7.25" to 8.25" driveshaft?

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Trevor B

Trevor B

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I'm upgrading from the 7.25" rear in my '73 Duster (904 transmission) to an 8.25."
From my reading on here, it would appear that I need to either shorten the Duster's driveshaft or find a different one. Can I pick one up from the junkyard? I can pull one off an '83 Cordoba or an '85 Diplomat, both with 8.25 rears and both with 904 transmissions. Those are slightly longer cars than the Duster but would one of them work?
 
It's really best just to measure and go have your Driveshaft shortened. I thinks it's like 3-3 1/2" difference. A driveline shop will probably only charge you about $95 and they will balance it for you to.
 
I think you are right. Looks like there are quite a few different lengths even in the same kinds of cars with the same years, engines, and transmissions!
What are the exact places to measure?
Thank you.
 
Center to center of the u-joint caps, look for any '73 & up duster/dart sport that had the 8.25 axle. The earlier '72 & down had 8.75 rears & an even shorter shaft, too short to
be correct by about 5/8", but use-able if you can't find the correct one. I've had some cut by shops, some good, some not so much. Proper indexing of the joint yokes and keeping
the yokes perfectly square upon completion of welding them is critical to having no vibration issues that won't go away no matter how well it is balanced after-wards.
 
Trevor B said:
I think you are right. Looks like there are quite a few different lengths even in the same kinds of cars with the same years, engines, and transmissions!
What are the exact places to measure?
Thank you.
Click to expand...
Had several shortened for 999 to 727 swap, and as mentioned, measured from center of u-joint cap to center cap. I couldn't find anywhere around my area to balance mine and never had an issue without having it balanced, FWIW. But if you have someone that can balance it, I would definitely have it done.
 
When I replaced the 7 1/4 with a 8 1/4, I had my driveshaft shortened by 1 5/8" (318 w 904). My wheelbase is 3" longer than yours (Scamp vs Duster) so the original length is different but the length that needs to be shortened is the same.
 
Thanks Everyone!
So once the 8.25" is attached, am I supposed to remove the u-joints from the original driveshaft and attach one to the diff and the other one w/ slip yoke into the transmission, and then measure between the two?
 
Trevor B said:
Thanks Everyone!
So once the 8.25" is attached, am I supposed to remove the u-joints from the original driveshaft and attach one to the diff and the other one w/ slip yoke into the transmission, and then measure between the two?
Click to expand...

How far in will you position the slip yoke in the transission when you take the measurement? Many folks have done well with having their existing driveshaft shortened by ~1 1/2" which allows enough space for the slip yoke not to hit the end of the travel in the transmission nor fall out.
 
I don't know how far in to go. It would be nice to be as exact as possible.

I measured the 7.25" on there now and it's about 51.5" center of one u-bolt cap to center of the other. And about 3.5" from the center of the front u-bolt cap to the end of the transmission tail housing.

whitepunkonnitro said:
The length you're needing is 49.9 inches. Give or take .250 will work
Click to expand...

So that would make the 8.25" driveshaft roughly 1.5" shorter, which is what H22A4 just gave me.
 
Thats why you really need to call and find a drive shaft shop first. They all seem to measure slighty different some center to center some wnat to know how far the splines come out from the tail housing. Go on youtube and Dr. Diff's website. There is a couple good examples out there. I Just took 7.25 out of my '67 cuda and put a 8.25 in. I think you have the same wheelbase. If I get a chance I will measure mine. I would have popped for a new one, but my intents are to put some kind of O/D in my car. So I will then.
 
Now that the 8.25 rear end is in, I measured from center of u-joint cap on the front to the center of the rear end's yoke and came up with 51" (rear axle on jack stands). This seems like it can't be right. I am measuring with the slip yoke about 3/4" out of the tail housing - that is to say that I see about 3/4" of metal extending past the rubber seal. Or is it supposed to be from the end of the metal on the transmission tail?

Also, I picked up a heavier driveshaft from a junkyard Cordoba with a 904 and 8.25" rear, thinking that it would be a better match (bigger diameter tube). But would you believe that when I had the dual exhaust done, the guy ran the pipes very close together using the stock driveshaft as a guide and the bigger one won't fit between them! Is it worth re-doing any part of the exhaust system for a beefier driveshaft?

Thanks!
 
The difference between a 7.25 and 8.25 is about 1.5 ( I think the chart shows 1.25 to be exact ).
If nothing other than the rear was changed that's all you need to know. Take the OEM driveshaft to a shaft shop and have it shortened by 1.5
The extra 1/4 wont hurt anything, just gets the tail shaft seal on new land.
 
RedFish said:
The difference between a 7.25 and 8.25 is about 1.5 ( I think the chart shows 1.25 to be exact ).
If nothing other than the rear was changed that's all you need to know. Take the OEM driveshaft to a shaft shop and have it shortened by 1.5
Click to expand...

That's what I did when I switched from a 7.25 to 8.25. As RedFish says, I see about 1/4" more shiny metal than before.
 
Thanks everyone - got the driveshaft to the machinist and am looking forward to getting this car back on the road (especially going from the 2.76 open gears to the 3.55 Suregrip!). 1.5" off the end and some new u-joints.
 
The nose on the 8.25 is 1 5/8" longer from the axle centerline, however the pinion is lower in the differential housing than the 7.25, thus taking back some of the extra length.
That may be where the 1.25" number comes from.
 
Thanks to Everyone!
Got the shortened driveshaft into the Duster today... with the "new" 3.55 SureGrip rear end and new leaf springs. Everything works great. And, as my old u-bolts were toast, there is no longer any grumbling or squeaking or weird vibrations under load. And with the new gears (went up from 2.76 open on the old 7.25") it feels AWESOME.

Now just have to adjust something in the speedometer - it's reading 12-15 mph over what I'm actually doing.
 
There's a speedometer gear in your transmission, easy to replace. There's a chart on My Mopar. You just need to know how tall your tire is and size of gear. I think there's a member on here who sells them. Glad you got it done.
 
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