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dmorris1200 05-13-2002 05:20 PM

driveshaft shortening
Well here's the deal guys. I just finished rebuilding a 9-inch rear for my '73 Fastback. I got it out of a '73 MachI so I didn't see any problem with her fitting right in. What I forgot to check is when I measured from the edge of the third member (where it bolts to the housing) to the end of the yoke the 9-inch is about 1 1/4 inches longer than the 8-inch member that's in her now. So I figure to switch from the 8-inch to the 9-inch rear I'll need to shorten my driveshaft 1 1/4 inch. How critical is that measurement, and what if I'm off an 1/8 or a 1/4 inch on the shortening? Originally I measured an inch and a quarter and was going to shorten the shaft an inch and a half to be safe (she sits almost fully into the trans tail housing now). Then someone told me it has to be cut exactly the right amount, no more no less. I've never had to shorten a driveshaft yet, how close must I be? Anybody with experience with this I would appreciate any feedback. Thanks.

Henry Highrise 05-13-2002 05:32 PM

Don, You need to have a little more sticking out of the tailhousing. Reason being when your rear suspension goes up it will be pushing the drive shaft futher into the yoke. If you do not allow for this you could beat the seal out of your tailhousing. What I always do is sitting on level ground measure how much stuck out on the old shaft , and fix the new shaft with that same measurement. Can you tell by looking when you slide your shaft out where it used to ride? There usally is a wear mark where the seal rode on it. Slide it back in untill that hits the seal and you can tell how much needs to be out. I know this is not real clear, but maybe you can understand what I mean. Good luck!

dmorris1200 05-13-2002 06:19 PM

Approx. 1 inch between where seal rides and flange starts. That's why I was thinking of cutting a little extra 'cause she's pretty far into the trans but wasn't sure if she needs to be in as far as possible without bottoming into the trans or could she be out a bit further and be ok. Also if I go causious and need to shorten more can the shaft be shortened twice or will I need to find another shaft? So many questions for one job eh? <img src="confused.gif" border="0">

johnnymopar 05-13-2002 07:02 PM

i may be running into this same situation soon, also. Would it be correct to assume, generally speaking, that the driveshaft length is determined if the yoke, shaft and rearend are all level and the shaft is fully inside the yoke?


Henry Highrise 05-13-2002 07:20 PM

Yes you can shorten twice if you need to. Johnny Mopar , You have to have some room for your driveshaft sliding yoke to move in and out a little bit in your tailhousing. When the rear suspension rides up the shaft slides in, when the suspension travels down the shaft slides out. So sitting parked you need to have a little bit of the sliding part of the yoke exposed, that way when the rear axle rides up you will have some to slide in with out battering your tailstock seal. About a inch or inch and a quarter should do it.

dmorris1200 05-13-2002 08:04 PM

Thanks for the help Dave, Don. ;)

gcomer3 05-15-2002 08:24 PM

I can tell you what I did on my T-Bucket. With the car sitting on the ground on all four wheels I slid the slip yoke into the tail shaft. There was a wear mark where it rode but if I remember it was 1/2"-3/4" from bottoming out. I then measured from the center of the bore where the u-joint cap would go in the slip yoke to the flat surface on the pinion flange-which would be the center of the u-joint cap that goes into the flange. Knowing this measurement, I chucked the driveshaft in a lathe and shortened it to that measurment. Worked great!

Joe S 05-19-2002 08:40 PM

Make sure you re align for the U-Joint before welding back together,best way is to draw a line (right angle to the u-joint) at the end you are cutting off then re align the mark before weling back together. If your U-Joints are not aligned properly it will viberate and take out your U-Joints.

gbovaird 05-21-2002 09:35 AM

It never hurts to have it balanced either.Seriously give this some thought i would hate for something to happen to you or your ride, if the shaft breaks and digs into the ground , or it smacks you in the *** . Oh ya , i always heard there should be @ 3/4 of an inch space in between the rear of the tailstock and the yoke, when the car is at ride height. Take care :-)

trees 06-06-2003 07:22 PM

DMorris, if you trust your measurement (I always do mine at least 3 times and two different ways to make sure I'm right) then shorten the shaft that amount. The in and out travel that your present drive shaft has been going thru is evident so you can see what kind of fudge factor you have available.


4 Jaw Chuck 06-06-2003 07:42 PM

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