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  #1  
Old September 18th, 2014, 09:30 PM
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Series vs Defender third members

I have a pristine S-IIa and have utilized a 2.5 NAD- obviously capable of turning 3.56's. So, can I just substitute 10 spline thirds from say a RRC, and keep the original Series axle shafts?
It seems that it should work, but my Series knowledge is limited and any help greatly appreciated.
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  #2  
Old September 19th, 2014, 12:04 PM
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Quote:
Originally Posted by rovertrader View Post
I have a pristine S-IIa and have utilized a 2.5 NAD- obviously capable of turning 3.56's. So, can I just substitute 10 spline thirds from say a RRC, and keep the original Series axle shafts?
It seems that it should work, but my Series knowledge is limited and any help greatly appreciated.
Spoiler alert - - The gear ratios are very different in a coiler transfercase. If you run the ratio numbers using the same gearbox, the overall ratios for a Series transfercase with 4.7:1 R&P and a coiler transfercase with 3.54 R&P rears you will find that the ratios are very comparable. All the coiler gearing does is slow the prop shaft rotational speed down to quiet the drive train noise a little. But you really screw up the gearing by mismatching the transfercase and the R&P.

With a series transfercase and 3.54:1 R&P you would be WAY over geared for the engine.

But yes a RRC differential housing will fit a Series axle housing. However you may need to do a little grinding on the housing where the ring gear fits on the earlier housing. Not a big deal as long as you clean up the chips.
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  #3  
Old September 19th, 2014, 12:46 PM
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No freaking way will that 2.5 pull the 3.54 gearing. Forgeddaboutit
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  #4  
Old September 19th, 2014, 01:13 PM
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As TeriAnn eluded to the gearing is different... the 2.5 only ever came with the 1.4 or 1.6 LT230. Series high ratio is 1.15.

1.15x4.7 = 5.40 effective final drive
1.4x3.54 = 4.96
1.15x3.54 = 4.07

So by changing your diffs to 3.54 you'll be changing your overall gear ratio by 25%

What size tire are you running? If you have a 235/85 or 7.50x16 It would be very steep with the 3.54s.

(but yes it is possible to swap a complete 10 spline 3.54 third member in place of a 4.7 series third... save the fill plug issue as mentioned above)
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  #5  
Old September 19th, 2014, 01:24 PM
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Well, thanks -I think ;-)
I am aware of the 3.56 vs 4.7 ratios, hence the consideration of the third swap.
Obviously I was being a bit lazy not doing the math, but felt others had been down this path. To clarify, I have mated the NAD to the Series tranny-Tcase, and am going to run 7.50/16's. Ultimately looking for a better road setup, as I don't see this truck getting wheeled initially given its condition-especially for the age.
I think at this juncture I will keep the original thirds, and see how she does. Not a biggie to swap out later, but if it made sense to do now, so much the better.
Thanks for the feedback and insight.
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  #6  
Old September 19th, 2014, 01:52 PM
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Overdrive
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  #7  
Old September 19th, 2014, 02:12 PM
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Install roamerdrive. Easier and quicker. Also off road you still have all that low end grunt of the 4.7 and diesel engine.
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  #8  
Old September 19th, 2014, 02:36 PM
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The bigger problem with 3.5s and the Series transfer case is low range, IMO. It would not be nearly low enough. High range would be manageable with the 2.5 NA as 4th gear would be similar to 5th gear with a 1.4 LT230.
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  #9  
Old September 19th, 2014, 03:13 PM
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I would only do it with a Series III box with 3.6:1 first gear, rather than the SIIA 3:1 first gear.

Otherwise 3:1x1.15:1x3.54:1 with 7.50x16s will make for a very, very tall first gear around town.

Edit: assuming idle speed of 800rpm, that would be 6.4mph idling in first gear... just a little quick

Quote:
Originally Posted by Red90 View Post
The bigger problem with 3.5s and the Series transfer case is low range, IMO. It would not be nearly low enough. High range would be manageable with the 2.5 NA as 4th gear would be similar to 5th gear with a 1.4 LT230.
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  #10  
Old September 19th, 2014, 03:23 PM
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True.
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  #11  
Old September 19th, 2014, 04:31 PM
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Great info- thanks!!
Overdrive makes sense, but will give it a go stock first as the changes are no biggie, other than sourcing a overdrive...
Appreciate all the help.
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  #12  
Old September 20th, 2014, 07:36 AM
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I swapped 3.54 3rds into that series cooler I built but I did an entire drive train from a defender and was running a hopped up Perkins prima diesel. Was a great setup with the lt 77 in the series too
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