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 Post Posted: Wed Aug 24, 2011 9:05 am 
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Location: St. Louis-Missouri
making the switch to a 9". when switching from an 8.5 does the driveshaft need to be cut or do i need a new shaft?

trying to get an idea of expenses
tia

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 Post Posted: Wed Aug 24, 2011 7:24 pm 
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Do your home work on drive shaft critical speed before your decide.
The Mark Williams sight has a nice chart.


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 Post Posted: Wed Aug 24, 2011 7:45 pm 
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I've done research on driveshaft critical speed vs rpm and length. but what im asking is, do i need to cut my driveshaft and have it rebalanced or or do i need a longer driveshaft when switching rear ends? i think the pinion sits closer to the front of car on a 9" then a 8.5 does.

what the difference in length for driveshafts when comparing 9" to 8.5"?

BTW i already have a 3.5" mark williams bonded mmc driveshaft. as mark williams suggested it beings my shaft needed to be 62-3/8" long.

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 Post Posted: Thu Aug 25, 2011 4:52 am 
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Excellent - mine is 52.5 (T400-9 in)


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 Post Posted: Thu Aug 25, 2011 12:54 pm 
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im not sure your understanding my question correctly. i originally had the mark williams driveshaft cut for an 8.5 grand national rearend but now im going to swap the 8.5 for a ford 9". will the driveshaft for the 8.5 be too long or short or will it bolt in.

i havnt been able to put the 9" in the car yet otherwise i would just measure. im sure somebody has swapped from a 7.5 or 8.5 to a 9" right?

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 Post Posted: Thu Aug 25, 2011 1:10 pm 
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It's been so long since I had the stock shaft I cann't remember thats why I posted the length with the 9 in 52.5 inches. I do believe it's shorter.


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 Post Posted: Thu Aug 25, 2011 4:54 pm 
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I did the 9" swap from a 7.5 with a muncie. Used the same drive shaft but just changed the u-joint. Don't go by what everyone says.... measure yours to make sure its right.

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79 Malibu Classic Landau
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4:11 9" Rearend
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 Post Posted: Sun Aug 28, 2011 6:28 pm 
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I agree^^, you will need to measure to be sure.
Engine/Trans/Rear mounting tolerance stack from car to car can be different, Aftermarket Control Arms are different lengths etc.. etc..

TH2004R/TH350 w/7.5" rear & Stock DS = 52.50", with 8.5" Rear = 51.50".
TH700R4 w/7.5" rear DS = 49.50", with 8.5" Rear = 48.50".


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