9" rearend
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Senior User
Joined: Dec 2009
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From: Alvin, Texas 77511
9" rearend
What type of 9" rearend would be better for my 52 flathead 8 for street running. The rearend that is in it now only goes about 55 mph with the engine rapping out. I would like to run at least 65 on the highway or better to keep from getting run over. What gear ratio do I need to get to help in this situation. Thanks as always for the help. What year of truck will fit a 52 with a bolt in and not any modifcations?
Lead Driver
Joined: Dec 2004
Posts: 7,669
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From: San Jose, CA
Here's a good article that you should read: 1948 through 1960 Ford F-1 and F-100 Rear Axle (differential) Swaps .: Articles
Basically, any 9" rear end from a 1957-72 Ford truck is a direct bolt in. 1968-72 requires minor modification of the spring perches. I bought a 9" for my truck from a 1957 F100.
The lower the gear, the faster you can go. Anything in the mid to high 3s will be an improvement over the 4.11 you likely have now.
Basically, any 9" rear end from a 1957-72 Ford truck is a direct bolt in. 1968-72 requires minor modification of the spring perches. I bought a 9" for my truck from a 1957 F100.
The lower the gear, the faster you can go. Anything in the mid to high 3s will be an improvement over the 4.11 you likely have now.
Hotshot
Joined: Oct 2009
Posts: 16,192
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From: Burbank, WA
Just be careful not to change the rear to something geared too high. Personally, I wouldn't go with anything lesser numerically than 3.50 with a flathead. They don't have big block torque, and need the low gearing to keep from burning up clutches. My 2c.
Post Fiend
Joined: May 2008
Posts: 7,641
Likes: 21
From: Poway, Ca.
Also, a lot of this gearing depends on what size you r back wheels and tires are too. I put 3" taller tires on my 51 and it improved the MPH by 10 at 2500 rpm.
Here's a good thread and an RPM Calculator:
https://www.ford-trucks.com/forums/7...alculator.html
Gear Ratio Calculator
But basically it boils down to nothing lower than 3.25 which seems to be prefered or 3.50 depending on tire size.
On the Tire Size for the RPM Calculator, it is much more accurate if you urse tire roll out instead of diameter.
Here's a good thread and an RPM Calculator:
https://www.ford-trucks.com/forums/7...alculator.html
Gear Ratio Calculator
But basically it boils down to nothing lower than 3.25 which seems to be prefered or 3.50 depending on tire size.
On the Tire Size for the RPM Calculator, it is much more accurate if you urse tire roll out instead of diameter.
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Senior User
Joined: Dec 2009
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From: Alvin, Texas 77511
I am going to try and find a 3:50 or a 3:75 9" rearend to put on the flathead. Called all day yesterday to junk yards and salvage yards around here and Houston but no one keeps anything as old as a 72 - 56 in their yards. I got a call from Junk Yard Dog that said he had some rearends that might work so I'm calling him back to see what he has that will work. The tag off of the one I have now has what looks like a 4:12 from what I can read from the old tag. I tried to clean it off as good as possible to make out the numbers. If there is such a rearend I guess that is what I have. Thanks for the information that you all have given me and I'll keep looking.
Elder User
Joined: Apr 2002
Posts: 692
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From: Gatesville, TX
I have had better luck driving around and finding a old truck sitting in someones back yard. Many time you can buy it for 2-$300. Take the rear end and anything else you need ans sell it for scrap metal. If your lucky you can get nearly what you paid for it back.
I have also had good luck finding older stuff at the Pull your part places.
I have also had good luck finding older stuff at the Pull your part places.
Cargo Master
Joined: Jun 2001
Posts: 2,571
Likes: 22
From: Oxford, Indiana
Where are you at? I know of at least half a dozen available '57-'72 9-inch axles in my neck of the woods - and I'm in the rust belt. Most stuff heads for the crusher after 20 years in the salt around here. They're around if you look. The '67-'72 trucks seem to be the easiest to find.
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From: Alvin, Texas 77511
I'm in Alvin Texas but I found two yesterday a 3.25 and a 3.70 that a guy has he will bring to Alvin free of charge for $400 for both of them. I thought that was a pretty good deal. Thanks for looking into this for me. I thought I would put the 3.70 in the flatty 8 and the other in the 52 302 engine. I think that will work fine don't you?
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Cargo Master
Joined: Jun 2001
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From: Oxford, Indiana
$400 for a pair?!? Well, I guess if they're scarce in your area, that might not be bad if he's going to deliver. I can buy them for $50 - $100 around here. Maybe those trucks are mostly still in use in Texas and that's why they're tougher to find. Who knows?!? As far as your plans go, yes, I think you're going to put them in the right trucks. Those ratios should work nicely with those engines.
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Senior User
Joined: Dec 2009
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From: Alvin, Texas 77511
Nobody keeps anything on about 10 years old around here. Parts are hard to find because they crush cars and trucks after they get about that old. I have had to order from on line most of my stuff. Spent two solid days trying to find the rearends and put a message out there on the internet and got lots of responces but they wanted from 500 to 950 for one so I think I did pretty good for two at 400 and free delivery to my house.
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