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Tech Question

todd sexton, Saint Paul, MN, 1975 Dodge Dart 225

My 1975 slant-6 Dart has a 7.25 rear axle (which is not uncommon,apparently). I’m switching to disc brakes up front, which means the 4.5″bolt pattern for the front wheels, which in turn means that I need toupgrade from 4″ to the 4.5″ bolt pattern for the rear. So I’m replacing myaxleshafts.

My questions:

(1) I’m assuming that axleshaft lengths vary between models and perhapseven particular cars. Is there a way to identify my axleshaft lengthwithout actually removing the axleshaft?
(2) Do you have any general advice for performing this axleshaftreplacement? I have quite a bit of general mechanic experience, but this ismy first vintage Mopar adventure.
(3) Would there be any advantages to replacing the entire differential(and therefore a different length driveshaft) instead?

I’m not planning on racing a slant-6 sedan, so serviceability, reliabilityand safety are my primary concerns. Thanks very much!

Todd-

Amazingly, the factory DID have 73-up A-body (Dart/Duster/Valiant, etc) 4.5″bolt circle 7.25″ axle shafts which were installed on cars with the slantsix and the disc brake option. You might luck out and find a set. If you get’em, just swapping axle shafts its very easy on the 7.25″, just unbolt theretainers and slide ’em out. But it may be easier to find an F-body (Aspen /Volare, which were all 4.5″ BC) with a 7.25″ axle and have those shaftsshortened and resplined. Nobody does this since most people wanna dump theblow-up-prone 7.25″ axle anyway so there’s little experience base. But ifyou’re staying with the stock slant six, that might be the cheapest way out.If I were you I’d call someplace like Moser Engineering and ask them aboutthis.

The most common swap is to use a 65-72 A-body 8.75″ axle, then use shortenedand resplined C-body axle shafts and matching brakes. Another plan, alsogood, it to use a complete rear axle from a 73-76 A-body, 8.25″ RG. Thisbolts in and already has the 4.5″ BC. Either of these swaps can handle a V-8swap should that day come.

Remember, you need the matching brake setups and drums, complete.

Rick

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