8.75 489 Case Swap – Best Place to Buy New Plug & Play (’71 road runner 383 4-speed)

[road robert]

Greetings Mopar Gurus (& Geeks lol):

After nearly 5 years of living with and loving my 3:91 sure grip the time has come to rebuild/replace AND do rear axles seals and bearings (one side leaking for a while). There is a lot of drivetrain noises getting worse and gets scary > 55 mph these days…

Even with the 4-speed I always avoid highways because I prefer too BUT when I need to get somewhere and get on the highway it’s painful and unpleasant ~ 4K RPM @ 70MPH. So looking for some suggestions:

• Thinking 3:55 or 3:23, what will 3:55 @ 70MPH RPMs be?
• Where is the best place to get a new one (I’ve been searching online – DoctorDiff, RMS (Ron’s Machining Service ebay), etc.
• Are the axle spline and pinion yoke U-joint coupling standard for 489 case?
• Axle bearings Green vs. tapered roller wheel bearings?

I can handle the 3rd member swap, but need a machine shop to remove and press on axle bearings, anyone know a good one in New England area? I don’t mind driving far for the right shop.

Thanks you!

road robert

 

[pnora]

Greetings Mopar Gurus (& Geeks lol):

After nearly 5 years of living with and loving my 3:91 sure grip the time has come to rebuild/replace AND do rear axles seals and bearings (one side leaking for a while). There is a lot of drivetrain noises getting worse and gets scary > 55 mph these days…

Even with the 4-speed I always avoid highways because I prefer too BUT when I need to get somewhere and get on the highway it’s painful and unpleasant ~ 4K RPM @ 70MPH. So looking for some suggestions:

• Thinking 3:55 or 3:23, what will 3:55 @ 70MPH RPMs be?
• Where is the best place to get a new one (I’ve been searching online – DoctorDiff, RMS (Ron’s Machining Service ebay), etc.
• Are the axle spline and pinion yoke U-joint coupling standard for 489 case?
• Axle bearings Green vs. tapered roller wheel bearings?

I can handle the 3rd member swap, but need a machine shop to remove and press on axle bearings, anyone know a good one in New England area? I don’t mind driving far for the right shop.

Thanks you!

road robert

DoctorDiff discuss it with him and everything will be at your door.

 

[hunt2elk]

Agree with the above, Cass has you everything you need.

 

[Speedbird]

I THINK the best available would be 3.23. I had to get used for a 2.76.
It may be a little off the beaten path, but what I did was go with a 2.76 ring gear and the Brewers low first gear ration set since it was a new trans anyway.
3.09 LOW GEAR RATIO 23 SPLINE GEAR SET RECONDITIONED - Mopar A833 4-Speed Transmission & Component Specialists
It's very, very rare that I need to get on a 1/4 mile or race a Tesla at a stop light.
I prefer tapered axel bearings. Lot's of opinions a debates. My thought on the matter is "why not"?
Mopar 8 3/4" (8.75) Helical Gear Sure-Grip ("Sure-Trac")

 

[slepr1]

I have the 4.10, 3.91, 3.55, 3.23. I like the 3.55 but prefer the 3.23. I find the 3.55 still scream at highway speeds but the 3.23 do not.

 

[Cranky]

Greetings Mopar Gurus (& Geeks lol):

After nearly 5 years of living with and loving my 3:91 sure grip the time has come to rebuild/replace AND do rear axles seals and bearings (one side leaking for a while). There is a lot of drivetrain noises getting worse and gets scary > 55 mph these days…

Even with the 4-speed I always avoid highways because I prefer too BUT when I need to get somewhere and get on the highway it’s painful and unpleasant ~ 4K RPM @ 70MPH. So looking for some suggestions:

• Thinking 3:55 or 3:23, what will 3:55 @ 70MPH RPMs be?
• Where is the best place to get a new one (I’ve been searching online – DoctorDiff, RMS (Ron’s Machining Service ebay), etc.
• Are the axle spline and pinion yoke U-joint coupling standard for 489 case?
• Axle bearings Green vs. tapered roller wheel bearings?

I can handle the 3rd member swap, but need a machine shop to remove and press on axle bearings, anyone know a good one in New England area? I don’t mind driving far for the right shop.

Thanks you!

road robert

There's calculator on the net that will tell you your rpm with different gear ratios and tire size etc. IIRC, Wallace Racing has one....heck, they have a calculator for just about everything automotive. Anyways, if you like to maintain the cool launch of a 3.55 but want to have better highway speeds, an easy way to bring the 3.55 down some without swapping it is to run a taller rear tire. Much easier to swap tires than swapping a 3rd member.

Both of my Dakotas have 3.55's but they really need a 3.90. Had a 92 Dakota 5.2 with 3.90's and it got better mileage in town and on the road than the V6's do. Awhile back the OD quit working on the V6 truck and since I was on the freeway a lot, I stuck on some 29" tall tires and they lowered the rpm down a good bit. but the truck was a slug in town. Right now that V6 truck has 25" tires and it responds much better and with the OD working and that makes the 25's a game player. Just looks goofy...

 

[ckessel]

My 67 wagon w/440 has 25.75" tires on it currently, spins at 2400 or so at 55 with 3.23's. I have some 15x7 cop wheels out getting coated. The new rears will be just under 29". Can't wait to see what it will be spinning then.