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Discussion Starter #1
Is there a way of knowing which 9" rearend is installed in my 69? The car is at the body shop and not readily available to pull an axle. Were certain ones used for certain years? I need to know if I have a 28 or 31 spline spool.
 

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Small block = 28 splines, Big block = 31 splines

I don't know of any exceptions to that...at least for 69.

Someone correct me if I'm wrong...
 

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9" spline count

Logan is correct, for the most part. Most small blocks - 28 and most big blocks - 31. I have seen some 390 cars with 28s (69 Mustang), as well as a 351CJ 4-speed car with 31s (73 Mach 1).

The only sure way to know for sure is to pull an axle, sorry.

My buddy's Boss 429 had 28s in in when he got a hold of it. Reason? The previous owner had drag raced the car extensively and found (the hard way) that 31-spline axles would snap without warning, whereas the 28s would twist first. The guy would paint a line the length of the axle shaft, and when they twisted more than a half-turn, he'd get new axles. Good enough for me.

I did the same thing when I put a 9" under my (shhhh) '74 Camaro drag car. A rear end from a 71-73 Mustang/Cougar fits a 70-81 Camaro perfectly! If you use an Econoline diff yoke, you can even use the stock Chevy driveshaft and u-joint! I used a Motorsport driveshaft and bigger Ford u-joints on mine since I was also replacing my 4-speed and going TH400 at the same time. My Chevy big block 396 put that Camaro down to [email protected] using 28 spline axles. Nothing wrong with them at all.
 

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Discussion Starter #4
Thanks guys. Logan jarred my way-back machine and I recall that I had heard that rule for BB/SB years ago.
The line painted on the axles is very handy. I used that one way back when too.
:)
 

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Rktmn, cougarcj is right about the bb/sb theory being only a rule of thumb. Trust me I have done a lot of junk yard exploring years ago as I tore up my share of axles stock car racing. I used to be able to tell by the tag on the bolt that holds the pig in but alas my memory is blank as to what the codes were now. Most Ford dealers parts or service dept can decode the tag for you. I do rember that a L in like 3L50 meant that it was a 3.50 locking diff. I will venture a guess that unless your 69 is a big block, cj or a 4 speed small block that you more than likely have 28 spline. It seems as if your pig was ever pulled chances of the tag being there is remote as every one is too lazy to put it back on. I also remember that late 60s to early 80s f150 4x4s were good sources for 31 spline pigs and a lot of them were traction locs. All you had to do was change the front yoke to bolt them right into a pass car.
 

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Discussion Starter #6
Looks like it's time to pull an axle. I've seen enough over the years to know better than to assume that what should be is. Hard telling what was done by the previous owners, let alone the factory. Thanks for the input.
 
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