rear end replacement

[robert.davis]

I have my truck which is an 86 kingcab and i also have a donor parts truck which is an 85 single cab, however it is a long bed so the chassis is the same. just wondering if the rear ends can get swapped mine took a dump a few days ago and rather then taking the axles out and pull the third piece out and trying to diagnose the problem i figured i just switch rear ends mine had water in the oil and a bunch of metal shavings the donor truck oil looked practically brand new and there wasn't any piece of metal just normal shaving from wear on the magnet. overall looks much better. the donor parts truck is also an automatic not sure if that will make a difference in the gearing. can anyone confirm any of this? can i just swap the rear end or am i making a big NO-NO 

[datzenmike]

All 2wd 720 differentials will interchange regardless of the frame type. Some of the early 4x4s will work on the 2wd but around '82 the pinion spline changed on the 4x4. and the 2wd drive shaft won't fit.

 

 

 

...Look under the passenger side hood hinge on the inner fender for the engine tag. Bottom line is TRANS/AXLE.... FS5W71B...... HF38 like the picture above. 38 means a 3.889 ratio. A 37 would be 3.70, 41 for 4.11 and so on. It would be best if the new ratio is the same as the one you have now. A miss match can cause sluggish performance and the speedometer will be wrong.  See what you have.

 

BTW water in the differential is most likely from submerging the axle. They get very warm inside and the air contracts from cooling and sucks water in the vent on top. A boat launch is bad for this or driving off road through streams.

[robert.davis]

ahh ok i checked that earlier one is 38 and one is 37 both 2wd though the first 7 numbers/ are different on both aswell though will that affect it ? 

[SLO720]

Axles are swappable, the only pain will probably be disconnecting the parking brake cables from each axle. One is a slightly lighter gear but not enough to matter. I would also suggest doing the leaf spring bushings depending on how you plan to remove the axle. Unbolting the u-bolts or removing at the spring? 

[datzenmike]

It's 5% different. Going from 3.7 to 3.889 you will feel slightly easier take offs and speedometer reading low by 2.5 MPH @ 50

 

47 minutes ago, robert.davis said:

  the first 7 numbers/ are different on both aswell though will that affect it ? 

 

As all '83 and up had 5 speeds or 3N71B automatic then the '85 must be an automatic.

[robert.davis]

you are right the 85 is the automatic and my 86 is stick shift. thanks for the info. i have some decisions to make lol. im already running smaller than stock tires so i have lost a lot of top end speed dont want to loose much more lol 

[datzenmike]

2.5 MPH @ 50 isn't much. But effectively raising the gear ratio through a gear swap or smaller diameter tires does give better acceleration which feels nice.