8" To 9" Conversion Question

[goldstar]

Iam changing the rear end to a 9" 28 spline out of a 1969 mach I (donor) . The unit is going in a 1968 convert mustang (recipiate) that currently has a 8". The question is will this require a change in driveline length ?. Iam aware there are 2 different rear yokes a long and short. And also the u-joint bearing caps are different diameters.

 

[Falcon79]

Are you putting the 9" third member into the 8" housing? I'm not sure you can do that if thats what I'm reading. In regards to your question, everything I've read says that the 9" is a little longer therefore you might need to change your driveshaft length, but I don't know about all the different types of yokes...etc.

 

[ks65stang]

I did a 9" Lincoln Versailles swap on my 65 coupe and the 3rd member is larger on the 8" so I had to shorten my driveshaft. Plan on having to do it unless you find a special yoke, I couldn't find one.......if a such thing even exist. I used the stock leaf spring plate and just elongated the holes. Good luck!!!

 

[dennis112]

The driveshaft for an original Mustang 9" is one inch shorter than the one for the 8". Unless it was changed in its life, the yoke is a long one. If the donor car still has the driveshaft, it will fit your car as will any driveshaft from a 67-70 Mustang/Cougar that was equipped with a 351w or larger engine (all of which happen to have 9" rears.) As mentioned, alternately the yoke on the rear can be swapped for the short one (with a new crush sleeve.) The short rear yoke (28 spline) is actually quite common as it is used on many aftermarket performance rears and can be found on some factory stuff, like the 57-59 full sized Ford. Yes there can be differences in the U-joints used, the 1310 and the 1330 or a combo of both.

 

[goldstar]

9" housing is bieng swapped also. Trans is a tremec 5 w/aluminuim driveline. They make a u joint two different size caps to adapt over.

 

[Falcon79]

I think now would be a good time to just go with a custom aluminum driveshaft.

 

[65FBE2]

The tranny has more to do with what drive shaft length is needed. You will need a 1" shorter drive shaft and the correct T 5 yoke. The 9" will need the short front yoke. The correct way to change to a short yoke is with a new crush sleeve but I have found it very difficult to get the preload correct if you don't completely dis assemble the pinion portion of the third member. Better is to carefully tighten the pinion nut until there is no slack, then give it about a 1/16 of a turn more to set some preload. To much preload will take out the pinion bearing in short order. Put the rear end in the car and then measure to get the proper drive shaft length. Modern Driveline is a great source for information and parts for this swap

 

[dennis112]

Yes, now knowing about the Tremec changes the situation. IIRC they are not the same length as the factory toploader or C4 so a different driveshaft will be in order irregardless. If you have a steel driveshaft and it is too long, a driveline specialist can shorten it to mate with the current 9" yoke. It may also take a conversion U-joint for the final mating. A driveline specialist can fix you up with that too.

If you have a new driveshaft made, the length of the 9" yoke is immaterial. No need to swap it out as the new driveshaft can be made to compensate for it.