I have a complete 8 3/4 A-body rear that I will be swaping into a car that currently has a 7 1/4 rear. The one thing I am not sure about is the shock plates. Do I need 8 3/4 shock plates or will the 7 1/4 plates work?
7 1/4 to 8 3/4 swap in A-body
- Thread starter Haney
- Start date
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GotDart
Well-Known Member
The 7-1/4 plates won't work. There is another thread about this here
http://www.forabodiesonly.com/forums/viewtopic.php?t=2787
and here
http://www.forabodiesonly.com/forums/viewtopic.php?t=2912
http://www.forabodiesonly.com/forums/viewtopic.php?t=2787
and here
http://www.forabodiesonly.com/forums/viewtopic.php?t=2912
dgc333
Well-Known Member
GotDart said:
The shock plates will work if you elongate the holes. The inside dimension on the 7.25 u-bolts is 2 1/2", on the 8.75 it's 3" so you will need to elongate the holes 1/4" to the outside on each side. Been there done that and it works just fine.
Also, don't forget you will need to shorten the drive shaft about 1 1/2".
dgc333 said:
Thanks for all the info..... I will elongate the holes.....
This car currently is a 340/904/7.25 rear. So going to the 8 3/4 rear I will need about a 1 1/2 inch shorter driveshaft?
Guitar Jones
aka Angry Johnny
Actually I think it's more like 2 1/4 inches it will need to be shortened.
As for the shock plates if your car is a 73 to 76 the axle tubes will already be the 3 inch size so you won't need to change the plates, the brakes will bolt on too but, you'll need the big bolt pattern axles.
As for the shock plates if your car is a 73 to 76 the axle tubes will already be the 3 inch size so you won't need to change the plates, the brakes will bolt on too but, you'll need the big bolt pattern axles.
dgc333
Well-Known Member
Guitar Jones said:
I don't recall it being that much but the easiest thing to do is take a piece of broom handle and lay it in the u-joint groove on the pinion and measure from the center of the broom handle to the center of the axle tube on both the 7.25 and 8.75. The difference is how much you will need to shorten the drive shaft. This is not a critical measurement, you need to have approximately 1" of travel left on your slip yoke in the tranny with the drive shaft installed. So 1/8" of precision is good enough.
You should be able to bring the drive shaft you have to a drive shaft shop and have it shortened, rebalanced and new u-joints insatalled for less than $100.
Guitar Jones
aka Angry Johnny
I just checked again, (it's pertinant because we are going to do this to my son's car in a few days) axle centerline to pinion u joint centerline for the 7 1/4 is 10.09 inches, 12.35 for the 8 3/4, difference is 2.26 inches or .010" more than 2 1/4 inches.
GotDart
Well-Known Member
Isn't there an angle between the driveshaft and the pinion?? That should figure into the equation somehow, shouldn't it?
Say the pinion is pointing down. When you lengthen it, yes the U-joint moves closer towards the transmission, but it also moves towards the ground which is away from the transmission. Maybe the angle is small enough it doesn't matter?
Say the pinion is pointing down. When you lengthen it, yes the U-joint moves closer towards the transmission, but it also moves towards the ground which is away from the transmission. Maybe the angle is small enough it doesn't matter?
dgc333
Well-Known Member
Yes, but that's built into the spring perches that are welded to the axle tubes. Unless you are cutting them off and then re welding them back on there is no need to worry about it.
GotDart
Well-Known Member
You missed my point. Refer to this diagram.
If the pinion angle does not change, but you lengthen the front attachment point, then the driveshaft angle must change. Agree?
And since the pinion CL and driveshaft CL are at and angle to each other, what you add to one side is not exactly what you subtract from the other side. However if the angle is small enough, it probably doesn't matter. But on a short wheelbase car with a short driveshaft, and if the pinion downward angle is set high, then it may very well make a difference.
If the pinion angle does not change, but you lengthen the front attachment point, then the driveshaft angle must change. Agree?
And since the pinion CL and driveshaft CL are at and angle to each other, what you add to one side is not exactly what you subtract from the other side. However if the angle is small enough, it probably doesn't matter. But on a short wheelbase car with a short driveshaft, and if the pinion downward angle is set high, then it may very well make a difference.
Guitar Jones
aka Angry Johnny
Yes that would make a difference but that drawing you posted is pretty exaggerated. In reality the angle isn't enough to really make much of a difference, and as has been stated earlier it doesn't need to be exact, within a 1/2 inch or so would be acceptable.
Red Beard
Active Member
Yes, it does change , but only a small amount.If it concerns you, check the pinion angle, and apply shims between the front of the perch and the springs. this will raise the front of the pinion to compensate for the shorter driveshaft
dgc333
Well-Known Member
Greg, The pinion angle is set by where the spring perches are welded to the housing (as Red stated it can be adjusted with shims but in a stock application the perches set the angle). Your concern would be true if the same housing was being used with a center section that was longer (requiring the drive shaft to be cut). But in this case the entire rear end is being swapped. An A-body 8.75 goin ginto another A-body will all ready have the perches welded on so the pinion angle will be where it needs to be.
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