1970 Duster base model slant 6 rear differential 2070051 spider kit

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joel erlandson

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I built a 1933 Dodge in the '80's & it has made it back to me for repairs on the rear end . Both spider gears are wrung off. This is the rear diff. considered to be junk by many but it fits in width perfectly. It is about 57.5" drum to drum. It is the 9 bolt 7.25 rear end. I bought a C7.25BI kit but 1 seller said I have to use an '80 model case for this kit to work. I don't know much about rear differ. but the seller said the case she is talking about changing is the case the pinion gear bolts to. Will the kit I bought fit the rear I have or do I have to change the case as well? If so what year, make, model cases will fit my casting #2070051? Could I use used spider gears & if so can you give me a source? Can anyone help me out here? I am in a real bind. A certified mechanic is coming tomorrow to see if the kit will fit but I'm anticipating it won't & hoping to get a response to my questions. Thank you to anyone who can help. Is there a source for NOS spider kit for a '70 Duster rear like the one I have? Thanks again. joel
 
Did the spider and side gear teeth shear off or did it take the axle splines off the side gear. Given parts availabilty, you could probably find another 7-1/4 easier than parts for one. What ratio is it? The real question here is, how much weight is there and how much power? The spiders are the weak link in these axles, and usually what shows trouble first. Finding an a body 8-1/4 is probably the best solution except it will have 5 on 4-1/2 instead of 5 on 4 bolt pattern. Here is what Randy's Worldwide has available on differential carrier interchange.
Diff Wizard | RANDYS Worldwide
 
Garrett Ellison said:
Did the spider and side gear teeth shear off or did it take the axle splines off the side gear. Given parts availabilty, you could probably find another 7-1/4 easier than parts for one. What ratio is it? The real question here is, how much weight is there and how much power? The spiders are the weak link in these axles, and usually what shows trouble first. Finding an a body 8-1/4 is probably the best solution except it will have 5 on 4-1/2 instead of 5 on 4 bolt pattern. Here is what Randy's Worldwide has available on differential carrier interchange.
Diff Wizard | RANDYS Worldwide
Click to expand...
 
When I chose this axle I was unaware of its weaknesses. The larger splined gears are ok while the smaller gears are 80% gone. It is a 293 ratio/ 3000 lb. car/ '82 Chevy V6. The unit came with the 5 on 4.5 pattern. The donor car was on a truck headed to the crusher in 1985 so I told the guy I'd give him $50 if he would pull it for me. I did not know the ratio until I pulled the cover last week. I was hoping someone had changed out a set of spiders but this site is more into power & they would have swapped out the axle for a beefier unit. That means there should be an original rear axle behind someones shop. I'll know more this Sat. when we take it apart. The axle splines appear to be ok. While it was the weakest of all their axles it was used extensively across the Chrysler line from what I can tell ending in about '74. The owner needs to get a heavier unit cut down but she says she can't afford it. I was also hoping someone might have a used set or NOS set of spider gears that I could buy if the set I bought doesn't fit. Not likely to find one local but you never know. I have a local mechanic checking with a MOPAR guy in my area. Thank you for helping me. joel PS I had talked to Randys on Mon. & he said "good luck"!
 
Members give 7-1/4 axles away fairly regularly here, but if it's a 4-1/2 bolt pattern from a '70 duster it had been changed at some point in time to a '73 up axle. The ones that ran in later models (I am wanting to say late70s or early eighties, I can't remember the exact year that ChryCo standardized the U bolts and backing plates) with 10 bolt covers had larger 3" tubes that had the inboard six or so inches swaged down to the 2-1/2 inch axle tube bore size in the center section. From what I have been able to uncover here on the interweb, AAM started making the 10 bolt cover axle to use either Dana 30 or 35 components. I'm pretty sure this is when they became a C-Clip axle as well.
On a side note, if worse comes to worse, '84-97 Jeep Cherokees ran Dana 35s. They have a width of of 43-3/4 pad to pad and a total width of 60-3/4 drum to drum, and a 2-5/8 tube diameter. They also have a 5 on 4-1/2 lug pattern. So far, this is the closest thing I have been able to find for a modern upgrade for a 7-1/4 AAM axle that could possibly use the stock spring backing plate/shock mounts with only slightly larger U bolts and ovaling the holes fore and aft. But it is not much stronger than a 7-1/4 and is only a good option if you can get it for no more than scrap price. If it could retrofit a 38-1/2 inch wide spring pad center to center distance, an S10 7-1/2 axle could be used, 2wd 54.5", 4wd 59" outside width.
 
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Circle pilot-That's an 8 hr drive for me so not out of the question. Normally I would tell the owner it was their problem but I am the builder of this car in the 80's & I feel responsible. Can you confirm the axle width drum to drum 57.5"/ spring pad ctr to ctr 43"? Have you ever had the rear cover off to know the ratio? Google "Axle identification, late 50's through early 80's" & it tells you how to determine ratio without removing the cover. I just have a feeling the kit I bought is not going to fit so that makes your offer my best bet as she can't afford to have one custom made & we just put 4 new smoothies with tires on the car. If I don't get this fixed all she has is a roller. The owner is female age 55- she inherited the car from her father & is a car gal. The car is a 1933 Dodge Rumble Seat coupe & was orig. rodded in the late 40's by a man in Mobile. I bought it in 1972 from behind a shop & rescued it so your rear end would live a long life. I am seriously considering your offer. please advise & thank you. joel If there is someone out there closer with a similar offer please notify. I am 71 I need the shortest drive I can get! But I doubt this deal can be beat & I thank Circle Pilot for the offer. joel
 
joel erlandson said:
Circle pilot-That's an 8 hr drive for me so not out of the question. Normally I would tell the owner it was their problem but I am the builder of this car in the 80's & I feel responsible. Can you confirm the axle width drum to drum 57.5"/ spring pad ctr to ctr 43"? Have you ever had the rear cover off to know the ratio? Google "Axle identification, late 50's through early 80's" & it tells you how to determine ratio without removing the cover. I just have a feeling the kit I bought is not going to fit so that makes your offer my best bet as she can't afford to have one custom made & we just put 4 new smoothies with tires on the car. If I don't get this fixed all she has is a roller. The owner is female age 55- she inherited the car from her father & is a car gal. The car is a 1933 Dodge Rumble Seat coupe & was orig. rodded in the late 40's by a man in Mobile. I bought it in 1972 from behind a shop & rescued it so your rear end would live a long life. I am seriously considering your offer. please advise & thank you. joel If there is someone out there closer with a similar offer please notify. I am 71 I need the shortest drive I can get! But I doubt this deal can be beat & I thank Circle Pilot for the offer. joel
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Circle Pilot- could you also verify the casting #? Axle ID lists 3- 2070051 (mine) / 3507881/ & 3723675. Thanks joel
 
The 7 1/4 is "ok" in its element. It's considered the "6 cyl" or "light duty" rear axle and as long as it's used in those applications, it's sufficient. It's when the rear axle is either neglected or used out of its element that it fails. Millions of Chrysler products came with them.
 
Thanks Norm. You may be my only answer. I'll know tomorrow if this kit will fit but I really have my doubts. The casting # on mine in on the bottom pass. side. I appreciate it. Joel
 
The only difference will be it has a 4 inch bolt circle diameter and the brakes I have seen on the large bolt circle 7-1/4 are usually wider but the overall wheel mount surface length has been the same. But I imagine your probably just going to use the spider and side gears from the donor. That’s the route I took unless I was changing the ratios, too.
 
Garrett Ellison said:
The only difference will be it has a 4 inch bolt circle diameter and the brakes I have seen on the large bolt circle 7-1/4 are usually wider but the overall wheel mount surface length has been the same. But I imagine your probably just going to use the spider and side gears from the donor. That’s the route I took unless I was changing the ratios, too.
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joel erlandson said:
I built a 1933 Dodge in the '80's & it has made it back to me for repairs on the rear end . Both spider gears are wrung off. This is the rear diff. considered to be junk by many but it fits in width perfectly. It is about 57.5" drum to drum. It is the 9 bolt 7.25 rear end. I bought a C7.25BI kit but 1 seller said I have to use an '80 model case for this kit to work. I don't know much about rear differ. but the seller said the case she is talking about changing is the case the pinion gear bolts to. Will the kit I bought fit the rear I have or do I have to change the case as well? If so what year, make, model cases will fit my casting #2070051? Could I use used spider gears & if so can you give me a source? Can anyone help me out here? I am in a real bind. A certified mechanic is coming tomorrow to see if the kit will fit but I'm anticipating it won't & hoping to get a response to my questions. Thank you to anyone who can help. Is there a source for NOS spider kit for a '70 Duster rear like the one I have? Thanks again. joel
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I did not have any luck with a housing number, tried to scrape off 50 plus years dirt and grime. Too windy and cold to do more. This differential was working good when I pulled it last year. I drove the car for several months after it had set since 1988. 40,000 miles on it. All new stuff.

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Did any of the early Dusters w/slant 6 come with 5 on 4.5 lugs? Here is more of the story- mid 1980's I was building the '33 Dodge coupe. At work a man had an early Duster on the back of a large truck that he was taking to the crusher so I asked him to pull the rear end for my build because I knew the width was exactly right. It had a 5 on 4.5 pattern which was what I had on the front. I don't know the year exactly & I didn't see the engine so all I know about the rear end is the casting #2070051/ 9 bolt cover/ width drum to drum about 57.5"/ & 2.93 rear gear. It couldn't have been much newer than 1970 because in 1985 it was only about 15 yrs. old & wouldn't have been on this guys truck. It could have been late 60's but it was just an old car- it had never been "hopped up" so the wheel pattern had not been drilled into it- it appeared to come that way from the factory. So because I chose that rear end the current owner is pretty much stuck with it because of the width. Another rear axle would be nice to have. Randys says "The 9 bolt rear uses a different case & spider assy. which is NOT compatible w/new components. Motive double checked & he felt the spider assy. would fit. I'll know tomorrow when we try it. It was nice of CirclePilot to offer a complete assy. & I'm looking forward to his measurements. I appreciate all the feedback. A new cut to fit Ford 9"rear end w/driveshaft is over $3000 or more now & she just can't afford that right now. I'm retired & just trying to help get the car I built 30+ years ago back on the road. I was done until the spiders gave up the good fight! Thank you all. Joel
 
It will be a little warmer tomorrow and not to windy. I'll look again for the number. It was getting late and I did what I could before I had to work on a customer's plane.
Norm
 
circlepilot said:
I did not have any luck with a housing number, tried to scrape off 50 plus years dirt and grime. Too windy and cold to do more. This differential was working good when I pulled it last year. I drove the car for several months after it had set since 1988. 40,000 miles on it. All new stuff.

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Thank you sir. If you get to it measure the drum to drum for total width. The 43" ctr to ctr for the springs is correct to my needs.. I'm sure thinking about it. You'll have TO HAVE AN OLD WIRE BRUSH TO SEE THE CASTING #. Should be on the lower right center casting not on the axle housing. Thanks again.
 
joel erlandson said:
Thank you sir. If you get to it measure the drum to drum for total width. The 43" ctr to ctr for the springs is correct to my needs.. I'm sure thinking about it. You'll have TO HAVE AN OLD WIRE BRUSH TO SEE THE CASTING #. Should be on the lower right center casting not on the axle housing. Thanks again.
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I'll look.
 
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