rear differential swap/driveshaft lengths

[mjtjnj]

I swapped in the AMC20 I rebuilt this weekend, and found that my driveshaft is a couple of inches too long now. Can I pop out the front U joint and replace the front part of the driveshaft, that slides over the TC output shaft, with a shorter one? I only need 2-3 inches or so.

 

Also, while I have the shaft out, where should I take the measurements from to add to the database? I have a 2.5/ax5/np231/long bed, so before I change anything I can get that measurement.

 

Thanks!

 

Tom

[mjtjnj]

OK, measured center-to-center of the U joints and it's 53.25". From the database, it looks like the HD rear axles always had a shaft 1" shorter. Going to the parts store to see if I can find a yoke 1" shorter!

 

Tom

[mjtjnj]

OK, local parts store wanted none of it. I looked up the parts on the pdf, and both the shaft and the yoke are different for d35 and amc20/dff. I was hoping to just get the yoke that came on the shaft for an 86 MJ 4cyl longbed 4x4 with the amc20 rear, part # 83500143. Would be a hell of a lot easier/cheaper than having the shaft shortened. The question, then, is, is that where I would make up the length difference?

 

anyone?

 

Thanks

Tom

[Bounty Hunter]

The yokes are generally the same length and you don't want to give up any spline engagement inside the yoke. Around here it runs about $60 to have a rear shaft professionally shortened and balanced.

[mjtjnj]

thanks. the guy around here I'm SURE would be twice that. I re-connected everything, put the wheels on, and I was able to sneak the driveshaft on by taking off the bearing caps, then putting them back on when it was clear of the pinion. I'm back on the road! Though we weren't totally happy with the backlash and the gear pattern (after going through 3-4 crush collars!), it's "good enough" and is nice and quiet. So I have a 4.10 LSD AMC20 under her now! Who wants the D35?!

 

[Pete M]

be careful about running a "too long" driveshaft. if it bottoms out hard, you can cause all sorts of damage in the t-case or worse. I don't advise it.

 

don't just guess that shortening a driveshaft would be too much money, GO FIND OUT.

[Bounty Hunter]

Hopefully the rear axle would move back slightly as the suspension compresses, but I wouldn't count on it to not break the transfer case when over-compressed.

[mjtjnj]

hmmm good points....thanks

 

would bottoming out cause the "over compression?"

 

Also, should I just take it to a shop and ask them to shorten it by an inch, since it appears from our database that all of the HD rear diffs had 1" shorter shafts? Or, should I take a measurement, and if so how to I measure what length is optimal?

 

Thanks

Tom

[ComancheKid45]

This chart seems likely to give you an answer :thumbsup:

 

lwb,_2.8,____tf 3speed,____np207,_amc20= 53"_____d35= 54"

lwb,_2.8,____ax5-ax4,______np207,_amc20= 53.75"__d35= 54.75"

lwb,_2.8,____ax5-ax4,______2wd,___amc20= 63.75"__d35= 64.75"

 

lwb,_4.0,____aw4,__________np231,___d44= 47.5"___d35= 48.25"

lwb,_4.0,____aw4,__________2wd,_____d44= ________d35= #

lwb,_4.0,____ax15-ba10/5,__np231,___d44= 49"_____d35= 50"

lwb,_4.0,____ax15-ba10/5,__2wd,_____d44= ________d35= #

swb,_4.0,____aw4,__________np231,___d44= 41.25"__d35= 42.35"

swb,_4.0,____aw4,__________2wd,_____d44= 52.125"_d35= 53.125" *

swb,_4.o,____ax15-ba10/5,__np231,___d44= 42.25"__d35= 43.25"

swb,_4.0,____ax15-ba10/5,__2wd,_____d44= 52.125"_d35= 53.125" *

 

lwb,_2.5,____ax5-ax4,______2wd,_____d44= 63.75"__d35= 64.75"

swb,_2.5,____ax5-ax4,______2wd,_____d44= 57.5"___d35= 58.5"

lwb,_2.5,____ax5-ax4,______np231,___d44= ________d35= #

swb,_2.5,____ax5-ax4,______np231,___d44= ________d35= #

lwb,_2.5,____tf 3speed,____np207,_amc20= 53"_____d35= 54"

[Bounty Hunter]

Here's how to measure, most all driveshaft shops would use this type of measurement.

[Bounty Hunter]

Yes, bottoming out would cause the catastrophic over-compression of the driveshaft. It's when the entire rear suspension compresses and moves the pinion upward toward the bed.