8 3/4 not what it seems

[BigFlo]

I'm posting this as a caution to novices (like me) and about the rat holes we can chase with these cars. I'm doing a restomod on a 65, 2 door post Belvedere. Originally a /6 with 3 speed manual and converting to 383 4 spd. I've been taking care of rust underneath and took the rear end off to get access to wheel wells. While the rear end is out, I plan to switch the open diff to a Sure Trac. So I purchased the seals, bearings and diff, etc from Dr Diff for my 741 8 3/4 rear end. THEN I decided to look at the fine print in the spec section of my Service Manual. It says my 225 cu in engine with manual trans and the 3.31 gears that it has in the rear end, contains an 8 1/4" ring gear. Arggg!
After talking with Cass at Dr Diff, the Sure Trac that I purchased will not mount to the 8 1/4 ring gear. So my options are to purchase a new ring & pinion, yoke and conversion U-Joint, or keep the peg leg, or buy a new/used 3rd member. None are really the options I wanted.
Like I said, I'm a novice when it comes to diffs - the first time I saw the inside of one is this week when I took mine apart, lol - and replacing the ring/pinion and adjusting the pinion depth is intimidating. I think I can get some more experienced help to do the ring/pinion replacement, so that is what I will probably do.
So do as much research as you can on things you don't know and when you think you have done enough, do more, unless you like wasting time and $, lol.
My questions - has anyone run into this?
Are there any solutions I am missing?

 

[skicker]

Your book is wrong...That is an 8 3/4...

 

[BeepBeepRR]

8.25 opens like a Dana from the back.

 

[beanhead]

Your book is wrong...That is an 8 3/4...

Uhh no it isn't.....for those early axles, the 8-1/4 was also a removable third-member style.
I just hope the OP measured it to confirm!

 

[beanhead]

He said '65 guys....

 

[skicker]

So a 65 has a pig that is an 8 1/4?

 

[beanhead]

So a 65 has a pig that is an 8 1/4?

Yeah they didn't actually call them "8-3/4" yet as we know them because they had the two different sizes, but same design. I 'think' they were called AR-1 and AP-1 or something like that.

 

[w.Hudson]

He is correct in what he is saying.

Early Mopars used a 741 (possibly few other casting numbers depending on year) that used an 8 3/4 rear axle center section that used a ring gear with an 8 1/4 inch bolt circle.
Any center section that had a sure grip used a 8 3/4 ring gear. So no sure grip for that gear.
Best bet will be to have Cass build you one or find a used unit ready to bolt in.

Also if you have tapered axles you will have another problem. The axles will be too long and will have to be machined on the spline end to get them to the right length.
The center block in the carrier is a different length from the open carrier to a Sure Grip.

 

[skicker]

New one on me...I've sold a crap ton of them as 8 3/4" center sections...

Nobody ever wants the 741's so they usually get sent to the scrapper...

 

[beanhead]

New one on me...I've sold a crap ton of them as 8 3/4" center sections...

Nobody ever wants the 741's so they usually get sent to the scrapper...

Tell me about it! I tried to give a '68 741 away at a Mopar meet....no takers.

 

[khryslerkid]

@BigFlo tell us what the last three numbers are.

 

[skicker]

Looking back at what your saying I can see how they could have been different...

I guess since I never had anything with a deep ratio none of them ever got saved to add a posi to.
Thanks all for the education...

 

[Kern Dog]

Tell me about it! I tried to give a '68 741 away at a Mopar meet....no takers.

Hmmm.....We obviously had not met yet. IF it is FREE, I find room for it!

 

[Kern Dog]

OP's name sounds like a robust waitresses name.

 

[skicker]

Scrounge...

 

[ruffcut]

I guess you do learn something new everyday!

 

[w.Hudson]

New one on me...I've sold a crap ton of them as 8 3/4" center sections...

Nobody ever wants the 741's so they usually get sent to the scrapper...

Tell me about it! I tried to give a '68 741 away at a Mopar meet....no takers.

I am running a 741 in my Dart. Mild 440 with an automatic. I have a couple of units with different ratios.
Most of the time they are a cheaper choice.

Everyone says that they are weak. I have never broken a pinion shaft. But I have broken teeth on several through the years no matter what housing number it had on it.

Plus a 741 will have a clutch type Sure Grip in it. So don't pass on looking at one.
They can have almost any gear ratio in it. 3.23, 3.55 and 3.91 can be found.

 

[beanhead]

Hmmm.....We obviously had not met yet. IF it is FREE, I find room for it!

Ha! I'm a parts HORR too but mostly with stuff I won't trip over. I only have so much room left to horde! And yeah, it was a couple of years before I metcha.

 

[beanhead]

I am running a 741 in my Dart. Mild 440 with an automatic. I have a couple of units with different ratios.
Most of the time they are a cheaper choice.

Everyone says that they are weak. I have never broken a pinion shaft. But I have broken teeth on several through the years no matter what housing number it had on it.

I agree with that, let's just say "in theory", because I've never put much power to one myself. I know guys that have though, that had them live just fine. (Then again I've also watched a buddy waste a Dana so ya never know!)

 

[dadsbee]

My 741 has taken 53 years of **** and abuse (out of a '67 Charger) and was clean as a whistle when I restored the rear axle and just threw the pumkin back in as it was. Dumping the **** out of the clutch against 2.94's has gotta be harder on it than 3.55's and I haven't broke it YET!