108 mm fine spline output flange options for a 915

[stownsen914]

I am switching from a coarse spline to a fine spline differential in a 915. I'm using 930 CVs in a race application, so will need 108 mm fine spline output flanges. I understand that size is a rare bird and that there are options like shortening a set of G50 output flanges. Are there other options?

[Rusty911]

I think CMS might make and sell these as well.

[winders]

https://californiamotorsports.net/collections/porsche-915-transmission-parts/products/copy-of-cms-porsche-997-turbo-billet-axle-flanges

[svcetiquette]

Given it's probably considered a downgrade to keep the coarse spline hubs, but another option is sourcing coarse spline bevel gears to swap into your new diff. Matt M was able to get me a pair of fine spline bevel gears to install in a Guard LSD I sourced from an older car to go in my racecar 915. Price was reasonable as well.

[speedracer07]

sorry to bust in... I have an early 915 with a 8:31 R&P that i want to drop in my 86, obviously the axels won't work since the early tranny has course splines and flanges are too small to fit the axles, what's the cheapest way to get this done?
My mechanic quotes me $1200, (CMS I think) is this reasonable?
Thanks!

[winders]

Originally Posted by speedracer07

sorry to bust in... I have an early 915 with a 8:31 R&P that i want to drop in my 86, obviously the axels won't work since the early tranny has course splines and flanges are too small to fit the axles, what's the cheapest way to get this done?
My mechanic quotes me $1200, (CMS I think) is this reasonable?
Thanks!

$1095

https://californiamotorsports.net/collections/porsche-915-transmission-parts/products/cms-911-930-conversion-axle-flanges

[speedracer07]

Ok, so $ 1200 installed it's not bad, thanks for the confirmation, this build has been a nightmare, I'm dropping a 3.6 in my 86, I had an early 915 with a Quaife LSD and 8:31 R&P that I want to use since I sold the original 915.

Thanks for, chiming in,
Merry Christmas!

[stownsen914]

Pretty sure there are cheaper options than $1095 for a pair of flanges.

[winders]

Originally Posted by stownsen914

Pretty sure there are cheaper options than $1095 for a pair of flanges.

Sure, if you can find them.......

[thetorch]

Most cost effective would be early 930 output flanges. Hard to find...

915 output flanges - please help me set this up for my 915 swap

[PeteKz]

I'm not sure if this info will apply to my planned conversion, so let me ask here. I have a 915 magnesium case with the 4-bolt/2-pin flanges in my 1973 911T (with 3.2 long stroke, etc.). It's not the original transaxle, but in that same timeframe. I want to add LSD to it and 8/31 ring and pinion. Will I need to swap output flanges or other stuff? Or does the LSD+8/31 R/P drop directly into my case and use the same flanges?

[winders]

You can buy 915 differentials from NPR or Guard with course or fine splines. Personally, if I had course spline output flanges and I were buying a differential, I would get a fine spline differential and get new flanges.

Also, when you install the new differential, you will need to get the ring and pinion adjusted (depth and backlash).

[LJ851]

Originally Posted by winders

You can buy 915 differentials from NPR or Guard with course or fine splines. Personally, if I had course spline output flanges and I were buying a differential, I would get a fine spline differential and get new flanges.

Also, when you install the new differential, you will need to get the ring and pinion adjusted (depth and backlash).

When changing only the differential, pinion depth is not affected and does not require setting. Only the backlash needs adjustment.

[G450X]

I think he wanted a LSD carrier swap, which requires R&P installation and set up.

I had a LSD/TBD installed in my 915 a few years ago and it was a somewhat pricey job, at least compared to a recent Truetrac installation in my old Toyota Tacoma 8.4” rear end!

[winders]

You might only check/adjust backlash...

[stownsen914]

If you replace only the diff, check/adjust preload and backlash. Preload is measured with an in-lb torque wrench.

[LJ851]

Originally Posted by G450X

I think he wanted a LSD carrier swap, which requires R&P installation and set up.

No differential carrier replacement, LSD of any type or open, require any pinion adjustments. The pinion is completely unaffected. Ring gear backlash and differential carrier preload are the only settings necessary.

[G450X]

Originally Posted by LJ851

No differential carrier replacement, LSD of any type or open, require any pinion adjustments. The pinion is completely unaffected. Ring gear backlash and differential carrier preload are the only settings necessary.

Yep, my mistake.

[Matt Monson]

Maybe it’s just me, but I wouldn’t install a replacement diff. into a 35-50 year old gearbox without first checking the pinion depth. Even if the box was previously rebuilt, with all the case bore issues and other long term problems these gearboxes are subject to, it just makes sense. I don’t know how many times people have reached out with a bad pattern on an install where the backlash and preload are within spec. It’s almost always a pinion depth problem. Assume nothing when you open up a gearbox for the first time.

[dannobee]

Matt brings up a good point, even if it's not glaringly apparent in his response.

Get some gear marking compound and check the ring gear pattern as a final check. If the pattern is off, for any reason, you'll quickly know if you must go deeper. And give the pinion gear a good shake and feel for any looseness when the diff is out. Most experienced people can hear or feel a clunk of even a of thousandth or two and know if something is wrong. The cases wear much more than that when worn. And if you lack the requisite "feel," you can measure the play with a dial indicator. This is irrespective of the pinion depth, which should not change (unless someone else really butchered it up).

FWIW, after building a LOT of transmissions and differentials, I found that the factory shims should be your starting point and if you vary much from that point, you'd better double check your own work first and find out why you're off from what the factory did. JMHO