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Old 06-17-2011, 06:40 AM   #1
Chevyman350
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HC8 3.21 Gears For Dana 70hd or 14 Bolt?

HC8 RPO 3.21 ratio gears....

Does anybody have info on what these came in?

Production numbers?

Standard on which trucks?

Where I could find some?

I want them for my 1987 R30 cummins conversion. I need a taller gear for the tire size I am running and expected cruse rpm.

I have a 3.73 Dana 70HD right now.

Thanks alot!
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Old 06-17-2011, 07:27 AM   #2
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

most likely a diesel application. Very hard to find, another board had a member looking for a set, thinking he was looking for over a year, I think he ended up buying new, don't really remember.

I'd be easier to get a 4500 to swap in....
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Old 06-17-2011, 02:27 PM   #3
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I already have an 4l80e to back it up Ibut I like to travel ant high speeds.

Any Idea where he got the New gears?
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Old 06-17-2011, 08:20 PM   #4
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I can't even remember what site it was. Either CK5 or Pirate. Check Randy's ring n pinion, they might, Summit too.
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Old 06-17-2011, 09:39 PM   #5
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I'd be careful of gears that long with big tires.
I have an 86 K20 with a 6.5 NA Diesel, 700R4, 31" tires (265 70 16 E) and, 3.73 gears. I turn 2300 RPMs at 80mph. With lower gears I'd run out of torque. The sweet spot on my 6.5 feels like its just over 2200 RPMs. Highway cruising is excellent even if it is an automatic.

The Cummins BT may actually have the sack to push 3.21 gears but I'd wanna be real sure. GM built some 1/2 ton trucks with tall 2.73 gears and the TH700R4 for fuel mileage. They were dogs.
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Old 06-17-2011, 09:55 PM   #6
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

A stock 6bt will have plenty to turn the tires with even less gear...
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Old 06-17-2011, 10:48 PM   #7
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

http://www.sixstates.com/products/pa...21&supplier=LA

Maybe.
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Old 06-18-2011, 03:04 PM   #8
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I understand that the cummins want 1500 - 1800 rpm for peak mileage.

I like to cruse interstate at 75 mph (Midwest is mostly 65 speed limit).

I will be using 225 70r 19.5" tires.
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Old 06-18-2011, 09:04 PM   #9
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I have 3.21 gears in my 14FF. They are stock in my 83 Suburban C20 454.
I was surprised to see them when I opened the rear diff for the first time.
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Old 06-18-2011, 09:18 PM   #10
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

Quote:
Originally Posted by Chevyman350 View Post
I understand that the cummins want 1500 - 1800 rpm for peak mileage.

I like to cruse interstate at 75 mph (Midwest is mostly 65 speed limit).

I will be using 225 70r 19.5" tires.

I'd call Randy's Ring and Pinion. You may need more than just gears to make this work.

Info I found from Yukon Gear says they make 3.54 to 7.17 gears for the Dana-70HD axle...
For the Dana 70 with a 5/8" Pinion offset (70, 70-U, 70 HD) you can get gears for the 4.10 carrier in 3.54 to 4.10 and 4.56 carrier in 4.56 to 7.17

More info states that the non 5/8" pinion offset housing can be found with (3.00 or 2.73 ). IDK how you would ID one.

I ran some quick what-if calcs through my RPM spreadsheet. You may actually be one of those unusual cases that would be well served by using a Gear Vendors overdrive unit.
225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 with your present 3.73:1 axle gears you will turn 2,210 RPMs at 75MPH. 1,724 RPMs when you engage the 0.78:1 GV overdrive.

225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 . Change to 3.54:1 axle gears and you will turn 2,097 RPM at 75MPH. 1,636 RPMs when you engage the 0.78:1 GV overdrive.

225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 . Change to 3.21:1 axle gears and you will turn 1,902 RPM at 75MPH. That's not quite in your sweet spot and you loose some of your ease-of-launch with a heavy trailer that the shorter 3.73 gears give you.

An Allison 1000 6 speed would get you where you want to be too... $$$$$
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Old 06-18-2011, 10:43 PM   #11
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

Using a GV is an option eventually but a costly one. I was looking for lower rpm crusing for a less bank draining price. I have issues with my project "expanding" already.

When did the 14bolt start to come out in Fleet side Pickup width?

I have read on some of the d***e diesel forums the gus swapping to the 14 bolt to get the 3.21 gears and alot of the d****'s hat 3.54's from the factory.
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Old 06-19-2011, 08:24 PM   #12
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

If I can get a deal on an complete axle that would be best thus trying to find a dually axle with the hc8 3.21's.

I have a dana 70hd and it looks like the 14 bolt is the only one with 3.21's so I need a 14 bolt dually axle.
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Old 08-08-2017, 05:32 PM   #13
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

Youll find these gears common in 14B full floater equipped C20 trucks that have 454/TH400 combo. I run 33's (295/75/16) 3.21 gear out back. Ive got factory 31's up front. I have a toggle switch shift 4L80 that makes the 454 turn 2K at 68 mph. I am able to hold gears longer to get up and going from a dead stop.

Factory 265/75/16 - 1st to 2nd 10mph, 2nd to 3rd 22mph,3rd to 4th 35mph
295/75/16 - 1st to 2nd 15mph, 2nd to 3rd 26mph, 3rd to 4th 40 mph.

I guess you could say she performs more like a 700r4 with 31s. But all in all, its all about your engine combo. If that diesel packs enough TORQUE, it will move these 3.21s with ease. I actually broke traction in this truck before with the Gen V 502/TH400 combo it had when i bought it.
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Old 08-08-2017, 10:02 PM   #14
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

A 6BT will have more than enough torque in front of a 3.23 or even a 3.08 rear end for what you want to do.

My Suburban listed in my signature is even geared steeper than if you used a 3.08 rear end and it does very well at high speeds. I mainly cruise at 60 mph for best fuel economy but it is definitely more peppy at 75 mph. I'm currently running 265/70/17 tires which are very similar in diameter to the 235/70/19 tires you plan on using. The NV5600 tranny coupled to a gear vendors gives me a final gear ratio of 0.57.
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Old 08-08-2017, 10:08 PM   #15
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

Well, being a 6 year old thread he may have made a choice by now. But 3.21 gear sets are available in the aftermarket by a couple of different manufacturers. Brand new set could run you close to 300.00 with shipping. Tallest 70hd gear available is a 3.54
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Old 08-12-2017, 12:34 AM   #16
Chevyman350
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Re: HC8 3.21 Gears For Dana 70hd or 14 Bolt?

I picked up a good used set from evil pay a year or 2 ago. I am goint to stuff the 70hd in and get it back on the road then swap. I am tired of not driving it.
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