915 coarse or fine spline flange recommendations...

[G450X]

I am buying parts for a pending rebuild of a 915 with close ratio gears (SCRS ratios except 5th) for my '82 SC. I plan to use a non-original "hybrid" 915 (mag final drive, alum main case) since it appears to be in good condition and was reportedly recently rebuilt and a low mile example.

The planned engine rebuild will be a 3.0 short stroke (a few years to find the MM P&C's) with other power boosting techniques like early 3.0 large port heads, early CIS, SSi's, etc.. (maybe 210 h.p. or so).

Since there was some contraversy over thin/thick flange 915's in some mid-year models, I pulled the flanges and they are normal 100mm coarse spline flanges.

I am planning on adding a LSD - would it be smart to source the elusive 108mm fine spline flanges from a late '85/'86 915 and add a fine spline LSD (since I have to buy a LSD diff anyway)?

Or...just keep my 100mm flanges with my modest power bump and buy a coarse spline LSD and be done with it?

Do I have to replace the entire differential with a fine spline version, or will the LSD just mount inside the current unit?

Am I forgetting anything in a coarse to fine spline swap? I assume the fine spline LSD is a bit more desirable since they made them for many more years...

Thanks in advance for any feedback.

[d962r]

BUMP! I would like to know where to find the 100mm fine spline flanges. I have the 108mm fine spline now and need 100mm.

[Joe Bob]

Dunno where t source the flanges but recommend the WEVO intermediate plate....it's a must have.

[G450X]

I ended up keeping the coarse spline 100mm flanges. Internal mods were a Wavetrac Diff, CMS one piece bearing plate, and Wevo internal gateshift. I also have a set of 108mm coarse spline flanges I could machine if I feel the changes to my 3.0 will need the extra strength. All gears but 1st were swapped out, 2nd (15:30), 3rd (21:31, 4th (24:27), and 5th (28:24). I hope to install it soon.

D962r, if you have 108mm fine spline flanges they are sought after for higher power applications and will be an easy sell. 100mm fine spline flanges are common and easily sourced.

[Tremelune]

If the weak point of the transmission is the gears and synchros, what does beefier flanges buy you?

[G450X]

Originally Posted by Tremelune

If the weak point of the transmission is the gears and synchros, what does beefier flanges buy you?

I somewhat agree, but I'm not even close to being an expert on the subject.

Many forum mechanics/experts expressed their dismay when Porsche went to 100mm axles due to their lack of strength (vs 108mm units). I don't know at what h.p. level they feel 108mm axles are needed - I assume that they have encountered some breakage with 100mm axles vs the stronger old (or "newer" 85.5/86) 108mm axles...