Transmission swap question: early vs. late 915

[Jim Williams]

Hope I used the right title... Here's the real question which takes a little longer...

I have an aluminum SC 915 ('80 or so version with 8:31) in my 73 RS look, mated to SC trailing arms, using the 6-bolt CVs on both ends. If I wanted to swap in a '74 transmission (4-bolt CVs), just as it came out of a '74, what would be the easiest path to take?

Will the transmission axle flanges swap between these two transmissions, or is this project more complicated? The different speedo drives are not a problem as I left the early mechanical drive cable in place. I do have both transmission types out of the car and on the garage floor, and I can just pull the flanges and compare, but I thought someone would surely have been down this road before who has a quick answer at hand, especially if there's something I might be missing.

This is the car with the *3.1* liter SC factory upgrade engine, and I just want to feel the difference in get-up-and-go with the different R/P ratio. (For those interested, you can read about this engine in the Feb issue of Excellence in a story titled "Point One" .)

[Eagledriver]

I don't know for sure but I think the splines are different. Course on the early tranny and fine on the late. What I did on my car was use 86 Carrera axels and the old 4 bolt flanges. You can actually drill and tap the blind holes on the flanges and use all 6 bolts on the Carrera axels. The taped holes have very shallow threads to don't fully torque those 2 bolts. Other option is to use longer bolts on those 2 holes and a thin nut on the back of the flange. I have this on one side and the taped holes on the other.

The old flages are larger than the SC flanges and use bigger bolts. They match with the 86 Carrera CV's. I think it makes a nice upgrade.

-Andy

[Eagledriver]

Bolts are available from Ace Hardware.

-Andy

[KTL]

Andy,

I thought the CV diameter and axle flange diameter grew from 100mm to 108mm in '85. So that's weird that you can get the '86 axles to bolt up to the earlier 4 bolt flanges!?

[Porsche84]

I just compared 72 flanges to my 86 flanges. They are the same diameter. The 72 uses 4 bolts and 2 pins. Swapping back to the 74 tranny would require using earlier axles or the 85.5 and later axles. I prefer the newer ones as they have the cap on the end and are much less messy to deal with, and never come loose, like the SC ones do. You still need to deal with 4 vs 6 bolts. I'd use the pins like the early cars. If you tap the old flanges and can not put full torque on the bolts, you'll go crazy trying to remember which ones are the weak ones.

[Eagledriver]

There are 2 sizes of flanges, big on the early 4 bolt flanges and late Carrera/turbo flanges, small on the SC and early Carrera. Don't know the exact sizes but 100 and 108 sounds right. Don't know the years but 85 1/2 sounds right. When they first went to 6 bolt flanges the made the flanges smaller.

-Andy

[Eagledriver]

If you look at the flanges you'll see that the back side where the pins go is "taller" than the other holes. It's easy to see which holes are which.

-Andy

[Jim Williams]

Thanks for the inputs...

So, as an option to the 85.5 axles, could I use the 4-bolt hubs from the early (steel) trailing arms (already have these) in the aluminum trailing arms? Then bolt up the early axles and 4-bolt CVs (which I also already have)?

Maybe someone knows if this is a viable option.

Thanks for any additional inputs....

[Eagledriver]

I'm not sure but I think the hubs are different between the steel and aluminum trailing arms.

-Andy

[Porsche84]

The early hubs are coarse spline and the 85+ are fine spline. You need to verify what you have for a diff spline. If it is stock 74 then they will be coarse. The aluminum trailing arms don't care which flange you use.

[Wangrande]

Keep looking. It's been a long time, but i think i found the one year of trans axle hubs where the splines are early style and the bolts are late-6. i think it is a 76 trans. I have a 72 915 mated w/ 3.2 and aluminum trailing arms/axles from an sc.

[Fishcop]

Originally Posted by Jim Williams

Thanks for the inputs...

So, as an option to the 85.5 axles, could I use the 4-bolt hubs from the early (steel) trailing arms (already have these) in the aluminum trailing arms? Then bolt up the early axles and 4-bolt CVs (which I also already have)?

Maybe someone knows if this is a viable option.

Thanks for any additional inputs....

Not sure if the ally swing arms have room for the larger diameter CVs... conversely I put SC hubs into my early steel swing arms in order to accept a full SC gearbox and drive shafts - I know that works.

But... you still have the early gearbox output shaft issues - I'm pretty sure there isn't a 'fine spline' early 4 bolt output shaft. You could swap diffs to alleviate this. I also recall Grady Clay mentioning Turbo CVs being turned on a lathe to remove a flange to give extra strong 6 bolt CVs in the early diameter.

Cheers

[Porsche84]

I installed aluminum arms on a 72 which uses the large flange 4 bolt set up. No problems

[Jim Williams]

Originally Posted by Wangrande

Keep looking. It's been a long time, but i think i found the one year of trans axle hubs where the splines are early style and the bolts are late-6. i think it is a 76 trans. I have a 72 915 mated w/ 3.2 and aluminum trailing arms/axles from an sc.

Any ideas on where to look, or who to check with, to see if this is a viable option? This is the best solution I've seen if this would work. I've looked through the parts catalogs for some hint as to where this change happened but I can't see where the numbers for the flanges for a standard (manual) transmission change any where in the range of 73 through 77. There is a different flange number for the standard trans vs. the Sportmatic.

[Fishcop]

You'd imagine it was the year they changed mech to electric speedo, mag to alloy, 7:31 to 8:31 etc... Maybe the 3.0 Carrera years?

[Jim Williams]

Originally Posted by Fishcop

You'd imagine it was the year they changed mech to electric speedo, mag to alloy, 7:31 to 8:31 etc... Maybe the 3.0 Carrera years?

That's quite a range of maybe's.

The rear drive ratio for the US cars changed from 1974 to 1975.
The speedometer drive changed from 1975 to 1976.
The case material changed from 1977 to 1978.

My question was in the light of the quote:

" Originally Posted by Wangrande -
Keep looking. It's been a long time, but i think i found the one year of trans axle hubs where the splines are early style and the bolts are late-6. i think it is a 76 trans. I have a 72 915 mated w/ 3.2 and aluminum trailing arms/axles from an sc."

The question now is if there exists a flange with early style splines and later 6-bolt flanges.

Given the wide range of options available with the 915 over the years, it certainly might be possible that a 6-bolt transmission flange exists with a coarse spline. But I am getting a little overwhelmed trying to cross-reference numbers in the PET. There are some early 915s (like in 1975, California versions) that had an 8:31 R/P available, but they show the same pinion gear set part numbers as the 7:31 R/P, and so would use the same axle flange.

I hope there might be someone out there who can put this to rest, that knows if this can be done, or has been down this road before and knows it can't be done.

[Fishcop]

Jim I extracted this from a thread I found...

Hi all:
Have a problem putting parts together and getting conflicting info.
QUESTION: Do authentic 914-6 stub axles have 4 bolts and two pins with 28 splines.

I had what was represented as 914-6 stub axles but they have 6 bolt and 28 spline shaft. My shop said that is from later car and that seems to be correct.
Having said that I NEED one or two 914-6 stub axles (4 bolt two pin) 28 spline.
Also need to know value of 4 cly stub axle (36 spline)
Thanks in advance for the input .
All the best
Terry

It sounds like 28 splines are "course" and 36 splines are "fine". I would seem this gentleman had what you are searching for...

[Jim Williams]

John,

Thanks for passing on this bit of info.

This motivates me to become a little more familiar with "coarse" and "fine". I have never counted the number of splines and and have both the 4-bolt and the 6-bolt 915 transmissions on the floor in my garage, so I'm going to pull axle flanges on both and see what I've been talking about.

I have a friend in the 914 club, maybe I need to contact him and follow up on this possible lead.

I've been hoping that a 915 expert would chime in here (like a Gary Fairbanks type, for example), someone who knows these transmissions backwards and forwards, and have the answer to this right off the top of his/her head.

[Flat6pac]

The splines on the hubs wont cross over to the early axel. Recommend the early axel as stock and change to the metal sealed halfshafts of the 85.5, everything will bolt right through because the splines of the sealed halfshaft out board will fit the trailing arms and the inside will bolt to the early hubs. Sent a PM

Bruce

[Jim Williams]

Originally Posted by Fishcop

Jim I extracted this from a thread I found...

Hi all:
Have a problem putting parts together and getting conflicting info.
QUESTION: Do authentic 914-6 stub axles have 4 bolts and two pins with 28 splines.

I had what was represented as 914-6 stub axles but they have 6 bolt and 28 spline shaft. My shop said that is from later car and that seems to be correct.
Having said that I NEED one or two 914-6 stub axles (4 bolt two pin) 28 spline.
Also need to know value of 4 cly stub axle (36 spline)
Thanks in advance for the input .
All the best
Terry

It sounds like 28 splines are "course" and 36 splines are "fine". I would seem this gentleman had what you are searching for...

I just finished pulling an axle flange from each transmission. The actual spline count for the transmission flange hubs is different from the 28 and 36 given above. The early spline count is 18, and the late spline count is 43 (odd number - pun intended, and I did it 4 times to make sure).

Maybe the the 28 and 36 were the spline counts of the stub axles on the wheel hubs.