I am going to be swapping the 7 1/4 rear axle out of my 84 Fifth Ave and replacing it with a 1969 8 3/4 out of a Belvedere.

Dippy.org says I can re-use the existing rear axle hardware, but are the axle tubes the same diameter on a 8 3/4 as on a 7 1/4? Something tells me they aren't, but I might be wrong.

Can I re-use the 7 1/4 mounting plates and u-bolts or will I need to fabricate/purchase M-body 8 1/4 u-bolts and mounting plates?

Someone please correct me if I'm wrong, but when I switched from a 7¼ to an 8¼ I had to get larger U-bolts and different shock mount plates as the axle tubes on my 7¼ were 2½" dia. and the tubes on the 8¼ were 3". Was told the axle tubes were the same between the 8¼ and the 8¾.

Hope this helps,

Nat

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1970 four-door Dart, 225/A-904/2.45 gears. 0-60 in twenty three minutes!

It does. My research indicated that the 7 1/4 axle used in 1984 in my 5th Ave has 3 inch tubes that drop down to 2 1/2 when they enter the carrier. I know going form 7 1/4 to 8 1/4 in an a-body requires new mounting pads and u-bolts, but that might not be neccessary on an M-body. That is what I am trying to confirm.

Thanks.

Reed,

If your 7 1/4 has 3" axle tubes they will taper down to 2 1/2" at the center section. Look at the center section to see if the tubes taper down or not and you'll know the tube size.

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Joshua

Yes the rear axle tubes taper on my 7 1/4. So I guess I have 3 inch tubes and the 8 3/4 rear will drop right in?

Well, you won't have to buy U-bolts and shock plates.

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Joshua

That swap just requires new U-bolts for the fatter axle tubes, and the bigger shock plates from the 8 1/4" or 8 3/4" axle... the pads on the axle itself are already correct to put into place.

-D.Idiot

Sorry, when I was referring to pads, I meant shock mounting plates. My bad.

Sure you will after you fight them and they still bust off; save yourself the hassle, use the "blue wrench", sawzall, cutoff wheel, etc to remove the old U bolts, and buy new ones! you can still use your current shock plates though.

I will probably have to do that, but I am going to at least try to get the nuts off my current u-bolts. Some pentrating lubricant, taking a wire wheel on a drill motor to the threads, and using an impact wrench ought to give me a fighting chance to get the nuts off without breaking the bolts.

I did the same to get my U-bolts off after 30+years, and my impact wrench didn't help at all - the U-bolt just twists and absorbs the shock. A deep socket and a big 3/4" drive ratchet did the trick - you need lots of torque over a big angle to get those nuts off.

Many times, the U-bolt sometimes just breaks, leaving you to fight the other side.

I gots lots of torque. I am 6'4. 300 pounds, and I lift weights regularly. Plus, I have a four foot cheater bar. So long as the bolts don't break, I should be okay. I plan on using a wire wheel on a drill motor to clean the threads then spray everything with Knock 'Er Loose (a penetrating lubricant). Hopefully that will get the job done.