Factory LSD installation into 915 - stub axle issue

[jager911]

I have a '74 915 which was rebuilt, all stock, now looking to install factory ZF LSD from '86 Carrera. What's the issue with fine-spline vs coarse-spline stub axles?

i.e. what parts are needed and are they readily available? Do the entire axles and CVs need to be changed? Anyone who's done, please share your experience.

[Tom '74 911]

This can be a bit of a bugger, depending on how deep you want to go. If your current CVs are still stock, then your '74 has the 108mm flanges w/2 pins & 4 bolts. Also, as you noticed, the output flanges are coarse splined. The path of least resistance, if you can find them, is to use 108mm fine splined flanges w/the LSD. This is basically the same flange you have now, but with fine splines instead of coarse. They are not easy to track down though. I ended up using a set of G50 output flanges that I machined to fit (the axle stub is too long to fit in a 915 diff.) because I was not able to locate a set of the later, 108mm fine splined flanges (which I think came on the turbos??). I did this all in an effort to retain the bigger 108mm flanges w/the bigger CV bolts.

A second alternative would be to swap all your CVs, axles, output flanges and stub axles over to the 100mm 6-bolt variety. These are pretty common and I bet it'd be similar in $$'s to the 108mm fine splined flanges... believe it or not.

A third alternative would be to frankinstein it and use 100mm, fine splined output flanges & CVs on the inboard side (next to the transmission) and keep your 108mm CVs & stub axles out at the wheels. This would require a mix-matched axle CV combo...

Clear as mud?

There is a thread that I started or participated in with Grady Clay where we discussed some of the pros & cons... I'll see if I can find a link...

Tom

[Tom '74 911]

Here's the thread:

CV Joint and Axle/Half Shaft Compatibility

[jager911]

Yikes! This is opening a can of worms! Thanks for the link.
If I swap the entire axles/CVs etc, does it impact the hub?

[Tom '74 911]

Originally Posted by jager911

Yikes! This is opening a can of worms! Thanks for the link.
If I swap the entire axles/CVs etc, does it impact the hub?

It's possible that there is a simpler solution. I tend to do things the hard, complicated way at least once! I don't know for certain about the hub and if it's compatible with all the various stub axle and CV options. I looked in PET and there seem to be similar part numbers between years/series for the rear hub, but I don't know from personal experience. Maybe someone else will chime in...

Tom

[mreid]

Originally Posted by Tom '74 911

This can be a bit of a bugger, depending on how deep you want to go. If your current CVs are still stock, then your '74 has the 108mm flanges w/2 pins & 4 bolts. Also, as you noticed, the output flanges are coarse splined. The path of least resistance, if you can find them, is to use 108mm fine splined flanges w/the LSD. This is basically the same flange you have now, but with fine splines instead of coarse. They are not easy to track down though. I ended up using a set of G50 output flanges that I machined to fit (the axle stub is too long to fit in a 915 diff.) because I was not able to locate a set of the later, 108mm fine splined flanges (which I think came on the turbos??). I did this all in an effort to retain the bigger 108mm flanges w/the bigger CV bolts.

I just went through this exact process where Jones Autowerks rebuilt and installed an lsd my 1984 915 for installation in my 1974. Bob knows someone who makes the 108mm fine splined and we used them so the tranny bolted right up to my existing axles. I suggest you give him a call.

[jager911]

Originally Posted by mreid

I just went through this exact process where Jones Autowerks rebuilt and installed an lsd my 1984 915 for installation in my 1974. Bob knows someone who makes the 108mm fine splined and we used them so the tranny bolted right up to my existing axles. I suggest you give him a call.

Mark,
Thanks for the tip. Got a number? How much was it for rebuild and install?
I have to make a decision to buy the LSD once I know it can be done without selling a kidney.

[Matt Monson]

Quote de Tom '74 911

This can be a bit of a bugger, depending on how deep you want to go. If your current CVs are still stock, then your '74 has the 108mm flanges w/2 pins & 4 bolts. Also, as you noticed, the output flanges are coarse splined. The path of least resistance, if you can find them, is to use 108mm fine splined flanges w/the LSD. This is basically the same flange you have now, but with fine splines instead of coarse. They are not easy to track down though. I ended up using a set of G50 output flanges that I machined to fit (the axle stub is too long to fit in a 915 diff.) because I was not able to locate a set of the later, 108mm fine splined flanges (which I think came on the turbos??). I did this all in an effort to retain the bigger 108mm flanges w/the bigger CV bolts.

I just went through this exact process where Jones Autowerks rebuilt and installed an lsd my 1984 915 for installation in my 1974. Bob knows someone who makes the 108mm fine splined and we used them so the tranny bolted right up to my existing axles. I suggest you give him a call.

Actually if it is the LSD I am thinking of, Bob called me up and had me retrofit course spline side gears into what was originally a fine spline LSD.

Regards,

Matt Monson
Guard Transmission llc

[mreid]

Matt, that may be exactly the solution. Bob and I discussed several solutions and I thought he landed on a fabricated output flange. I'll ask him when I see him on Monday.

[jager911]

Originally Posted by mreid

Matt, that may be exactly the solution. Bob and I discussed several solutions and I thought he landed on a fabricated output flange. I'll ask him when I see him on Monday.

Very interested in the answer to this as it seems easiest way to solve.

[LJ851]

Originally Posted by jager911

Very interested in the answer to this as it seems easiest way to solve.

I used to have a PDF showing the actual parts but the link is no longer active. This company offers their own fine spline, 108mm drive flanges.

http://www.albinsgear.com.au/

[Matt Monson]

Jäger,
Just be aware that if I'm correct its not an option I can offer to you. The LSD in question was one of mine and not an original ZF unit. My gears won't mesh with a factory unit.

[KTL]

The 100mm fine spline output flange is the simple way to go IMO. The later SC and 84-85 Carrera used these flanges. MUCH easier to track down a pair of these than it is to find those fine spline 108mm flanges used on the 930 and '86 Carrera. As Tom said, you can use a later set of G50 flanges from 87-98 and have them machined. I know Rothsport does this modification.

If one wants to go the G50 flange route, just be careful with the 964 output flanges! The G50 964 (2WD Carrera 2 = G50.03, .04, .05) flanges are same part number as 87-89 (G50.00/01/02/50) flange 950.332.209.01 But the G64 trans (4WD Carrera 4 G64.00/01/02) are a different number and differ from left to right on the trans as well. Stay away from the Tiptronic too because those are a completely different p/n that starts with 943! However, the 993 C2 (G50.20/21) and C4 (G64.20/21) both use the desired flange 950.332.209.01

Anyhow........... I think the 100mm CV axle is not as bad as it's been made out to be. Yes the M8 mounting screws are a problem if they come loose. It does happen and when it does it can cause a lot of damage. But as long as they're checked occasionally (especially if car is tracked, it should be part of your pre-event tech inspection w/out question) or locked with some Schnoor, Nord-Lock or Stage 8 lock washer setup atop the CV torque distribution washer "moon plates," VW P/N 87-5081-0, they are good for a long time. I've had the 100mm CV on my '79 racecar for a number of years (many more years than i've owned it) and also a 1970 GT class 911 (cstreit's former car) for a long time with no problems in terms of losing axle shafts to faulty tightening.

I'm of the opinion that the 100mm CV is plagued by its screw size and the CV itself has good torque and angularity capacity. The six M8 screws are torqued to only 30 ft-lbs = 180 ft-lbs of total clamping torque. 30 ft-lbs isn't all that tight. Whereas the 108mm CV has 6 M10 screws torqued to 60 ft-lbs ea. = 360 ft-lbs of total clamping. So the 108mm CV certainly is secured much better than the 100mm CV. And Porsche had to have good reason for providing the 108mm CV on the 930 (not to mention the early cars w/out a lot of power) and on the 86-up cars. So maybe the 100mm CV was a VW-based cost cutting approach? Either way, its proven to be sufficient in my limited experience.

I have a personal interest in this because i've researched the coarse 100mm output flange (what i've currently got, unfortunately) vs. 108mm coarse conversion on my racecar. I found a coarse set of 108mm output flanges which appear to have the same shoulder, shaft, spline & end recess dimensions as my 100mm coarse flanges. It's just that they have four M10 bolts and two roll pins. I suspect they may be from a Sportomatic trans?

Since mine's a dedicated racecar I like the idea upgrading to the stronger M10 bolts (i've yet to see a M10 screw be loose during a nut & bolt check) and larger CV having more angularity. Plus one can improve on the factory axles by getting floating axle shafts. One of the downsides to factory axles is their positive stop/shoulder that puts a lot of stress on the inner race.

Floating axles & a bulletproof 930 CV would be ideal for a racecar IMO. Offroad VWs are big fans of the 930 CV. But they're also heavier, which is counterproductive on a racecar. Of course there are finned/lightened 930 CVs available. All it takes is moar money?

Sorry for the rant. Just adding more info to the can of worms Jager is opening!

[jager911]

Originally Posted by Matt Monson

Jäger,
Just be aware that if I'm correct its not an option I can offer to you. The LSD in question was one of mine and not an original ZF unit. My gears won't mesh with a factory unit.

Thanks Matt, if I can't figure this out, I will raid the kids college fund and buy one of your nice GT LSDs which I assume is more plug and play with the correct coarse splines for my '74.

[jager911]

Originally Posted by KTL

Sorry for the rant. Just adding more info to the can of worms Jager is opening!

Thanks Kevin. Wow, all these years of tinkering with 911s and I thought the axles and LSDs were just interchangeable at least in the '74-'86 range. I helped with the rebuild of my 915 but haven't seen the guts of a LSD. Of course, I have the 4 M10 bolts with the 2 locating pins.
Now to pick your brain since you've let the worms out of the can.
1. I have a lead on SC axles with the 6 Allen bolts (M8?) and it includes the outside stub axles to the hubs. Are there any issues with the outside splines mating to the hub?

>The 100mm fine spline output flange is the simple way to go IMO. The >later SC and 84-85 Carrera used these flanges. MUCH easier to track down a >pair of these than it is to find those fine spline 108mm flanges used on the 930 >and '86 Carrera. As Tom said, you can use a later set of G50 flanges from >87-98 and have them machined. I know Rothsport does this modification.

2. I'm still not sure there is a fine spline stub axle readily available w/o mods which fits my '74 915 7:31 R&P and mates up with the '86 LSD I'm looking to install. Are you saying the stock stub axles from an SC (100mm fine spline) will work? If so, and if these are more readily available, I will take the plunge and buy the axles and LSD and then search for these, rather than the machining option.

[Alex973]

blackbird here have the axles you want

[jager911]

Originally Posted by Alex973

blackbird here have the axles you want

Yes, that's my source, I still need to ensure the SC outside stub axles will fit the splines in the wheel hub for a '74. I'm learning not to trust that everything is interchangeable.

[jager911]

Originally Posted by Tom '74 911

A third alternative would be to frankinstein it and use 100mm, fine splined output flanges & CVs on the inboard side (next to the transmission) and keep your 108mm CVs & stub axles out at the wheels. This would require a mix-matched axle CV combo...

Tom

Tom,
I'm desperate to try anything to make this work but I'd rather not go down the machining path. A buddy of mine located a pair of 6 bolt fine-spline stub axles in his parts bin so I hope they're the right ones. I located a set of SC axles end-to-end to the outside wheel stubs. What I don't know is if the wheel stub splines fit the hubs on a '74 so I'm entertaining your hybrid frankenstein option 3 as an alternative. I would have 6 bolt flange on inside and 4 bolt flange on the outside. So the only parts needed in this scenario are the inner CVs (and the fine spline stub axles for the LSD)? My car has a Carrera engine and 993TT brakes and even a Mazda cooler so what's some more Frankenstein bits.

[mreid]

Steve, the hubs appear to be the same 74-89:

Pelican Parts - Product Information: 911-331-065-33-M260

I'm in VA right now at my brothers 50th birthday party, but I'll let you know what Bob says on Monday.

[newms]

Hi Steve. I'm looking forward to your feedback after you get the LSD sorted. I found an alarming amount of noise and vibration at first. I also have wevo blue mounts which adds to it. I've gotten used to it and think its obvious in my car as I have no sound deadening anywhere. Also I am interested to hear your thoughts on the performance of the LSD. For me (I have a US 915 from an 83 with factory LSD). I'm not completely sure my track times are better but I do feel way more stability and safety in braking and trail braking turn in. And on the autox course ,where I have been known to go, an incredible improvement in off throttle stability and high speed decreasing radius entry. So overall I love it but not convinced my track times are a lot better. Maybe see you at pocono and we can compare notes.
Rick