[QUOTE=MILD THUNDER;4460484]They are not interchangeable .

Thanks for the clarification. I'm pretty sure the spacers are the same so I'm assuming the bolt patterns are the same? What exactly prevents them from mating up?

The shaft is different. The #4 used a polygon shaped verticle shaft, where the #5 uses a splined shaft.

So the empty cases would probably mate up?

another speedmaster question will a small shaft 4 upper interchange with a 3 drive. the 3 has a fill cap on the back and the 4 doesn't have a cap it has the regular screw fitting.also the the upper gears change the ratio or is it done just in the lowers like the rotation is thanks for any info didn't mean to change the subject

The 4 upper is entirely different than a #3. They dont mate up.

There were two lower gear ratios sets, and many upper ratios, which equated to, alot of available ratios at the time.