What 915 trans do I have?

[PatrickM]

Hi there, this might seem like a very dumb question, but, without any code or serial # on the bottom rib of the housing, how can I tell what year my transmission is? The numbers have been ground off and well, there are some suspect looking numbers in their place. Is there a way to tell by looking at the housing, certain things on the housing that would clue me in to what year my transmission is?

Thanks, Patrick

[Bill Verburg]

Some fairly obvious external features
mechanical speed used through '75

Mg case used thru part of '77, Al after

The driver side plates became more robust over the years

beyond that You have to dis assemble or note the turn ratios between input and out

[PatrickM]

Thanks Bill. My car is a 1977, but since like I mentioned, the numbers on the bottom rib are fishy looking. Here are some photos from what I could get last night without jacking up the car. Perhaps you can tell what year it might be.
Thanks for your time.
-Patrick

[Bill Verburg]

Originally Posted by PatrickM

Thanks Bill. My car is a 1977, but since like I mentioned, the numbers on the bottom rib are fishy looking. Here are some photos from what I could get last night without jacking up the car. Perhaps you can tell what year it might be.
Thanks for your time.
-Patrick

looks like a stock '77 electric speedo, al, case, unreinforced sideplate, first year of the horseshoe spring assist
It should be a 915/61, /60 was the Mg case version & w/o spring assist w/ the same gearing
new that year was
double lock syncros for 1st & 2nd
asymmetric syncro teeth for first
smaller syncro ring 1st
stronger guide sleeve for 1/2 syncro

large friction lock area on syncros to slow the gears spin more quickly

[PatrickM]

Thanks Bill, so one of the al-you-min-eeeium 915s from later in the year. Both magnesium and , aluminum were made in '77, right? So a decent 915 but not as desired as the mag ones? 8:33 final drive.

[gled49]

If it’s 77, it is Mag. Late ones might have ALU side cover, or pivot cover or shift cover, maybe one maybe all. 8:31 R&P

[Rodney Nelson]

Why are the magnesium transmissions more desirable?

[Walt Fricke]

The mag tranny cases have a cast iron girdle embedded into the bulkhead which separates the diff from the part with gears 1-4. That holds the two big bearings for the input and pinion shafts in position better (or at least longer) than the aluminum cases do. The alu cases can hog out some over time, allowing the outer race to spin, which eventually leads to a failure.

Alu cases eventually need expensive machine work to install a steel girdle in this area. Usage may have some effect here: car heavily tracked in hot climate vs daily driver up North? Eventually probably the right, indefinite, term.

My '77 was mag case with the straight bar clutch release arm, but it rolled out of the factory in August, 1976. Bill V tends to know these interesting details, but as Porsche shifted from model to model, sometimes the newer model (or nominal year) got some of the old stuff that was still in the bins (why waste it?), and sometimes the dies for castings were getting too worn, and Porsche anticipated the next model and used compatible parts from them on the last of the older models if they had run out. This happened to the last of the SC engine cases - they came with the side drain plug which is emblematic of the 3.2 Carrera. So I could see this being so here.

Usually you can tell magnesium alloy from aluminum - it tends to be a duller gray. I believe another feature is the knob circled here:



If this knob is drilled through, I would suspect a mag case. The '76 912 used a 915 transmission, but the clutch was a push clutch - like the 901 or 356. So the clutch cable went through the hole in the knob, allowing it to push on the throw out bearing. The subsequent aluminum cases must have used the same, positive, master for making new molds, but didn't bother to drill this knob, so it is sort of a vestigial organ.

I don't have handy a list of transmission numbers. My little white spec book for the '77 and earlier lists chassis numbers for the several different transmissions, not transmission numbers.

[PatrickM]

Hey Walt, it's drilled through. So does that mean that it's magnesium?

[Walt Fricke]

I suspect so, but the real test is the appearance. You can buff mag, but I don't think it ever gets quite to aluminum's silvery color, and it oxidizes pretty quickly (which is a reason not to bother).

Your 2.7 for sure is mag, so you should be able to tell after cleaning grime and dirt off if the tranny is the same material as the engine.

[winders]

That case looks too bright and shiny to be magnesium. If it were magnesium, it would have a dull gray look to it.

[Bill Verburg]

Here's 4 transmissions
from eft to right, G50, 915, 011, 901, the middle 2 are Mg, the 2 ends are Al.





while it's possible that the OP's trans is Mg it looks more like Al to me, but that's JMO

[RWebb]

there is a lot of noise in that photo which gives it a 'shiny' look

can the OP clean an area and give it some side lighting for another pic?

[Walt Fricke]

Agree - I didn't trust the photos for color - they look like dust was sprayed on the case, though some of the background looks more normal. And even the gunk on the CV looks lumpier than usual.

[winders]

Sure...but look at the output flange in the photos. It is way duller and darker than the transmission case. When I look at my magnesium 915 case and the output flange they are a lot more similar in their lack of brightness and shininess than in these photos.

[IROC]

Originally Posted by Walt Fricke

Bill V tends to know these interesting details, but as Porsche shifted from model to model, sometimes the newer model (or nominal year) got some of the old stuff that was still in the bins (why waste it?), and sometimes the dies for castings were getting too worn, and Porsche anticipated the next model and used compatible parts from them on the last of the older models if they had run out. This happened to the last of the SC engine cases - they came with the side drain plug which is emblematic of the 3.2 Carrera. So I could see this being so here.

Yep, also on this era of 915s. My '76 has a mechanical speedo, for instance. Dec 1975 build date.

The gearbox above looks aluminum, to me.

[winders]

Hasn't this part always been aluminum:



If so, it matches the case perfectly.

[Walt Fricke]

I think that part has always been aluminum also. The output flange just looks a bit rusty (common) on its rim.

The most aluminum looking part is the rib where the palindromic 696 969 numbers he distrusts have been stamped.

Telemechanics can be a challenge.

[PatrickM]

Thanks to everyone that has chimed in and taken an interest. Sorry for the bad photos, but my garage is currently overrun with "stuff" from my basement as we are about to embark on a basement remodel. I haven't been able to make room to jack the car up to get a better photograph. Here are some more.

[Matt Monson]

That’s gearbox is aluminum.