of bearings and seals. Less than $100. They worked perfect.

That should get you started on the correct road but another option I would suggest that you can do is just drop the Avanti off and let them tear it apart. The parts are only a day or two away and they will have the bearing and seal numbers directly of the parts.

Lastly, if you don't like the rear ratio just talk to the shop about changing it to a better number.

Bob

Bob, many thanks that is a ton of good info...you are a treasure. I spoke with a good differential shop (only thing they do) east of Houston in Baytown. They made a reasonable plan, so i will let them do it and get a guarantee (good reports about their work and stand-by their work).

I am confident it is a 3:70 ratio based on road speed. With my 700R4 OD whatever my the ratio it is good. About 2400 rpm at 75-80 mph

Again, thanks for info.

Joe

[Avanti83]

Joe

Good plan.

Interesting data point though. I found the specs for 1975 and they say the standard gearing is 3:54 with 3:73 and 3:31 as optional.

The 3:70 (3:73) you have should be a great choice with the 700R4.

Bob

[Devildog]

Just to complete the question, I pulled my differential and it was a 3.3. My tach and speedo are off a little so my estimate was wrong.

However, I did install a 3.70 ration. With my 700R4 tranny's 3.05 first gear x the 3.3 yields a 10 gear ratio to launch nicely. The 700R4 is 0.7 in over drive gear which yields 70+MPH @ 2200 rpm, 80 mph@ 2400, and 100 mph @3000 based on GPS ground speed.

If someone searches in future this may help them.

Joe