901 Simplified Differential Fix Question

[Jeff Higgins]

I'm in the middle of correcting the problems associated with the simplified diff in my '68 901 five speed tranny. The teats on the ends of the tension bolts are showing significant wear.

First off, thanks to John Walker for saving my bacon on yet another potential problem. I wasn't even aware of this issue until he filled me in, and also supplied me with a 914 diff to raid for the parts to correct the problem.

Grady Clay did a good job of explaining the issue in a couple of other threads, so I won't rehash all of that here. I will, however, use the part numbers in one of the figures he posted to explain my question:



Grady mentions that both parts 5 and 6 need to be replaced. The reason, as I discovered, is that the counterbore in the new part 6 that accepts part 5 is deeper than in the old part 6. Using the old part 6 with the new part 5 results in inadequate clearance between the two part 5's. Neither parts 2 nor part 3 will fit between them.

Here is a photo of the assembled diff to show how it all goes together:



Here is a photo showing the 914 diff parts on the left, and my '68 901 parts on the right. You will note how much larger the 914 spyder gear is than the 901, so I cannot use the 914 part. You will also notice the differences between the new, improved 914 part 5 and the old 901 "simplified diff" part 5:



Fortunately, the new improved part 5 mates with the splines in my old 901 part 6 just fine. So here is my question: Rather than get a new part 6 with a deeper counterbore, why not just face off the inboard flanges of the two part 5's? The flanges that mate into the part 3. I would need to take .150" of of each, like this:



This will not impinge upon the fillet radius at the base of that flange. I checked to see if the tension bolts would hit the -2 shaft, and they do not. In other words, I can't see a down side. Any thoughts? Can I just face of the new part 5's .150" and avoid having to source part 6's with deeper counterbores?

[Jim_Brown]

Jeff-
I did exactly what you describe above to part 5 with no problems to date.

Cheers,
Jim

[Jeff Higgins]

Thanks Jim, that's the answer I was looking for. Seems like the obvious thing to do to avoid having to replace the part 6's as well. No one had mentioned this, however, so I was afraid I was missing something equally obvious.

[jluetjen]

I'm going to do the same simplified diff update on my aluminum 901 transaxle when I rebuild it this winter. I was looking at my inventory of diffs that I've collected with various transaxles, and I noticed the significant difference between the 914 ring-gear (front) and the 911 ring-gear behind it.



It's not as apparent in the picture as when you look at them in person, but the 914 ring gear is about 29 mm across the teeth while the 911 ring gear is 35 mm across. Without doing any sophisticated measurements of the active surface area of the gear, it would appear that the 914 CWP set has about 18% less area then a 911 CWP.

This got me thinking about options -- Certainly the 911 ring gear will be better for larger capacity engines (and the associated higher torque levels) in order to reduce premature wear. But I wonder if the 914 CWP gear set would be better for smaller engine (< 2.5 liter?) race applications where the smaller CWP set would have less drag. Has anyone tried this? I wonder if an anti-friction coating would help?