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Discussion Starter · #1 · (Edited)
Working on buttoning up my 2 speed to c4 swap in my 64 and have a question concerning the slip yoke depth into the rear of the transmission. I’m using the stock length driveshaft from the 2 speed and not sure if it’s long enough based on the yoke depth at the rear of the transmission. Looks like it should be in further. Does this swap normally require the driveshaft to be lengthened? The pic was taken with the weight of the car compressing the rear springs.
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Definitely needs a longer driveshaft, which will put the travel the correct 3/4"-1" (with the car compressing the springs.)

BTW, If the rear was freely hanging on the springs, the yoke will come out even more via the arc rotation of the spring at its front eye.
 

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Just a little info update for you, these are yokes that both fit the C-4 and 3/4 spd trans. Notice on the longer one how the splines are deeper inside with a collar to go deeper on the shaft. This is the output shaft from a C-4 trans. The rubber collar on the shaft stops the longer yoke from going on far enough.
The longer yoke, to the best of my knowledge is for a manual trans.
 

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I checked a couple of the "longer" Ford yokes I have on the shelf and measured the total spline length. They measured 4", which is average for Ford or GM vehicles. The "added" length beyond the splines is just a hollow tube with no contact with the output shaft. The U-joint end area is not relevant in the short vs long discussion as there is no discernible differences in that location.

I can relate from experience that the splines in the yoke are not overly hardened and they will twist given enough force.

I personally would not trust using the longer yoke to make up what looks like 2" (or more) of missing driveshaft length, although I would be tempted to use it to move a vehicle around in the yard as needed.
 

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Agreed. Measure with load on the rear axle. If you are still short, then one option is to use a different rear axle yoke, and many truck yokes were about an inch longer. I've used this option for various tranny swaps and it solved the issue for cost of a used yoke, seal and new pinion nut.

The long un-splined tube section on some slip yokes is intended to ride over a multi-lip seal a few inches up the trans output shaft. This seal is important, as they also have a small hole in the rear plug to allow air to escape. Without the seal, you lose trans fluid. In later years, they deleted the seal section and instead removed a spline to allow fluid or air to return to the trans, avoiding hydrolock. Verify you have the right parts for what you've got.

I took this photo of the shaft seal required for long-tube yokes. The smooth section slides over this to prevent fluid from traveling through the yoke splines and out the little hole in the rear plug. If using a short yoke (with missing spline), then the seal should be removed or the yoke splines will jam into it:
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