FJ40 Diffs in a BJ70

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Feb 28, 2007
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Im getting ready to tear apart both axles on my 85 BJ70 and Im considering putting it back together with center sections from a 78 FJ40.

I dont see any issues to address besides the driveshaft bolt pattern is different between the two. I dont think this is much of an issue though because I can use the end from a FJ40 shaft on each of the 70 shafts to mate to the 40 diffs.


The main reasons to use the 40 parts.
1. The stock BJ70 diffs are 3.70 and the rear needs a rebuild - the 40 diffs are 4:10. The 70 needs diff work and lower gearing would be nice.
2. I already have the 40 diffs and they are in good shape with one of them rebuilt and the other a little used front diff. This would save $ vs rebuilding the 70 parts.

What am I missing? Why shouldnt I use the 40 diffs for the 70?


Chris
 
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1978 FJ40 diffs are 3.7 unless you changed the out...

This is an age-old TLC debate (myth?). 78 must have been a cossover year for the gearing, but I have never seen a 78 with 3:70.

Both 78 FJ40s I have owned (still own one) and the 78 FJ55 I owned have all been 4:10 - also indicated by the axle code on all these mentioned vehicles.

The diffs I have sitting on my back porch are 9X37 stock gears with toyota markings on them.
 
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and i have only seen a couple 4.11 diffs on 1978 40 series, and i have seen a lot of 78s.

not to argue the point, as long as you are sure of what you have then swap away...there are no issues to deal with.
 
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1978 FJ40 diffs are 3.7 unless you changed the out...
Maybe in canada but not the US.
Depends on the month and year but every 78 i've seen has been a fine spline 4.11. then the 79 cross to fine 3.7's.

And honestly those and the fj62 4.11's are the easiest to swap because you can pull the pinion flange on your 3.70 out of the bj70 which are fine spine and run them on the fine spline 4.11's If not you most likely have to get a blank flange and drill it if you have good DS's. If not you could mix and match yoks. The fine splines are stronger than the course too.
 

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