Identifying 901/911 Gears versus 915 Gears

[jluetjen]

If I've got a Porsche gear set in front of me, how can I tell if it is a 901/911 gear set versus a 915 gear set? Are the ID's the same? Are there other visual ways to tell them apart? Do the 915's still use letters for identification?

[PCA7GGR]

901 gears are identifed by a stamped alpha character (i.e. "ZA") and numbers that a written in black marker pen (i.e. "22") that indicated matched sets. Below is an example of a 901 "ZA" set:

[Grady Clay]

John,

Good question. The basic difference between the two transmissions are size.

First lets be clear about which two transmissions we are talking about. Additionally we are only talking the gear pairs that make up gears 1-5, 1-4 or 1-3.

The “901” starts in 911 production in 1965 although it includes the 1964 Type 904 5-speed. Essentially a similar transmission was used in the 906. For most purposes the transmission stayed the same from ’65-’69 when mid-year the castings were changed from cast aluminum to magnesium pressure die-castings. The transmissions also include the Type 902 common to many 912s as well as 911s. The Type 911 transmission from ’70-’71 were the same except slightly wider 1st & 2nd gears. In all the production transmissions there were both 4-speed and 5-speed versions.

All 914-4 and the ’70-’71 914-6 shared the same gears with the 901/904/906/911 (901-911)transmissions. The ’68-‘73 Type 905 4-speed Sportomatic shared the same gears with the 901-911 transmissions although some aren’t interchangeable.

The defining dimension for these 901-911 transmission is the spacing between the input shaft and the pinion shaft. It is nominally 67.36 mm.

The big change came with the Type 915 in ’72. The shaft spacing was increased to 76 mm, the gears were also wider, the shaft splines larger, and there were six bearings (two 4-point & four roller) to support the two shafts in place of the prior four bearings (two 4-point & two roller.) The 915 was used in production from ’72 through ’85 and came in both 5-speed and 4-speed versions.

Additionally there was the Type 925 Sportomatic in both 4-speed and 3-speed versions that shared the 915 dimensions.

There was a Type 916 used in the ’72 916 version of the 914-6. It was based on the 915.


All the gears in both these series have identifying matching numbers hand etched. The gear that is mounted on the input shaft is stamped with the Roman numeral “I” and the gear mounted on the pinion shaft is stamped “II”. All of the gears have a date stamped in at least one of the matching pair in the form of MM/YY. Some had the complete part number stamped in the gear (example: 904.302.101.00). In Porsche parlance the one gear that is splined onto a shaft is the “fixed” gear and the other one that is supported by a bearing (and has the syncro) is the “free” gear.

For 901-911 gears the ratio was given a letter designation. The “A” was the lowest. Some additions required two-letter designations, for example there was an existing “G” and an addition was designated “GA”. I think ZD was the tallest ratio gear pair. The 901-911 gears can have either one or two letters. In a normal 901-911 5-speed there are four types of non-interchangeable gears; 1st gears, second gears (mostly intergral with the input shaft), 3rd gears, and the interchangeable 4th&5th gears.

The 1st, 2nd and 3rd gears are “fixed” on the input shaft. The 4th and 5th gears are “fixed” on the pinion shaft. The angle of the cut of the gears is reversed from the 1st-2nd-3rd gears to the 4th-5th gears. You cannot use a 3rd gear as a 4th-5th gear or vice versa.

The gear pairs in the 914-4 transmissions were not marked.

For the 915/925 gears the number of teeth on each gear was designated with a letter. For example an 11-tooth gear is an “A” and a 35-tooth gear is a “Z” giving the first gear ratio “AZ” for the 11:35=3.18…:1 ratio. Another example is the 4th gear “QQ” for the 26:26= 1:1 ratio. All 915 gears have two letters. In a normal 915 5-speed there are four types of non-interchangable gears; 1st gears, 2nd gears, interchangeable 3rd&4th gears, and 5th gears.


When discussing the ratio of gear pairs it is wise to include the type, gear, letter designation, tooth count and numeric ratio (901, 3rd, M, 22:29=1.318.) While this may seem like overkill, it avoids confusion. There is no substitute for taking a Magic Marker and counting the teeth.

Best,
Grady

[Zeke]

Originally posted by Grady Clay


The 1st, 2nd and 3rd gears are “fixed” on the input shaft. The 4th and 5th gears are “fixed” on the pinion shaft. The angle of the cut of the gears is reversed from the 1st-2nd-3rd gears to the 4th-5th gears. You cannot use a 3rd gear as a 4th-5th gear or vice versa.


Best,
Grady

Thanks for the post, Grady. As per usual, I printed it out and saved it in my 3-ring binder in the trans section.

However, could you please clarify something? In the past, you have recommended not "flipping" a 5th gear for use as a 3rd. (I did this in my 914 tranny and it shifted great) The reason you gave is the is the angle of the gear cut is not consistant with the gears on the same shaft. So, the force created by the gear is to the wrong direction, is that right?

But here you say the gears are cut differently for 4-5 as opposed to 1,2 & 3. To my poor non-engineering mind, that would make 5th correct in either location. I will add that after I flipped the 5th gear in the 914, I could clearly see the gear cut and how it differed from it's neighbors. Now, I don't understand why it is so.

[Grady Clay]

Milt,

As always you help me clarify the details – and the devil
is in the details.

Here is the gear cluster of the 901/02, S/N 103379. It is
a virgin that I’m the first to inspect anything inside since
Porsche assembled and delivered it in a ’67 911. This
has been sitting in my possession for almost 40 years.
Pelican is the first to see it.
GearCluster67_01_IMG_1986a.jpg


Note that P-260 is installed.
Also note the angle of all the gears on the pinion
shaft (upper in this image).

OK, the point is understanding the subtractive or additive
forces due to the gear placement. The issue is if it is
appropriate to use a 901-911 4th or 5th gear in place
of a 3rd gear. Many people have done this. You can
take a 23:29=0.793 “Z” 5th gear and install it as a
29:23=1.261 “ “ 3rd gear.

Here is a 5th gear in the normal position as illustrated
above. The “fixed” gear II (top) is on the pinion shaft
and the “free” gear I (bottom) is on the input shaft.
Note the axial force (green arrow to the left) is opposite
direction to the force (blue arrow to the right) when the
gears are loaded under acceleration. This axial force
from the gear angle counteracts the axial force from the
pinion that the 4-point bearing must absorb.
GearCluster01cForce5th01.jpg




The same is true if the same 5th gear is positioned as
a 4th gear.
GearCluster01dForce4th01.jpg



Now a big deal if the same 901, 23:29=0.793:1
“Z” 5th gear is repositioned and used as a 29:23=1.261:1
“ “ 3rd gear position, the gear force (horizontal green
arrow to the right) becomes additive with the pinion
axial force (horizontal blue arrow to the right). This can
cause the pinion 4-point bearing to become overloaded
and fail.
GearCluster01eForce3rd01.jpg



The same is true if (with serious modifications) a 4th or
5th gear were used in the 2nd gear position.
GearCluster01fForce2nd01.jpg



Can you do this? Yes, so long as you replace this
expensive 4-point pinion bearing very regularly.
Yes, you probably can get away with it using a
1.7 914 engine. No, you can’t if using any powerful
6-cylinder.

I hope this is clear enough so everyone understands.
If anyone has questions, feel free to post. Every
question is welcome.

Thanks Milt.

Best,
Grady

[randywebb]

Ok, it's because the gears are helical cut, not straight cut...

oui?

[Zeke]

Well, sure, Randy, that would make sense. And thanks, Grady. Now I see the answer. I didn't realize that I was turning the gear around facing the other way. Sure, the force is wrong now. Well, my short ratio box was indeed for a 4 banger and I wasn't planning on spending a lot of time in 3rd or driving the hiways at all. It might have worked in my application. I don't know, the motor didn't last long enough for me to spend much time at all in any gear.

[Fahren Cars]

This has been a super helpful and terrific thread for learning. Are randywebb and Grady Clay still around? My son and I are rebuilding a 901 side shift for 914, and we have a few questions -- would love to hop on a quick call.

- Jason and Jacques #914fsp

[Matt Monson]

Originally Posted by Fahren Cars

This has been a super helpful and terrific thread for learning. Are randywebb and Grady Clay still around? My son and I are rebuilding a 901 side shift for 914, and we have a few questions -- would love to hop on a quick call.

- Jason and Jacques #914fsp

Grady passed about 7-8 years ago. I haven’t seen Randy post in years.

[Matt Monson]

I’ll also add that there’s a guy named Mike, who goes by Dr Evil. If you google search him you’ll find both a YouTube channel and a rebuilding dvd he sells that is quite good.