Hot Rods Minimum length of driveshaft inside trans

I have a 56 f 100 with a C4 trans and 8 inch rear.Right now the yoke is 2 1/8th inches inside of the tail shaft of the trans.That's about half of the total length of the yoke.Due to a clearence issue I need to move the engine forward 3/8 of an inch.This would only leave about 1 3/4 inches inside of the tailshaft.Is this safe.Whats the minimum safe length that should be inside the tailshaft.Thanks for any help.

 

With full vehicle weight on ground, 3/4" minimum yoke float.

 

Jack up the frame until the rear end is off the ground at full droop, see what's left in the trans.
Is there a longer yoke to put on the driveshaft, I'm not into ford knowledge ?

 

When I have driveshafts done, they suggest bottoming the yoke, then withdrawing it 3/4" and measuring the new length there.
Measuring this way might give an indication of how close or far you are from the ideal engagement.

 

Yep
Also, It kind of depends on the transmission and the brand/application of the yoke, some yokes have a substantial spline undercut at the start.

 

I have the jack under the rear.I'll try it hanging free and see whats left in the trans.

 

All right,with the frame supported and the weight off the rear.The drive shaft goes into the tail of the trans.The measurement is now 1 1/4".Not what I expected to happen.Just to clarify there is 3 1/8 inches of the drive shaft into the trans.Right now the back of the truck sits higher than the front when the weight on all the springs.

 

I just had a tailshaft shortened and was told to take the measurements with the weight on the wheels, the tailshaft should have 25mm clearance before bottoming out. Haven't had it out for a test drive/inspection yet, but I have stuck to this figure for a couple of previous ones and so far so good.

 

Most transmission tailshafts have a bushing that the outer diameter of the yolk rides in. If not in far enough you may not be into the bushing and the drive shaft will vibrate badly.

 

Thanks for all the replys.I think it might be to short.

 

Check with Circle Track suppliers. With all The movement of the rear end there at many lengths of trans yokes. I’ve bought the longest I could and cut it with a chop saw to get the correct length inside on the splines and bushing.

 

That's a good suggestion.Didn't think of that.Thanks

 

Aren't we all !!

 

Had a truck with a lot of yoke exposed. Would beat up seal and leak.

 

Current build has 5/8 slack.
Had em as close as 1/2, no issues.
Sometimes depends on how the rear end set up articulates.
I’ve seen rear axles not centered with the wheel opening. I’ve moved em to correct both a drive shaft and visual issue.
Make sure you are happy with how the rear tire fits before messing with a drive shaft.

 

Now that's using the old noodle!

 

I agree with Sandcrab ^^^^^^^^^^ Thats where you should check first. Then if you need a longer yoke, buy one and cut it to fit like Jimmysix said.

 

To avoid having to buy a new driveshaft when I built my '39 Chev coupe I found a 3rd Gen Camaro driveline was almost perfect, but not quite enough yoke into the trans splines. So I ordered a long 7" yoke since my 3.5" yoke only went in the trans a little over 1". I stuck the 7" yoke into the trans and pulled it back 3/4", then measure to the rear axle mounting surface, and cut the yoke off at the front in my lathe to make it exactly what I needed. Works great, and if I ever have to replace the driveline I can order a factory stock length.