7.25 to 8.75 conversion 1974 Valiant

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Dillon Crowell

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I’ve got a 1974 Valiant with front discs so I’ve got bbp. I have an e body housing im having shortened and I have a body axles that have been drilled for bbp. I’m assuming I have 10” in drums in the rear which I’m hoping to use. Can I use the backing plates from my 7.25? And is there anything else I need to complete the swap?
 
If you have re drilled axles you will need the factory small bolt 10x1.75 brake parts along with re drilled small bolt drums
 
doug 371 said:
If you have re drilled axles you will need the factory small bolt 10x1.75 brake parts along with re drilled small bolt drums
Click to expand...

Exactly.

To use your BBP backing plates and brakes you need axles with the factory BBP flange offset. I’d buy a new set of axles from DoctorDiff and use the BBP brakes you’ve already got.

By the time you’ve sourced a decent set of small bolt pattern brakes and had everything re-drilled you’ll probably be into it almost as much as new axles.

IMG_7856.jpeg
 
No, you can't use the '74 drums for your application.
Since you've got redrilled A body axles, you have two choices:
1.) Find some small bolt pattern 10" drums AND BACKING PLATES, have them redrilled to 4.5" B.C. to match your redrilled axles. Use '72 and earlier brake hardware ('69-'72 preferred). New drum availability is spotty, at best, and not cheap when you do find them. MUST be paired with the correct backing plates (NOT interchangeable with ANY others).
2.) Get aftermarket axles with the 4.5" bolt circle. Doctor Diff has them for under $300 with new bearings installed. Mopar Axle Package 5 X 4 1/2" (5 X 4.5) Bolt Pattern 30/35 Spline Now you can use your '74 drums and backing plates.
Why can't I use my drums, you ask? Because the A body (4" bolt circle) flange offset (do a forum search on it, lots of threads explaining it) is different from the 4.5 axles. Plus, the center register on your axles is substantially smaller than the center register on your '74 drums... wobble wobble wobble.
 
Yeah there’s no way I’d use re-drilled axles without already having a complete SBP brake set up in hand.

And as the professor said, SBP brake stuff isn’t always available, it’s dropped out of production for years in the past. I’d just get the proper BBP axles and run the BBP brakes you’ve already got. More braking surface and much easier to find parts, and you can use everything off the shelf. With the SBP drums, every time you need new drums you’ll need to have the BBP pattern drilled before you can use them.
 
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So if I order the axles off dr diff is there anything else I need? I heard that the u bolts and shock plates won’t fit
 
Dillon Crowell said:
So if I order the axles off dr diff is there anything else I need? I heard that the u bolts and shock plates won’t fit
Click to expand...

Correct. The 7.25” has smaller axle housing tubes than everything else so the u-bolts and shock plates won’t fit.

You should also check the u-joint dimensions.
 
Dillon Crowell said:
So if I order the axles off dr diff is there anything else I need? I heard that the u bolts and shock plates won’t fit
Click to expand...
Good news and bad news.
IN A PINCH you can elongate the holes in your shock plates so they will accept 3" U bolts, but you may need to do a bit of grinding to the plate to allow the nuts to seat in their new positions.
You DO know you'll need a new driveshaft (or have your current one shortened)? The nose of the pinion is further forward on an 8.75 than it is on a 7.25 rear. And depending on the carrier and yoke you have, it may require a different (or adapter) U joint.
 
I saw someone say I’d have to shorten it so I’ll probably just have my driveshaft shortened. I’ve got a 489 case and the trans came out of a 74 challenger.
 
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