Rebuilding 915 transmission

[jld]

I have replaced the diff and am working on setting the preload with shims under the bearings
I have made a bracket to attach to an output stub where I can use a torque wrench to read the resistance to turning
Does anyone have any information about what values I'm looking for?
Thanks...

[dannobee]

Porsche Wiki

I think Pete covers this in part 5

[jld]

I’m not familiar with him
Anymore information would be helpful
Thanks

[jbauto]

I just bought an '83 SC Cab and am snooping around for pattern failures and good advice. Thanx Pete. Nice site

[jld]

???

[jld]

Found it
Thanks

[Mark Salvetti]

Originally Posted by jld

Found it
Thanks

The preload is different depending on whether you are using SKF or FAG bearings, but I believe Pete's Wiki covers that.

Mark

[jld]

👍
Thanks

[jld]

P wiki helped but I need more information about the actual typical shim thickness on the top side. There's discussion about the lower two shims but nothing about the top and I'm starting from scratch there
So going the "conventional" way of measuring the gap between the case and a correctly placed cover plate, what would that measurement be ?
Thanks
Happy Fourth ��

[kevingross]

Originally Posted by jld

P wiki helped but I need more information about the actual typical shim thickness on the top side. There's discussion about the lower two shims but nothing about the top and I'm starting from scratch there
So going the "conventional" way of measuring the gap between the case and a correctly placed cover plate, what would that measurement be ?
Thanks
Happy Fourth ��

Not sure I fully understand your question. Typically, you install a pair of 2.50 mm shims, one on each side, along with the magnet carrier disc if your 915 is of that vintage. You then measure the freeplay and, as the WSM instructs, increase the S2 shimming to the freeplay plus recommended preload. Next, you measure the turning torque of the preloaded differential (without the pinion shaft installed) and adjust S2 to get the turning torque in the proper range.

A note of caution: if you take apart the transmission and find the original S1 shim is less than 2.50, don't start with a 2.50 S1 -- you'll bind the ring and pinion on assembly, which is not a good thing. I try to keep the initial S1 say 0.50 mm below where it ultimately will be and work my way up.

[jld]

That was my question… starting with what, on the top side
Because I’m mating different diff ( disassembled ) to a new ( to me) trans
So I will start with a 2.0 and adjust from there
Thanks 👌

[jld]

The other question was…
If I’m going the less technical way to set preload by measuring the gap between the, properly set top cover and the case, What would that measurement be??

[shoooo32]

Are you installing a LSD? If you have the original diff you can just measure the two units and add or subtract shims based on those measurements. That usually puts you in tolerance for preload/backlash.

[shoooo32]

[jld]

No LSD unfortunately
Different trans and different diff
Only point of reference is the two shims that were on the Speedo magnets side
Just bearing and race and no shims on the other side

[kevingross]

Originally Posted by jld

The other question was…
If I’m going the less technical way to set preload by measuring the gap between the, properly set top cover and the case, What would that measurement be??

The WSM calls for 0.3 to 0.4 mm of preload in a 915. So in theory you would want to have that much of a gap between the case and side cover. However, my experience has been that this method is not at all accurate and repeatable. It can be a good way to get an initial stab at preload. You'll generally find that this method does not reliably yield a turning torque value in the WSM's specified range. Turning torque is a much better, stable, repeatable indicator of preload.

Hope this helps.

[jld]

THANK YOU
This gives me a starting point 👍
I have extra shims to start with, plus an extra bearing to use should I need to put on and off the axle multiple times
Also made the setup for the torque wrench to use afterwards

[jld]

Thank You for all the information.
Over loaded the shims, put it together and used the feeler gauge method with the cover plate.I was able to work backwards by removing shims until I came up with 0.45. I assembled everything and then did the torque test. Which put me at a Rolling setting of 37.