I decided to rebuild an extra transmission that I had, but since it was in pieces I did not know what size gasket/shim should be installed on the intermediate plate. I had this issue once before and successfully used dye on the crown and pinion gear to set the proper depth. It took a couple of tries, but I eventually got the proper mesh pattern and the transmission worked nicely.

I decided to use the same procedure on this transmission, but the result was noisy. So I added 0.005 and tried again without success. I then subtracted 0.005, again without success.

So I dug out the shop manuals and read up on the recommended procedure, which is to set the proper distance between the face of the pinion gear and the centerline of the crown gear. The proper distance is determined as follows:

The original design dimension is 63.20mm, but this must be tweaked for quietest operation on each individual transmission.
When the quietest operation is determined at the factory, they etch the variance from the design dimension on the end of the pinion gear, which in my case was 16, (see pic #1 below) so the target distance from face of pinion gear to centerline of crown for my transmission was (63.20 + 0.16), or 63.36mm.

The challenge is accurately measuring this distance, but I came up with this method which seems to work great:

I noticed that the main body of the differential, right above the crown gear had a few machined surfaces, so I chucked it up on a lathe and checked for consistency. Although I don’t recall what the exact runout was, it was very consistent.

The diameter of the machined area was 108.24mm, which meant that from the outside to the centerline is (108.24 / 2), or 54.12mm. (see pic #2 below)

I them assembled the transmission without a gasket and took a machined steel bar and shim stock (see pic #3 below) to measure the distance from the face of the pinion gear to the machined surface, which was 8.75mm. This meant that the distance between the face of the pinion gear and the centerline of crown gear was (54.12 + 8.75), or 62.87mm. (see pic #4 below)

So the required gasket/shim is (target distance, 63.36 – distance without shims, 62.87), or 0.49mm, or 0.019 inch.

I reassembled the transmission with a 0.020 gasket/shim and it performed beautifully.

I hope you find this helpful!

Pic#1
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Pic#2
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Pic#3
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Pic #4
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I will, thanks

Fantastic work, thanks for sharing! Have you created your own procedure for side shim selection, in those instances where a non-original differential is being fitted?

I don't know what you just said but I am going to read it over and over until I do.
Thank you

Very clever. Great idea.
I'm going to look at this this on a 915
I hope a ZF LSD has those machined flats.
It's important to have the box level to insure the diff sits square on the S1 side.
A bit more difficult on a 911 with the ring gear in the way.

Before getting it right you said it was "noisy". What was kind of noise was it making? Curious if it's the same noise I'm hearing on mine post rebuild.

Thank you for sharing this awesome procedure!! Definitely going in my notebook!

As others have asked, any tricks for the side to side adjustment?

Thinking of replacing the crown and pinion. Actually, would like to swap the crown and pinion for one I have from a late sportomatic.

Montreal914
If you are looking at replacing your R&P from a set from a 911 sportomatic, it won't work. The sporto pinion shaft is shorter. I looked into this a few years ago.
I'll post pictures if you are interested.

I would call it a whine whose pitch changes with speed and is loudest when coasting. Much quieter while accelerating.

This is all based upon having a matched pair of pinion shaft, differential and the case that the factory used during original assembly. If any of these are changed I think the only alternatives are checking mesh with blue dye, or trial and error until quiet operation is achieved.

I think I might actually have a 7:27 R&P from a later 925 Sportomatic, not a 905.

Here you can see it next to a standard 914 pinion shaft. Sporto on the bottom, 914 on top. It is longer and will need to be EDM cut. Also, the splines are shorter and the depth of the inner thread might be an issue. I will need to further investigate to see if it could work.

I believe having read somewhere it was done before but can't find the reference.

All fun!

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Look for any identifying numbers on your R&P set and also check the ring gear diameter. The 905 sporto used a small diameter ring gear with a special differential carrier; I guess that was because they needed space for the torque convertor.
A few years ago, a guy brought me a 7:27 R&P set from a 905 transmission and asked to have it installed in a 901 transmission for his Fiat Abart sport racer. He wanted to get more top speed.
I could not do it because the 905 pinion shaft was shorter by 12mm. I threw together a couple of gear stacks and took some pictures to show the difference.Click to view attachment Click to view attachment Click to view attachment